Brazilian banded iron formations

Brazilian banded iron formations: a geological and geotechnical

characterisation from hard and fresh to weak and completely

weathered rocks

TEÓFILO AQUINO VIEIRA DA COSTA

This thesis is presented for the degree of

Doctor of Philosophy

The University of Western Australia

School of Civil, Environmental and Mining Engineering

June 2021

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THESIS DECLARATION

I, Teófilo Aquino Vieira da Costa, certify that:

This thesis has been substantially accomplished during enrolment in the degree.

This thesis does not contain material which has been submitted for the award of any other

degree or diploma in my name, in any university or other tertiary institution.

No part of this work will, in the future, be used in a submission in my name, for any other degree

or diploma in any university or other tertiary institution without the prior approval of The

University of Western Australia and where applicable, any partner institution responsible for the

joint award of this degree.

This thesis does not contain any material previously published or written by another person,

except where due reference has been made in the text and, where relevant, in the Declaration

that follows.

The works are not in any way a violation or infringement of any copyright, trademark, patent, or

other rights whatsoever of any person.

The research involving geotechnical data reported in this thesis was assessed and approved by

The University of Western Australia and Vale S.A.

The work described in this thesis was funded by Vale S.A.

Technical assistance was kindly provided by the Australian Centre for Geomechanics (ACG),

E-precision Laboratory, Geocontrole Br. Sondagens S.A. and the Petrophysics laboratory of

Federal University of Campina Grande for laboratorial experiments that is analysed in this thesis.

This thesis contains published work and work prepared for publication, all of which has been

co-authored.

Date: June 21, 2021

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PUBLICATIONS ARISING FROM THIS THESIS

Conference papers – already published:

Costa, TAV, Dight, PM, Mercer, K & Marques, EAG 2015, ‘An overview of the weathering process

and preliminary density and UCS correlations for fresh itabirites in Vale mines on the western

side of the Iron Quadrangle, Brazil’, Proceedings of the Iron Ore Conference, Australasian

Institute of Mining and Metallurgy, Perth, Australia.

Costa, TAV, Mercer, K, Dight, PM & Marques, EAG 2015, ‘Weathered banded iron formations in

Vale iron ore mines on the western side of the Iron Quadrangle, Brazil: weak hematitite and

weathered argillaceous itabirite geotechnical characteristics and implications of matric suction

effects on slope stability’, Proceedings of the 25th International Symposium on Slope Stability in

Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, Australia.

Prepared as manuscript – not published:

Intact rock strength characteristics and elastic static properties of fresh Brazilian banded iron

formations.

Petrophysical characteristics and elastic dynamic properties of fresh to moderately weathered

Brazilian banded iron formations.

Weak rock behaviour of highly to completely weathered Brazilian banded iron formations.

Weathering profile, intact rock strength and elastic characteristics of Brazilian banded iron

formations.

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STATEMENT OF CANDIDATE CONTRIBUTION (%)

This thesis contains co-authored published and unpublished papers. The percentage of the work

of co-authors is presented below.

Costa, TAV (70%), Dight, PM (20%), Mercer, K (5%) & Marques, EAG (5%) 2015, ‘An overview of

the weathering process and preliminary density and UCS correlations for fresh itabirites in Vale

mines on the western side of the Iron Quadrangle, Brazil’, Proceedings of the Iron Ore

Conference, Australasian Institute of Mining and Metallurgy, Melbourne, Australia.

Costa, TAV (70%), Mercer, K (20%), Dight, PM (5%) & Marques, EAG (5%) 2015, ‘Weathered

banded iron formations in Vale iron ore mines on the western side of the Iron Quadrangle, Brazil:

weak hematitite and weathered argillaceous itabirite geotechnical characteristics and

implications of matric suction effects on slope stability’, Proceedings of the 25th International

Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for

Geomechanics, Perth, Australia.

Costa, TAV (80%), Dight, PM (10%) and Marques, EAG (10%) Intact rock strength characteristics

and elastic static properties of fresh Brazilian banded iron formations – Brazil. Not published.

Costa, TAV (80%), Marques, EAG (10%), Dight, PM, (5%) and Lima, P (5%) Petrophysical

characteristics and elastic dynamic properties of fresh to moderately weathered Brazilian

banded iron formations. Not published.

Costa, TAV (80%), Dight, PM (10%) and Marques, EAG (10%) Weak rock behaviour of highly to

completely weathered Brazilian banded iron formations. Not published.

Costa, TAV (80%), Mercer, K (10%), Dight, PM (5%) and Marques, EAG (5%) Weathering profile,

intact rock strength and elastic characteristics of Brazilian banded iron formations. Not

published.

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ABSTRACT

Brazilian Proterozoic banded iron formations (BIF), classified as low-grade ore, ‘itabirites’ are

divided into quartzitic, dolomitic and amphibolitic and high-grade ore ‘hematitite’, together are

the main iron host rock of the Iron Quadrangle mines in Brazil. Their genesis is controversial, but

it is agreed that metamorphic and tectonic events as well as the supergene and hypogene

enrichment are responsible for reconcentrating the iron, and weathering processes are

responsible for reducing the original high intact rock strength, generating deep and

heterogeneous weathered profiles with low strength rocks (weak rocks) reaching 400 m depth.

Based on field investigation and laboratory tests, from 15 different mines, this thesis aims to

determine intact rock strength parameters and petrophysical proprieties (macro and micro

scales) in different weathering profile levels (horizons and zones), highlighting geological and

geotechnical characteristics considering the degree of anisotropy defined by the compositional

metamorphic banding (heterogeneity). Establishing relationships between petrophysical, rock

strength and elastic parameters, proposing empirical correlation equations.

To reach the thesis goals, rock laboratory tests (triaxial, unconfined compressive strength – UCS,

P and S wave and Brazilian test) and soil laboratory tests (Atterberg limits, particle size

distribution and soil-water characteristic curves, saturated and unsaturated direct shear test,

triaxial – CIU and permeability test) were undertaken for each typology in different anisotropy

directions. Additionally, petrographic thin sections, geological and geotechnical field

investigation, and permeability in situ tests were assessed.

All test results and Vale’s internal database were assembled and evaluated, and a complete

failure envelope for each typology, was determined to describe the intact rock and shear strength

parameters variance in association with the geological and geotechnical characteristics along

the weathering profile. For each weathering level it was concluded that:

For fresh typologies, the anisotropy ratio and index, respectively based in the UCS tests and Vp

measures, are low to isotropic unless for fresh dolomitic itabirites that present a fairly to

moderately anisotropy ratio. Even with a moderate UCS results dispersion, there is a direct

correlation between iron content, bulk density and UCS parameters for hematitite and itabirites,

and inverse correlation with total porosity. For these types, the main characteristics responsible

for defining the rock strength and anisotropy are the mineralogy and the rock fabric. On this

matter, hard hematitite is the stronger strength typology followed by fresh quartzitic,

amphibolitic and dolomitic itabirites. Hard hematitite also presents extremely high elastic

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parameters followed by amphibolitic and quartzitic itabirites and dolomitic itabirites presented

the lower elastic behaviour.

Also, for fresh rocks positioned at the bedrock of the BIF weathering horizon, empirical

correlations equations for UCS, Young´s Module, bulk density and P and S wave velocity were

established, which indicate a reliable, straightforward, and low-cost method which can be used

to predict, with acceptable accuracy, intact rock strength and elastic parameters.

Moderately weathered typologies, positioned at saprorock and saprolite horizons, even with a

small number of tests, showed fairly to moderately anisotropy index due to the higher total

porosity and lower bulk density, behaving like a soil when highly weathered (saprolite horizon),

or rock when moderately weathered (saprorock horizon). For these types, the heterogeneity is

defined mainly by the total porosity, however the mineral compositing plays an important role.

The completely weathered rocks are characterised as saprolite or in situ residual soil horizons.

For these BIF rock-like soil types, the bulk density, particle size distribution, permeability, total

porosity, and water content are the most important parameters for typology shear strength

variation. The low anisotropic ratio generally obtained affect minimally the low shear strength

values of weathered BIF types. The weathered argillaceous itabirite presents the lowest

permeability and highest clay content that induces a matric suction effect (up to 80 kPa) and as

an aquiclude can keep the negative porewater pressure (suction) describing an important

unsaturated behaviour.

This thesis concludes that for BIF rocks each weathering horizon and level presents a typical

intact rock strength, elastic parameters, and intrinsic petrophysical proprieties defining a specific

geomechanical behaviour mainly controlled by the binomials iron content/bulk density, total

porosity/permeability, and the mineral composition/bands thickness. Ultimately, the weathering

profile horizon and level control slope stability and failure mechanisms not only for long-term

excavations but also for temporary slopes from shallow to deep iron ore mines.

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TABLE OF CONTENTS

THESIS DECLARATION ........................................................................................................... III

ABSTRACT ........................................................................................................................... VII

LIST OF TABLES ................................................................................................................ ..XXV

LIST OF GRAPHS ............................................................................................................. . XXVII

LIST OF SYMBOLS ............................................................................................................. XXIX

ACKNOWLEDGEMENTS .................................................................................................... .XXXI

DEDICATION ................................................................................................................... . XXXII

AUTHORSHIP DECLARATION: CO-AUTHORED PUBLICATIONS .......................................... XXXIII

CHAPTER 1. INTRODUCTION ............................................................................................... 1

1.1 Problem statement ....................................................................................................... 1

1.2 Research objectives ....................................................................................................... 3

1.3 Research limitation ....................................................................................................... 5

1.4 Thesis contributions ...................................................................................................... 7

1.5 Thesis organisation ........................................................................................................ 8

CHAPTER 2. LITERATURE REVIEW ..................................................................................... 11

2.1 Regional geological settings ........................................................................................ 11

2.2 BIF geological settings ................................................................................................. 12

2.2.1 Itabirites and hematitites geotechnical and geological settings ......................... 13

2.3 Weathering profile settings ......................................................................................... 18

2.3.1 Failure criteria evaluation for the BIF’s weathering profiles ............................... 21

2.3.2 BIF weathering profile determination and implications for slope stability ........ 23

2.4 Rock mechanics approaches for fresh to moderately weathered BIF ........................ 25

2.5 Soil mechanics approaches for completely weathered BIF ........................................ 27

CHAPTER 3. METHODOLOGY ............................................................................................ 33

3.1 Fieldwork and sampling .............................................................................................. 34

3.2 Database consistency approachs and definitions ....................................................... 34

3.3 Laboratory tests .......................................................................................................... 38

3.3.1 Sampling validation and laboratory test grouping .............................................. 38

3.3.2 Rock mechanics tests .......................................................................................... 40

3.3.3 Soil mechanics tests ............................................................................................ 44

3.3.4 Petrographic thin sections .................................................................................. 48

3.4 Software used.............................................................................................................. 49

CHAPTER 4. INTACT ROCK STRENGTH CHARACTERISTICS AND ELASTIC STATIC PROPERTIES OF FRESH BRAZILIAN BANDED IRON FORMATIONS .............................................................. 51

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ABSTRACT ........................................................................................................................... 51

4.1 Introduction................................................................................................................. 52

4.2 Objectives and approaches ......................................................................................... 53

4.3 Geological and geotechnical settings .......................................................................... 55

4.3.1 Regional geological settings ................................................................................ 55

4.3.2 BIF geological and geotechnical settings ............................................................. 57

4.3.3 Intact rock strength parameters, anisotropy and petrophysical properties correlations ......................................................................................................................... 62

4.4 Methodology ............................................................................................................... 68

4.4.1 Laboratory tests .................................................................................................. 73

4.5 Results ......................................................................................................................... 76

4.5.1 Mineralogical and fabric overview ...................................................................... 76

4.5.2 BIF, heterogeneity and anisotropy ...................................................................... 80

4.5.3 Intact rock strength anisotropy ........................................................................... 83

4.5.4 BIF characterisation of geomechanical properties and parameters ................... 88

4.6 Discussion .................................................................................................................. 100

4.6.1 BIF compositional metamorphic banding heterogeneity and strength anisotropy ........................................................................................................................... 100

4.6.2 Petrophysical, geological and geomechanical properties characterisation and correlations ....................................................................................................................... 103

4.7 Conclusion ................................................................................................................. 108

CHAPTER 5. PETROPHYSICAL CHARACTERISTICS AND ELASTIC DYNAMIC PROPERTIES OF FRESH TO MODERATELY WEATHERED BRAZILIAN BANDED IRON FORMATIONS ................. 113

ABSTRACT ......................................................................................................................... 113

5.1 Introduction............................................................................................................... 114

5.2 Objectives and approaches ....................................................................................... 116

5.3 Geological and geotechnical settings ........................................................................ 117

5.3.1 Regional geological settings .............................................................................. 117

5.3.2 BIF geological and geotechnical settings ........................................................... 119

5.3.3 P and S wave velocities, dynamic elastic and petrophysical properties correlations ........................................................................................................................... 126

5.4 Methodology ............................................................................................................. 133

5.4.1 Laboratory tests ................................................................................................ 136

5.5 Results ....................................................................................................................... 140

5.5.1 Mineralogical and fabric overview .................................................................... 140

5.5.2 Heterogeneity and anisotropy evaluations ....................................................... 146

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5.5.3 Anisotropy evaluations of dynamic elastic parameters and petrophysical properties .......................................................................................................................... 151

5.5.4 Isotropic evaluations of dynamic elastic parameters and petrophysical properties ........................................................................................................................... 159

5.6 Discussion .................................................................................................................. 175

5.6.1 BIF compositional metamorphic banding heterogeneity and the anisotropy behaviour .......................................................................................................................... 175

5.6.2 Correlations between wave velocity propagation, dynamic elastic, and petrophysical proprieties .................................................................................................. 178

5.6.3 Comparing dynamic with static parameters .................................................... 181

5.7 Conclusion ................................................................................................................. 181

CHAPTER 6. WEAK ROCK BEHAVIOUR OF HIGHLY TO COMPLETELY WEATHERED BANDED IRON FORMATIONS FROM IRON QUADRANGLE – BRAZIL .................................................. 185

ABSTRACT ......................................................................................................................... 185

6.1 Introduction............................................................................................................... 187


6.3 Geological and Geotechnical settings ....................................................................... 190

6.3.1 Regional geological setting ................................................................................ 190

6.3.2 Banded iron formation geological settings ....................................................... 191

6.3.3 Banded iron formations weathering profile ..................................................... 199

6.4 Saturated and unsaturated approaches brief literature review ............................... 202

6.4.1 Industry overview .............................................................................................. 202

6.4.2 Application of unsaturated theory to open pit mining ..................................... 205

6.4.3 Soil–water characteristic curves ....................................................................... 210

6.5 Methodology ............................................................................................................. 212

6.5.1 Overview ........................................................................................................... 212


6.6 Results ....................................................................................................................... 222

6.6.1 Fabric and mineralogical thin section overview ................................................ 222

6.6.2 Unified soil classification system ....................................................................... 228

6.6.3 Bulk density ....................................................................................................... 234

6.6.4 Coefficient of permeability ................................................................................ 234

6.6.6 Saturated shear strength characteristics .......................................................... 239

6.6.7 Unsaturated shear strength characteristics ...................................................... 252

6.7 Discussion .................................................................................................................. 256

6.8 Conclusion ................................................................................................................. 261

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CHAPTER 7. WEATHERING PROFILE, INTACT ROCK STRENGTH AND ELASTIC CHARACTERISTICS OF BRAZILIAN BANDED IRON FORMATIONS ......................................... 265

ABSTRACT ......................................................................................................................... 265

7.1 Introduction............................................................................................................... 267


7.3 Geological and geotechnical settings ........................................................................ 269

7.3.1 Regional geological settings .............................................................................. 269

7.3.2 BIF geological and geotechnical settings ........................................................... 271

7.3.3 Banded iron formation weathering profiles ..................................................... 284

7.3.4 Failure criteria evaluation for the BIF’s weathering profiles ............................. 286

7.4 Methodology ............................................................................................................. 288


7.5 Results ....................................................................................................................... 297

7.5.1 Banded iron formation weathering profiles ..................................................... 297

7.5.2 BIF mineralogical and fabric overview .............................................................. 302

7.5.3 Heterogeneity and anisotropy for BIF weathering profiles, a micro overview . 308

7.5.4 Evaluation of the BIF anisotropy throughout the complete weathering profile .... ........................................................................................................................... 312

7.5.5 BIF isotropic approach throughout the complete weathering profile .............. 322

7.5.6 Strength envelope of best fit curve for completely weathered BIF profile ...... 328

7.6 Discussion .................................................................................................................. 340

7.7 Conclusion ................................................................................................................. 346

CHAPTER 8. CONCLUDING REMARKS .............................................................................. 351

8.1 Fresh to moderately weathered BIF geological and geomechanical characterisation ................................................................................................................................... 351

8.2 Completely weathered to residual soils and weak BIF geological and geomechanical characterisation .................................................................................................................... 356

8.3 BIF completely weathered profile geological and geomechanical characterisation 359

CHAPTER 9. RECOMMENDATIONS FOR FUTURE WORK .................................................. 365

9.1 BIF hard rock behaviour ............................................................................................ 365

9.2 BIF weak rock behaviour ........................................................................................... 366

9.3 Future approaches for thesis limitations .................................................................. 366

REFERENCES ...................................................................................................................... 367

APPENDIX I ........................................................................................................................ 387

APPENDIX II ....................................................................................................................... 419

APPENDIX III ...................................................................................................................... 447

APPENDIX IV ...................................................................................................................... 451

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APPENDIX V ....................................................................................................................... 469

APPENDIX VI ...................................................................................................................... 475

APPENDIX VII ..................................................................................................................... 481

APPENDIX VIII .................................................................................................................... 487

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LIST OF FIGURES

FIGURE 1.1 RESEARCH AIMS INTERCONNECTION .............................................................................. 5

FIGURE 1.2 STUDIED MINES LOCALISATION, STRATIGRAPHIC COLUMN, AND IRON QUADRANGLE GEOLOGICAL

SETTINGS (MODIFIED FROM MORGAN ET AL. 2013) ......................................................................... 7

FIGURE 2.1 SUMMARY FOR ALL STUDIED BIF TYPES, SHOWING IN THE GREEN DOTTED SQUARE THE HIGH UCS

VALUES, IN BLUE THE INTERMEDIATE, AND RED THE LOW UCS VALUES (AFTER MARTIN & STACEY 2018) 18

FIGURE 2.2 SCHEMATIC DIAGRAM OF TYPICAL TROPICAL RESIDUAL SOIL PROFILE PRESENTING THE HORIZONS

ON THE LEFT SIDE AND ZONES ON THE RIGHT SIDE (MODIFIED FROM DEERE & PATTON 1971) ............... 19

FIGURE 2.3 THE THREE REGIONS FOR THE HOEK–BROWN STRENGTH CURVE AND THE EQUIVALENT MOHR–

COULOMB STRENGTH LINE (LIN ET AL. 2014) ................................................................................ 22

FIGURE 2.4 MOHR–COULOMB FAILURE ENVELOPE FOR A SATURATED SOIL ........................................ 28

FIGURE 2.5 EXTENDED MOHR–COULOMB FAILURE ENVELOPE FOR UNSATURATED SOILS (GAN & FREDLUND

ET AL. 1988) ........................................................................................................................... 30

FIGURE 3.1 CLASSIFICATION BASED ON ANISOTROPIC RATIO, SINGH ET AL. (1989) AND ΒANGLE DEFINITION

AFTER MCLAMORE & GRAY (1967) ............................................................................................ 37

FIGURE 3.2 P WAVE VELOCITY ANISOTROPIC INDEX, MODIFIED FROM PERUCHO ET AL. (2014) ..................

............................................................................................................................. 38

FIGURE 3.3 WEATHERING GRADE AND ESTIMATION OF THE ROCK STRENGTH TABLE PLOTTED FOR THE

THREE MAIN GROUPS. THE APPLIED LABORATORY TESTS FOR EACH STRENGTH LEVEL ARE GROUPED BY THE

GREEN DOTTED SQUARE FOR HARD ROCKS, A BLUE DOTTED SQUARE FOR MODERATE ROCK STRENGTH AND

THE RED DOTTED SQUARE FOR WEAK ROCK AND SOIL-LIKE MATERIAL STRENGTH (AFTER MARTIN & STACEY

2018) ......................................................................................................................... 39

FIGURE 3.4 THE MECHANICAL AND ELECTRONIC APPARATUS (AUTOLAB-500®) USED FOR P AND S WAVE

VELOCITIES AND EFFECTIVE POROSITY TESTS (LIMA & COSTA 2016) .................................................. 44

FIGURE 3.5 THE SCHEMATIC MODIFIED SUCTION CONTROLLED DIRECT SHEAR TEST APPARATUS FOR

TESTING THE SHEAR STRENGTH OF UNSATURATED SOILS, FROM GAN ET AL. (1988) ............................. 48

FIGURE 4.1 MINE LOCATIONS (RED CIRCLES) AND IRON QUADRANGLE GEOLOGICAL SETTINGS (MODIFIED

FROM MORGAN ET AL. 2013) .................................................................................................... 55

FIGURE 4.2 A (LEFT), HHE AT MICROSCOPE VIEW WITH GRANULAR CRYSTALS OF HEMATITE (LIGHT GREY)

AND SMALLER CRYSTALS OF HEMATITE MICROPLATES (LIGHT GREY) (HORTA & COSTA 2016). B (RIGHT),

OUTCROP OF FRACTURED HHE AT CAPITÃO DO MATO MINE ........................................................... 59

FIGURE 4.3 A (LEFT), SHOWS MICROPHOTOGRAPHY OF FQI PRESENTING TYPICAL QUARTZ BANDING

(WRITE CRYSTALS), GRANULAR TO TABULAR HEMATITE (LIGHT GREY) AND POROUS (BLACK) (HORTA & COSTA

2016). B (RIGHT), TYPICAL OUTCROP PRESENTING FRACTURES IN TAMANDUÁ MINE ........................... 60

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FIGURE 4.4 A (LEFT), SHOWS MICROPHOTOGRAPHY OF A FAI HIGHLIGHTING THE PRESENCE OF FIBROUS

GOETHITE AN AMPHIBOLITE ACICULAR OLD CRYSTAL (DARK FIBRE MINERALS) IMMERSE ON QUARTZ BANDS

(HORTA & COSTA 2016). B (RIGHT), SHOWS A TYPICAL SLOPE OF FOLDED AND FRACTURED FAI AT JANGADA

MINE ......................................................................................................................... 61

FIGURE 4.5 A (LEFT), SHOWS A MICROPHOTOGRAPHY OF FDI HIGHLIGHTING THE TYPICAL BANDING OF

IRON DOLOMITE AND QUARTZ (BLUE LIGHT COLOUR), AND FERROAN DOLOMITE AND TABULAR HEMATITE

BANDS (DARK GREY) (HORTA & COSTA 2016). B (RIGHT), TYPICAL HAND SAMPLE OF FOLDED FDI ......... 62

FIGURE 4.6 CLASSIFICATION BASED ON ANISOTROPIC RATIO, SINGH ET AL. (1989) AND ΒANGLE DEFINITION

AFTER MCLAMORE & GRAY (1967) ............................................................................................ 72

FIGURE 4.7 P WAVE VELOCITY ANISOTROPIC INDEX, MODIFIED FROM PERUCHO ET AL. (2014) .......... 72

FIGURE 4.8 A (LEFT), FQI BANDS OF HEMATITE (LIGHT GREY) WITH SMALLER CRYSTAL SIZE IN CONTACT

WITH BANDS OF LARGER CRYSTALS OF GRANULAR QUARTZ (COLOURED) AND A ROUGH CONTACT BETWEEN

(DASHED RED LINE) (HORTA & COSTA 2016). B (RIGHT), HHE CONTACT LESS ROUGH (DASHED RED LINE)

BETWEEN LARGER GRANULAR HEMATITE (DARK BAND) WITH SMALLER CRYSTALS OF TABULAR HEMATITE

(LIGHT BAND) (HORTA & COSTA 2016) ....................................................................................... 81

FIGURE 4.9 A (LEFT), TYPICAL SPECULARITE ORIENTATION CRYSTAL (HORTA & COSTA 2016). B (LEFT),

FDI BANDS OF ORIENTED GRANULAR IRON DOLOMITE WITH LARGE CRYSTALS (LIGHT BROWN) AND BANDS OF

TABULAR HEMATITE MIXED WITH IRON DOLOMITE AND QUARTZ (DARK BROWN) (HORTA & COSTA 2016) ..

......................................................................................................................... 82

FIGURE 4.10 A (LEFT), PORE BANDS IN MASSIVE HHE. B (RIGHT), FAI THIN SECTION WITH PORES ALONG

BANDING (TABULAR HEMATITE – LIGHT GREY, AND QUARTZ – WHITE) FILLED BY GOETHITE (LIGHT RED)

(HORTA & COSTA 2016) ........................................................................................................... 83

FIGURE 4.11 HETEROGENEITY VARIATION ACCORDING TO THE INCREASE OF IRON CONTENT FROM POOR BIF

(IRON RICH COUNTRY ROCKS) TO RICHEST BIF (HEMATITITE) .......................................................... 102

FIGURE 5.1 STUDIED MINES LOCALISATION AND IRON QUADRANGLE GEOLOGICAL SETTINGS (AFTER

BAARS & ROSIÈRE 1997) ........................................................................................................ 119

FIGURE 5.2 A (LEFT), HHE AT MICROSCOPE VIEW WITH GRANULAR CRYSTAL OF HEMATITE (LIGHT GREY)

AND SMALLER CRYSTAL OF HEMATITE MICRO PLATES (LIGHT GREY) (HORTA & COSTA 2016). B (RIGHT),

OUTCROP OF FRACTURED HHE AT CAPITÃO DO MATO MINE ......................................................... 122

FIGURE 5.3 A (LEFT), MICROPHOTOGRAPHY OF FQI PRESENTING TYPICAL QUARTZ BANDS (LARGE

COLOURED CRYSTAL), GRANULAR TO TABULAR HEMATITE BAND (LIGHT GREY) AND PORES (BLACK) (HORTA &

COSTA 2016). B (RIGHT), TYPICAL OUTCROP PRESENTING FRACTURES IN TAMANDUÁ MINE ................ 123

FIGURE 5.4 A (LEFT), PWQI MICROPHOTOGRAPHY HIGHLIGHTING PRESENCE OF GOETHITE CEMENTING A

FRACTURE (HORTA & COSTA 2016). B (RIGHT), AN OVERVIEW OF TYPICAL PWGI SLOPE AT TAMANDUÁ

MINE ....................................................................................................................... 124

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FIGURE 5.5 A (LEFT), FAI MICROPHOTOGRAPHY HIGHLIGHTING FIBROUS GOETHITE AND AMPHIBOLITE

ACICULAR OLD CRYSTAL (DARK FIBRE MINERALS) IMMERSED IN QUARTZ BANDS (HORTA & COSTA 2016). B

(RIGHT) TYPICAL MINE SLOPE FOLDED AND FRACTURED FAI AT JANGADA MINE ................................. 125

FIGURE 5.6 A (LEFT), PWGI AT MICROSCOPE VIEW SHOWING IN RED ORANGE THE LARGE AMOUNT OF

GOETHITE (HORTA & COSTA 2016). B (RIGHT), TYPICAL PWGI FROM JANGADA MINE SLOPE ............. 125

FIGURE 5.7 A (LEFT), FDI MICROPHOTOGRAPHY HIGHLIGHTING TYPICAL BANDING OF DOLOMITE AND

QUARTZ (LIGHT COLOUR) INTERLAYERED BY BANDS OF HEMATITE AND QUARTZ (DARK COLOURS) (HORTA &

COSTA 2016). B (RIGHT), TYPICAL FOLDED FDI HAND SAMPLE ....................................................... 126

FIGURE 5.8 P WAVE VELOCITY ANISOTROPIC INDEX MODIFIED FROM SAROGLOU & TSIAMBAOS (2007),

AND ΒANGLE DEFINED AFTER MCLAMORE & GRAY (1962) ............................................................ 135

FIGURE 5.9 A (LEFT), PORO-PERMEAMETER PRESSURE GAUGES. B (CENTRE), PRESSURE TRANSDUCTORS.

C (RIGHT), COMPRESSION CHAMBER (LIMA & COSTA 2016) ......................................................... 138

FIGURE 5.10 THE MECHANICAL AND ELECTRONIC APPARATUS USED TO MEASURE P AND S WAVE VELOCITIES

(LIMA & COSTA 2016) ............................................................................................................ 139

FIGURE 5.11 A (LEFT), FQI MICROPHOTOGRAPHY SHOWING BANDS OF LARGER GRANULAR QUARTZ (LIGHT

BROWN CRYSTALS) IN CONTACT WITH MICROPLATES OF HEMATITE (SMALL GREY AND LIGHT GREY CRYSTALS).

B (RIGHT), HHE THIN SECTION SHOWING CONTACT BETWEEN LARGER GRANULAR HEMATITE CRYSTALS (DARK

BAND) WITH SMALLER TABULAR HEMATITE CRYSTALS (LIGHT BAND) ................................................ 147

FIGURE 5.12 A (LEFT), HHE THIN SECTION PRESENTING AN ORIENTATED SPECULARITE LARGE PLATELETS

(LIGHT GREY ELONGATED MINERALS) ORIENTATED ACCORDING TO THE BANDING. B (RIGHT), FDI THIN

SECTION SHOWING BANDS OF GRANULAR DOLOMITE (LIGHT BANDS) AND BANDS OF TABULAR HEMATITE

(BROWN) AND PORES IN CONTACT (BLACK) HORTA & COSTA (2016) .............................................. 148

FIGURE 5.13 A (LEFT), PORE BANDS IN MASSIVE HHE. B (CENTRE), FWQI MICROPHOTOGRAPHY SHOWING

POROSITY (DELIMITED IN RED LINES) BETWEEN QUARTZ (LIGHTLY COLOURED) AND HEMATITE (LIGHT GREY)

CONTACT. C (RIGHT) PWGI PRESENTING HEMATITE (WHITE) AND QUARTZ (GREY) BANDS WITH DIFFERENT

CRYSTAL SIZES AT CONTACT (DARK); HORTA & COSTA (2016)............................................................ 150

FIGURE 5.14 A (TOP LEFT), GRAPH SHOWS VP VALUES FOR EACH ΒANGLE. B (TOP RIGHT) SHOWS VS FOR EACH

ΒANGLE. C (LOWER LEFT), GRAPH SHOWS EDYN VALUES FOR EACH ΒANGLE. D (LOWER RIGHT) SHOWS ΝDYN FOR

EACH ΒANGLE. IN ALL FIGURES, THE DASHED LINE IS THE ANISOTROPIC CURVE, COLOURED TRIANGLES ARE THE

FDI VALUE RESULTS AND THE BLACK TRIANGLES REPRESENT Β0°, RED Β45° AND BLUE Β90° ..................... 154



EACH ΒANGLE. IN ALL FIGURES, THE DASHED LINE IS THE ANISOTROPIC CURVE, COLOURED TOP-DOWN TRIANGLES

ARE THE FAI VALUE RESULTS AND THE BLACK TRIANGLES REPRESENT Β0°, RED Β45° AND BLUE Β90°.......... 155

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EACH ΒANGLE. IN ALL FIGURES, THE DASHED LINE IS THE ANISOTROPIC CURVE, COLOURED SQUARES ARE THE FQI

VALUE RESULTS AND BLACK SQUARES REPRESENT Β0°, RED Β45°AND BLUE Β90° .................................... 157



EACH ΒANGLE. IN ALL FIGURES, THE DASHED LINE IS THE ANISOTROPIC CURVE, COLOURED CROSSES ARE THE

PWQI/PWGI VALUE RESULTS AND THE BLACK CROSSES REPRESENT Β0°, RED Β45° AND BLUE Β90° .......... 158

FIGURE 5.18 A (LEFT), SHOWS THE DISPERSION GRAPH FOR VP. B (RIGHT), SHOWS THE VS DISPERSION

GRAPH. FOR THESE GRAPHS, EACH COLOUR AND SYMBOL REPRESENT A SINGLE LITHOTYPE AS PRESENTED IN

THE LEGEND. DASHED LIGHT BLUE LINES ARE THE GROUP LIMITS AND DASHED DARK BLUE CIRCLES ARE

HIGHLIGHTED SAMPLES GROUPING ............................................................................................ 163

FIGURE 5.19 THE CORRELATION BETWEEN VS/VP (WAVE VELOCITY RATIO) FOR THE SET OF SAMPLES

ANALYSED. THE DASHED LINE IS THE LINEAR ADJUSTED CURVE WITH ITS EQUATION. POINT ’A’ SHOWS THE

LIMITS BETWEEN MODERATELY WEATHERED AND FRESH ITABIRITES AND POINT ‘B’ SHOWS THE LIMITS

BETWEEN FRESH ITABIRITES AND HHE. EACH COLOUR AND SYMBOL REPRESENT A SINGLE LITHOTYPE .... 164

FIGURE 5.20 A (TOP LEFT) AND C (LOWER LEFT) SHOW, RESPECTIVELY, THE GRAPHS VP WITH EDYN AND VP

WITH ΝDYN FOR ALL EVALUATED LITHOTYPES. B (TOP RIGHT) AND D (LOWER RIGHT) SHOW, RESPECTIVELY,

GRAPHS FOR VS WITH EDYN AND VS WITH ΝDYN FOR ALL OF THE LITHOTYPES EVALUATED. THE DASHED LINES

ARE THE CORRELATION LINES FOR FRESH ITABIRITE (RED) AND MODERATELY WEATHERED ITABIRITE (BLACK)

MATERIALS, AND THE LEGEND SHOWS THE COEFFICIENT OF DETERMINATION. EACH COLOUR AND SYMBOL

REPRESENT A SINGLE LITHOTYPE ................................................................................................. 165

FIGURE 5.21 THE DISTRIBUTION OF ΡB FOR THE SET OF SAMPLES ANALYSED. EACH COLOUR AND SYMBOL

REPRESENT A SINGLE LITHOTYPE AND THE DASHED LIGHT BLUE LINES REPRESENT THE GROUPING LIMITS . 167

FIGURE 5.22 A (LEFT), SHOWS THE CORRELATION BETWEEN ΡB AND VP. B (RIGHT), SHOWS THE

CORRELATION BETWEEN ΡB WITH VS FOR ALL TESTED SAMPLES. FOR THESE GRAPHS, EACH COLOUR AND

SYMBOL REPRESENT A SINGLE LITHOTYPE AS PRESENTED IN THE LEGEND. THE DASHED LIGHT BLUE LINES

REPRESENT THE GROUP LIMITS AND CIRCLES AND SQUARES ARE HIGHLIGHTED SAMPLE GROUPINGS ...... 168

FIGURE 5.23 A (LEFT), SHOWS THE GRAPH FOR ΡB AND EDYN FOR ALL TESTED SAMPLES. DIFFERENT MARKS

AND COLOURS REPRESENT DIFFERENT LITHOTYPES. B (RIGHT), SHOWS THE CORRELATION BETWEEN ΡB AND

ΝDYN FOR ALL THE SAMPLES TESTED. FOR THESE GRAPHS, EACH COLOUR AND SYMBOL REPRESENT A SINGLE

LITHOTYPE AS PRESENTED IN THE LEGEND. THE DASHED LIGHT BLUE LINES ARE THE GROUP LIMITS ........ 169

FIGURE 5.24 DISPERSION GRAPH OF TOTAL POROSITY. FOR THIS GRAPH, EACH COLOUR AND SYMBOL

REPRESENT A SINGLE LITHOTYPE AS PRESENTED IN THE LEGEND. THE DASHED LIGHT BLUE LINES ARE THE

GROUPING LIMITS (WEATHERING GRADES) .................................................................................. 170

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FIGURE 5.25 DISPERSION GRAPH SHOWING THE RELATIONSHIP BETWEEN BULK DENSITY AND TOTAL

POROSITY FOR ALL TESTED SAMPLES. FOR THIS GRAPH, EACH COLOUR AND SYMBOL REPRESENT A SINGLE

LITHOTYPE AS PRESENTED IN THE LEGEND. THE DASHED LIGHT BLUE LINES ARE THE GROUP LIMITS AND THE

DASHED COLOURED CIRCLES ARE HIGHLIGHTED SAMPLE GROUPINGS ................................................ 171

FIGURE 5.26 A (LEFT), SHOWS THE RELATIONSHIP BETWEEN TOTAL POROSITY AND VP. B (RIGHT), SHOWS

THE RELATIONSHIP BETWEEN Ø AND VS FOR ALL THE SAMPLES. FOR THESE GRAPHS, EACH COLOUR AND

SYMBOL REPRESENT A SINGLE LITHOTYPE AS PRESENTED IN THE LEGEND THE DASHED LIGHT BLUE LINES ARE

THE GROUP LIMITS AND THE DASHED BLACK LINE IS THE ADJUSTED CURVE ......................................... 172

FIGURE 5.27 A (LEFT), SHOWS THE RELATIONSHIP BETWEEN DYNAMIC YOUNG’S MODULUS AND TOTAL

POROSITY FOR ALL OF THE SAMPLES TESTED. B (RIGHT), SHOWS THE RELATIONSHIP BETWEEN DYNAMIC

POISSON’S RATIO AND Ø FOR ALL OF THE SAMPLES TESTED. FOR THESE GRAPHS, EACH COLOUR AND SYMBOL

REPRESENT A SINGLE LITHOTYPE AS PRESENTED IN THE LEGEND. THE DASHED LIGHT BLUE LINES ARE THE

GROUP LIMITS AND THE DASHED LINE IS THE ADJUSTED CURVE ........................................................ 173

FIGURE 6.1 MINE LOCATIONS AND IRON QUADRANGLE GEOLOGICAL SETTINGS (MODIFIED FROM

MORGAN ET AL 2013) ............................................................................................................ 190

FIGURE 6.2 A (LEFT), MICROPHOTOGRAPH OF WHE AT SLOPE SCALE SHOWING TYPICAL BANDING. B

(RIGHT), IS A MICROPHOTOGRAPH SHOWING MICRO-BANDING IN WHE, GRANULAR AND LARGER HEMATITE

CRYSTALS (LARGER, LIGHT GREY) AND MICRO-PLATES OF TABULAR HEMATITE (SMALLER, LIGHT GREY) (COSTA

2009) ....................................................................................................................... 193

FIGURE 6.3 A (LEFT), MICROPHOTOGRAPHY OF HEMATITE CRYSTAL, WITH PORES AND GRANULOBLASTIC

TEXTURE DEFINING THE BANDED SUBTYPE (COSTA 2009). B (CENTRE), ORIENTED MICRO-PLATES OF

HEMATITE, DEFINING THE FOLIATED SUBTYPE (COSTA 2009). C (RIGHT), CLAST TEXTURE DEFINING THE

BRECCIATED SUBTYPE (COSTA 2009) ......................................................................................... 195

FIGURE 6.4 A (LEFT), WAI AT SLOPE SCALE SHOWING LAYERS OF CLAY MINERALS. B (RIGHT),

MICROPHOTOGRAPHY SHOWING BRECCIATED TEXTURE OF WAI (HORTA & COSTA 2016) .................. 197

FIGURE 6.5 A (LEFT), PHOTOGRAPH OF WQI AT SLOPE SCALE SHOWING SUBVERTICAL BANDING

(AUTHOR’S PERSONAL ARCHIVE). B (RIGHT), MICROPHOTOGRAPHY SHOWING BANDING OF TABULAR

HEMATITE (LIGHT GREY) AND GRANULOBLASTIC CRYSTALS OF QUARTZ (DARK GREY) OF WQI (HORTA &

COSTA 2016) ....................................................................................................................... 198

FIGURE 6.6 A (LEFT), PHOTOGRAPHY OF WGI AT SLOPE SCALE SHOWING BANDING OF OCHREOUS

GOETHITE, HEMATITE, AND QUARTZ. B (RIGHT), MICROPHOTOGRAPH SHOWING LAYERS OF GOETHITE,

QUARTZ AND GIBBSITE AND LAYERS OF HEMATITE AND GOETHITE OF WGI (HORTA & COSTA, 2016) ... 199

FIGURE 6.7 BILINEAR UNSATURATED MOHR–COULOMB ENVELOPE, MODIFIED FROM NEJAD &

MANAHILOH (2017)............................................................................................................... 207

xx

FIGURE 6.8 A (LEFT), SCHEMATIC MODIFIED SUCTION CONTROLLED DIRECT SHEAR TEST APPARATUS FOR

SHEAR STRENGTH OF UNSATURATED SOILS TESTING (GAN ET AL. 1988). B (RIGHT), EXTENDED MC FAILURE

ENVELOPE FOR UNSATURATED SOIL (GAN ET AL. 1988)................................................................. 208

FIGURE 6.9 DEFINITION OF SWCC VARIABLES (ZHAI & RAHARDJO 2012) ................................... 212

FIGURE 6.10 CLASSIFICATION BASED ON ANISOTROPIC RATIO, RAMAMURTHY ET AL. (1993) AND ΒANGLE

DEFINITION AFTER MCLAMORE & GRAY (1967) .......................................................................... 216

FIGURE 6.11 A (LEFT), LARGE CRYSTALS OF GRANULAR HEMATITE, SURROUNDED BY HEMATITE

MICRO-PLATES AND INTERCONNECTED POROSITY FILLED BY RESIN (WRITE) (HORTA &COSTA 2016). B

(RIGHT), GRANULAR AND HEMATITE MACRO-PLATES DEFINING A MODERATE VISUAL TOTAL POROSITY BAND

(HORTA &COSTA 2016) .......................................................................................................... 223

FIGURE 6.12 A (LEFT), MICROPHOTOGRAPHY OF MICRO-PLATES OF HEMATITE, MARTITE IN SUB-EUHEDRAL

LARGER CRYSTALS WITH THE PRESENCE OF OCHREOUS GOETHITE (BROWNISH) (HORTA &COSTA 2016). B

(RIGHT), BRECCIATED TEXTURE WITH LARGE PORES (BLACK) IN A FRAGMENT OF HEMATITE SURROUND BY

GOETHITE, MICRO-PLATES OF HEMATITE, GIBBSITE, AND A QUARTZ MATRIX (HORTA & COSTA 2016) ... 224

FIGURE 6.13 A (LEFT), OCHREOUS GOETHITE (RED), QUARTZ (YELLOW) AND GIBBSITE CEMENTING

CRYSTALS OF HEMATITE (BLACK) (HORTA &COSTA 2016). B (RIGHT), OCHREOUS GOETHITE AND GOETHITE

(RED AND ORANGE) CEMENTING THE MICRO-PLATES OF HEMATITE (HORTA & COSTA 2016) .............. 225

FIGURE 6.14 A (LEFT), BAND WITH GRANULAR HEMATITE (LARGE CREAM CRYSTALS), PORES (BLACK),

QUARTZ (LIGHT GREY) AND HEMATITE MICRO-PLATES (SMALL CREAM CRYSTALS) (HORTA & COSTA 2016). B

(RIGHT), MICRO-PLATES OF HEMATITE LAYER WITH PORE CONCENTRATION HIGHLIGHT AT WHITE DOTTED BOX

(HORTA & COSTA 2016) ......................................................................................................... 225

FIGURE 6.15 A (TOP), WAI PSD CURVES – GREEN. B (CENTRE), WHE PSD CURVES – RED. C (BOTTOM),

WQI AND WGI PSD CURVES – CYAN. BLACK CURVES ARE THE MEAN VALUE FOR EACH TYPE.................. 230

FIGURE 6.16 BOX PLOT GRAPH FOR BULK DENSITY RESULTS FROM PSD TESTS, PRESENTING FOR EACH

WEATHERED BIF TYPE THE MEAN LINE AND THE INCLUSIVE MEDIAN. DOTTED LINES REPRESENT THE LIMITS OF

EACH TYPE ................................................................................................................... 234

FIGURE 6.17 SHOWS THE SWCC ADJUSTED FITTING CURVES FOR ALL EVALUATED TYPOLOGIES AS

PROPOSED BY ZHAI & RAHARDJO (2012) ................................................................................... 236

FIGURE 6.18 A (LEFT), BEST FIT CURVE FOR VOLUMETRIC WATER CONTENT USED TO DETERMINE THE WAI

SAMPLE SWCC STATISTICAL MIDPOINT. B (RIGHT), BEST FIT CURVE FOR PERMEABILITY FUNCTION ........ 239

FIGURE 6.19 A (LEFT), WAI – NORMAL DISPLACEMENT VERSUS SHEAR DISPLACEMENT (BL-1). B (RIGHT),

WAI – EFFECTIVE SHEAR STRESS VERSUS SHEAR DISPLACEMENT. STAGE 1 = 100 KPA, STAGE 2 = 400 KPA

AND STAGE 3 = 800 KPA ......................................................................................................... 240

xxi

FIGURE 6.20 A (LEFT), WHE – NORMAL DISPLACEMENT VERSUS SHEAR DISPLACEMENT (BL-3). B

(RIGHT), WHE – EFFECTIVE SHEAR STRESS VERSUS SHEAR DISPLACEMENT. STAGE 1 = 100 KPA, STAGE 2 =

400 KPA AND STAGE 3 = 800 KPA ............................................................................................ 241

FIGURE 6.21 A (LEFT), WQI/WGI – NORMAL DISPLACEMENT VERSUS SHEAR DISPLACEMENT (BL-12).

B (RIGHT), WQI/WGI – EFFECTIVE SHEAR STRESS VERSUS SHEAR DISPLACEMENT. STAGE 1 = 100 KPA,

STAGE 2 = 400 KPA AND STAGE 3 = 800 KPA ............................................................................. 242

FIGURE 6.22 A (UPPER LEFT), CIU SHEAR STRESS VERSUS STRAIN CURVE FOR WAI AT Β0°. B (UPPER RIGHT),

EFFECTIVE POREWATER PRESSURE VERSUS AXIAL STRAIN FOR WAI AT Β0°. C (BOTTOM LEFT), CIU SHEAR

STRESS VERSUS STRAIN CURVE FOR WAI AT Β90°. D (BOTTOM RIGHT), EFFECTIVE POREWATER PRESSURE

VERSUS AXIAL STRAIN FOR WAI AT Β90°. SAMPLE – BL 1 – 10388 .................................................. 243

FIGURE 6.23 EFFECTIVE STRESS PATH CURVE (P-Q’) FOR 0° AND 90° WAI (SAMPLE – BL 1 – 10388)244

FIGURE 6.24 A (UPPER LEFT), CIU SHEAR STRESS VERSUS STRAIN CURVE FOR WHE AT Β0°. B (UPPER

RIGHT), EFFECTIVE POREWATER PRESSURE VERSUS AXIAL STRAIN FOR WHE AT Β0°. C (BOTTOM LEFT), CIU

SHEAR STRESS VERSUS STRAIN CURVE FOR WHE AT Β90°. D (LOWER RIGHT), EFFECTIVE POREWATER PRESSURE

VERSUS AXIAL STRAIN FOR WHE AT Β90°. SAMPLE – TAM BL-08 -10395 ....................................... 245

FIGURE 6.25 EFFECTIVE STRESS PATH (P-Q’) CURVE FOR 0° AND 90° WHE. SAMPLE – TAM BL-08 -10395

....................................................................................................................... 246

FIGURE 6.26 A (TOP LEFT), CIU SHEAR STRESS VERSUS STRAIN CURVE FOR WQI/WGI AT Β0°. B (TOP

RIGHT), EFFECTIVE POREWATER PRESSURE VERSUS AXIAL STRAIN FOR WQI/WGI AT Β0°. C (BOTTOM LEFT),

CIU SHEAR STRESS VERSUS STRAIN CURVE FOR WQI/WGI AT Β90°. D (BOTTOM RIGHT), EFFECTIVE

POREWATER PRESSURE VERSUS AXIAL STRAIN FOR WQI/WGI AT Β90°. SAMPLE – BL 12 – 10420 ....... 247

FIGURE 6.27 EFFECTIVE STRESS PATH (P-Q’) CURVE FOR 0° AND 90°WQI. SAMPLE – BL 12 – 10420 .....

....................................................................................................................... 248

FIGURE 6.28 A (LEFT), WAI LINEAR REGRESSION (BEST FIT) LINES IN Σ1 Σ3 STRESS SPACE FOR TOTAL

STRENGTH VALUES. B (RIGHT), LINEAR REGRESSION (BEST FIT) LINES IN Σ1Σ3 STRESS SPACE FOR EFFECTIVE

STRENGTH OBTAINED FROM ROCDATA 5.0 (ROCSCIENCE 2021) TRIAXIAL CIU TESTS CONSIDERING

DIFFERENT ANISOTROPY DIRECTIONS (45° – BLUE, 0° – RED, 90° – GREEN, AND ISOTROPIC – BLACK) ... 249

FIGURE 6.29 A (LEFT), WHE LINEAR REGRESSION (BEST FIT) LINES IN Σ1Σ3 STRESS SPACE FOR TOTAL

STRENGTH VALUES. B (RIGHT), LINEAR REGRESSION (BEST FIT) LINES IN Σ1Σ3 STRESS SPACE EFFECTIVE

PRINCIPAL STRESS OBTAINED FROM ROCDATA 5.0 (ROCSCIENCE 2021) TRIAXIAL CIU TESTS CONSIDERING

DIFFERENT ANISOTROPY DIRECTIONS (45° – BLUE, 0° – RED, 90° – GREEN, AND ISOTROPIC – BLACK) ... 250

FIGURE 6.30 A (LEFT), WQI/WGI LINEAR REGRESSION (BEST FIT) LINES IN Σ1 Σ3 STRESS SPACE FOR TOTAL

STRENGTH VALUES. B (RIGHT), LINEAR REGRESSION (BEST FIT) LINES IN Σ1Σ3 STRESS SPACE FOR EFFECTIVE

STRENGTH OBTAINED FROM ROCDATA 5.0 (ROCSCIENCE 2021) TRIAXIAL CIU TESTS CONSIDERING

DIFFERENT ANISOTROPY DIRECTIONS (45° – BLUE, 0° – RED, 90 – GREEN, AND ISOTROPIC – BLACK) .... 250

xxii

FIGURE 6.31 A (TOP LEFT), WAI NORMAL DISPLACEMENT VERSUS SHEAR DISPLACEMENT. B (BOTTOM

LEFT), SHEAR STRESS VERSUS SHEAR DISPLACEMENT. C (RIGHT), WATER DISCHARGE VERSUS SHEAR

DISPLACEMENT FOR SAMPLE BL_1 (Β0°), WITH MATRIC SUCTION LEVELS: STAGE 1 – 0 KPA, STAGE 2 – 20

KPA, STAGE 3 – 70 KPA AND STAGE 4 – 150KPA ........................................................................ 253

FIGURE 6.32 A (TOP LEFT), WHE NORMAL DISPLACEMENT VERSUS SHEAR DISPLACEMENT. B (BOTTOM

LEFT), SHEAR STRESS VERSUS SHEAR DISPLACEMENT. C (RIGHT), WATER DISCHARGE VERSUS SHEAR

DISPLACEMENT. SAMPLE BL_3 (Β0°). MATRIC SUCTION LEVELS: STAGE 1– 0 KPA, STAGE 2 – 20 KPA, STAGE

3 – 70 KPA AND STAGE 4 – 150KPA ......................................................................................... 254

FIGURE 6.33 A (TOP LEFT), WQI/WGI NORMAL DISPLACEMENT VERSUS SHEAR DISPLACEMENT.

B (BOTTOM LEFT), SHEAR STRESS VERSUS SHEAR DISPLACEMENT. C (RIGHT), WATER DISCHARGE VERSUS

SHEAR DISPLACEMENT. SAMPLE BL_12 (Β0°). MATRIC SUCTION LEVELS: STAGE 1 – 0 KPA, STAGE 2 – 20

KPA, STAGE 3 – 70 KPA AND STAGE 4 – 150 KPA ....................................................................... 254

FIGURE 7.1 MINE LOCATIONS AND IRON QUADRANGLE GEOLOGICAL SETTINGS (MODIFIED FROM

MORGAN ET AL. 2013) ........................................................................................................... 269

FIGURE 7.2 A (LEFT), HHE AT MICROSCOPE VIEW WITH GRANULAR CRYSTAL OF HEMATITE (LIGHT GREY –

B1) AND SMALLER CRYSTAL OF HEMATITE MICROPLATES (DARK GREY – B3) (HORTA & COSTA 2016). B

(RIGHT), OUTCROP OF FRACTURED HHE AT CAPITÃO DO MATO MINE ............................................. 274

FIGURE 7.3 MHE OUTCROP FOLDED IN A TAMANDUÁ PIT FACE SHOWING TYPICAL INTERLAYER BETWEEN

HHE AND WHE ..................................................................................................................... 274

FIGURE 7.4 A (LEFT), MICROPHOTOGRAPHY OF BRECCIATED WHE SHOWING GRANULAR LARGER

HEMATITES CRYSTALS (LIGHT GREY) AND CEMENT OF MICROPLATES OF HEMATITE (COSTA 2009). B (RIGHT),

WHE AT SLOPE SCALE SHOWING TYPICAL BANDING ...................................................................... 276

FIGURE 7.5 A (LEFT), MICROPHOTOGRAPH OF FDI SHOWING TYPICAL BANDING OF DOLOMITE AND

QUARTZ (LIGHT COLOUR), (HORTA & COSTA 2016); B (RIGHT), SHOWS TYPICAL HAND SAMPLE OF FRESH

FOLDED FDI ....................................................................................................................... 277

FIGURE 7.6 A (LEFT), WAI UNDER THE MICROSCOPE SHOWING BANDS OF MICROPLATES OF HEMATITE

AND SPECULARITE (LIGHT GREY) AND BANDS OF SMALLER CRYSTALS OF GIBBSITE AND OCHREOUS GOETHITE

(LIGHT BROWN) (HORTA & COSTA 2016). B (RIGHT), WAI IN AN EXPOSURE ................................... 278

FIGURE 7.7 A (LEFT), MICROPHOTOGRAPH OF FAI, ILLUSTRATING THE PRESENCE OF FIBROUS GOETHITE

AND ACICULAR AMPHIBOLE CRYSTALS (DARK FIBRE MINERALS) INCLUDED IN QUARTZ BANDS (HORTA & COSTA

2016); B (RIGHT), TYPICAL SLOPE OF FOLDED AND FRACTURED FAI (W2) IN JANGADA MINE .............. 279

FIGURE 7.8 TYPICAL PWGI SLOPE IN JANGADA MINE ............................................................... 280

FIGURE 7.9 A (LEFT), WGI UNDER THE MICROSCOPE SHOWING GOETHITE (ORANGE) AND ERODED

QUARTZ (YELLOW) (HORTA & COSTA 2016); B (RIGHT), A SLOPE AT TAMANDUÁ MINE SHOWING FOLDED

WGI INTERLAYERED WITH WHE ............................................................................................... 281

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FIGURE 7.10 A (LEFT), TYPICAL BANDING UNDER THE MICROSCOPE SHOWING HEMATITE BANDS (LIGHT

GREY) AND QUARTZ BANDS (RED), (HORTA & COSTA 2016); B (RIGHT), TYPICAL FQI SLOPE AT TAMANDUÁ

....................................................................................................................... 282

FIGURE 7.11 AN EXAMPLE OF LEACHING IN A TYPICAL PWGI SLOPE AT TAMANDUÁ MINE ................ 283

FIGURE 7.12 A (LEFT), WQI UNDER THE MICROSCOPE, SHOWING QUARTZ (GREY) AND MICROPLATES OF

HEMATITE (LIGHT GREY) (HORTA & COSTA 2016); B (RIGHT), WQI SLOPE DETAIL AT TAMANDUÁ MINE ....

................................................................................................................... 284

FIGURE 7.13 THE THREE REGIONS FOR THE HOEK–BROWN STRENGTH CURVE AND THE EQUIVALENT

MOHR–COULOMB STRENGTH LINE (LIN ET AL. 2014) ................................................................... 288

FIGURE 7.14 WEATHERING GRADE AND ESTIMATION OF THE ROCK STRENGTH TABLE PLOTTED FOR THE THREE

MAIN WEATHERED GROUPS. THE APPLIED LABORATORY TESTS FOR EACH STRENGTH LEVEL ARE GROUPED BY THE

GREEN DOTTED SQUARE FOR HARD ROCKS (BEDROCK), A BLUE DOTTED SQUARE FOR MODERATE ROCK

(SAPROROCK) STRENGTH AND THE RED DOTTED SQUARE FOR WEAK ROCK (SAPROLITE) AND SOIL-LIKE MATERIAL

STRENGTH (AFTER MARTIN & STACEY 2018) ............................................................................... 289

FIGURE 7.15 CLASSIFICATION BASED ON ANISOTROPIC RATIO, RAMAMURTHY (1993) AND Β ANGLE

DEFINITION AFTER MCLAMORE & GRAY (1967) .......................................................................... 292

FIGURE 7.16 RELATIVE POSITION OF FUNDAMENTAL ROCK STRENGTHS ON THE HOEK–BROWN FAILURE

ENVELOPE IN PRINCIPAL STRESS SPACE (MODIFY FROM SARI ET AL. 2010) ........................................ 293

FIGURE 7.17 ESTIMATED UCS SUMMARY FOR ALL BIF TYPES, SHOWING IN LIGHT GREEN THE HIGH, IN

BLUE THE INTERMEDIATE AND RED THE LOW UCS VALUES (AFTER MARTIN & STACEY 2018)............... 299

FIGURE 7.18 A (LEFT), ABRUPT CONTACT BETWEEN WEATHERED HEMATITITE AND FDI AT CPX MINE. B

(RIGHT), A RARE, DECIMETRE THICK SLIGHTLY WEATHERED ZONE AT FDI CONTACT WITH WEATHERED

HEMATITITE (WHE) AT MAC MINE ........................................................................................... 300

FIGURE 7.19 A (LEFT), NARROW TRANSITIONAL CONTACT ZONE, SHOWING METRE-SCALE PWQI, WQI

AND FQI AT TAMANDUÁ MINE; B (RIGHT), TRANSITIONAL CONTACT ZONE, SHOWING METRE-SCALE OF

PWQI BETWEEN WQI AND FQI AT TAMANDUÁ MINE .................................................................. 301

FIGURE7.20 TYPICAL GEOLOGICAL CROSS-SECTION HIGHLIGHTING TWO DIFFERENT WEATHERING

PROFILES INDUCED BY BIF MINERALOGICAL COMPOSITIONS AND THEIR INTERFERENCE PATTERNS. A (TOP

LEFT), CROSS-SECTION FOR CONTINUOUS PROFILE WITH FAI AND FQI PROTO-ORE PRESENTING MODERATELY

WEATHERED MATERIALS. B (TOP RIGHT), CROSS-SECTION FOR DISCONTINUOUS PROFILE WITH FDI PROTO-

ORE AND ABSENCE OF MODERATELY WEATHERED MATERIALS ......................................................... 302

FIGURE 7.21 A (LEFT), FQI MICROPHOTOGRAPH SHOWING THE HETEROGENEITY WITH CRYSTALS OF

HEMATITE (LIGHT YELLOW) AND CRYSTALS OF QUARTZ (LIGHT GREY) (HORTA & COSTA 2016). B (CENTRE),

FDI MICROPHOTOGRAPH SHOWING FERROAN-DOLOMITE CRYSTALS AND LEVELS OF LEPIDOBLASTIC HEMATITE

AND FERROAN-DOLOMITE LEVELS (HORTA & COSTA 2016). C (RIGHT), WAI SHOWING LAYERS OF WEAKER

xxiv

CLAY MINERALS (LIGHT BROWN) INTERLAYERED WITH TABULAR HEMATITE (LIGHT GREY) (HORTA & COSTA

2016) ................................................................................................................... 309

FIGURE 7.22 A (LEFT), FDI MICROPHOTOGRAPH SHOWING TYPICAL BANDING OF GRANOBLASTIC

QUARTZ (LIGHT BLUE) AND BAND OF TABULAR HEMATITE AND IRON DOLOMITE (DARK BLUE) (HORTA &

COSTA 2016). B (RIGHT), FQI QUARTZ BAND (LIGHT BLUE) WITH GRANULOBLASTIC ORIENTATION AND SOME

TABULAR HEMATITE INTERLAYERED, AND BAND OF HEMATITE (LIGHT GREY) WITH ELONGATED HEMATITE

CRYSTALS (HORTA & COSTA 2016) ........................................................................................... 309

FIGURE 7.23 A (LEFT), PWQI SHOWING HEMATITE QUARTZ BANDS WITH HIGH PORES CONTENT (BLACK)

ORIENTED ACCORDING TO THE BANDING. (HORTA & COSTA 2016). B (RIGHT), WHE MICROPHOTOGRAPH

ILLUSTRATING THE PORES ORIENTATION ALONG THE METAMORPHIC BANDING (HIGHLIGHTED BY RED DOTTED

LINES) OBLIQUE TO A BRECCIAED LAYER (HORTA & COSTA 2016) ................................................... 310

FIGURE 7.24 A (LEFT), WAI MICROPHOTOGRAPH SHOWING TABULAR AND SPECULARITE HEMATITE

(LIGHT GREY ELONGATED CRYSTALS) AND GIBBSITE (LARGE DARK CRYSTAL AT CENTRE) (HORTA & COSTA

2016); B (RIGHT), WGI MICROPHOTOGRAPHY ILLUSTRATING CRYSTAL OF GRANULAR HEMATITE AND

QUARTZ, AND TABULAR HEMATITE TOTALLY COVERED BY GOETHITE FILLING (LIGHT BROWN) (HORTA & COSTA

2016) ................................................................................................................... 312

FIGURE 7.25 ROCDATA 5.0 (ROCSCIENCE 2021) LINEAR REGRESSION (BEST FIT) LINES IN Σ1 Σ3 STRESS

SPACE FOR WQI SHOWING FITTED STRENGTH PARAMETERS FOR EACH AVAILABLE ΒANGLE. A – TOP LEFT AT

(90°), B – TOP RIGHT AT (45°) AND C – BELOW (0°) .................................................................... 317


SPACE FOR WHE SHOWING FITTED STRENGTH PARAMETERS FOR EACH AVAILABLE ΒANGLE. A – TOP LEFT (90°),

B – TOP RIGHT (45°) AND C – BELOW (0°) ................................................................................. 318


SPACE FOR WAI SHOWING FITTED STRENGTH PARAMETERS FOR EACH AVAILABLE ΒANGLE. A – TOP LEFT

(90°), B – TOP RIGHT (45°) AND C – BELOW (0°) ........................................................................ 319


SPACE FOR WAI, WGI, WHE AND WQI SHOWING FITTED STRENGTH PARAMETERS FOR EACH AVAILABLE

ΒANGLE. A – LEFT (90°), B – RIGHT (0°) ........................................................................................ 320

FIGURE 7.29 MOHR–COULOMB LINEAR REGRESSION (BEST FIT) LINES IN Σ1 Σ3 STRESS SPACE FOR WAI

(A – TOP LEFT), WQI (B – TOP RIGHT), WHE (C – BOTTOM LEFT) AND WGI (D – BOTTOM RIGHT), SHOWING

MC PARAMETERS OBTAINED BY THE ADJUSTED FITTING CURVE ....................................................... 327

xxv

LIST OF TABLES

TABLE 4.1 SUMMARY TABLE WITH MICROSCOPIC INFORMATION FROM THIN SECTIONS DESCRIPTION FOR

FRESH ITABIRITES (HORTA & COSTA 2016) ................................................................................... 78

TABLE 4.2 STATISTICAL RESULTS OF EACH TYPE IS PRESENTED FOR FAI, FDI, HHE AND FQI FOR THE THREE

MAIN ΒANGLES ............................................................................................................................. 84

TABLE 4.3 RC SUMMARY TABLE FOR EACH STUDIED LITHOTYPE IN THREE TESTED DIRECTIONS, AS CLASSIFIED

(CLASS) BY SINGH ET AL. (1989), PRESENTING THE NUMBER OF TESTED SAMPLES (N) .......................... 86

TABLE 4.4 IVP SUMMARY TABLE FOR EACH STUDIED TYPE IN THREE TESTED DIRECTIONS, AS CLASSIFIED

(CLASS) BY SAROGLOU & TSIAMBAOS (2007), PRESENTING THE NUMBER OF TESTED SAMPLES (N) ........ 87

TABLE 4.5 BASIC STATISTIC SUMMARY TEST RESULTS FOR STRENGTH AND ELASTIC PARAMETERS EVALUATED

FOR ALL FRESH BIF .................................................................................................................... 89

TABLE 4.6 EVALUATED PARAMETERS TREND SUMMARY TABLE FOR ALL FRESH BIF ............................. 98

TABLE 4.7 EVALUATED PARAMETER CORRELATIONS SUMMARY TABLE FOR ALL FRESH BIF ................... 99

TABLE 5.1 A SUMMARY OF THE RESULTS OF THE MICROSCOPIC EXAMINATION FROM HORTA & COSTA

(2016) ........................................................................................................................... 142

TABLE 5.2 BASIC STATISTICAL SUMMARY TEST RESULTS AND PARAMETERS EVALUATED FOR ALL FRESH AND

MODERATELY WEATHERED BIF, CONSIDERING ANISOTROPIC RESULTS .............................................. 152

TABLE 5.3 SUMMARY TABLE OF DYNAMIC PROPRIETY ANISOTROPY INDEX FOR ALL LITHOTYPES .......... 153

TABLE 5.4 BASIC STATISTICAL SUMMARY TEST RESULTS AND PARAMETERS EVALUATED FOR ALL FRESH AND

MODERATELY WEATHERED BIF, CONSIDERED AS ISOTROPIC RESULTS ............................................... 160

TABLE 5.5 EVALUATED PARAMETERS TREND SUMMARY TABLE FOR ALL FRESH BIF ........................... 162

TABLE 5.6 SUMMARY TABLE FOR ELASTIC PROPRIETY CORRELATIONS PRESENTING COEFFICIENT OF

DETERMINATION ABOVE MODERATE (R2 ≥ 0.5) AND THE BEST-FITTED CURVE EQUATIONS ................... 174

TABLE 5.7 CORRELATION COEFFICIENT (R2) FOR EACH TYPE CORRELATING WAVE VELOCITIES, ELASTIC

DYNAMIC PARAMETERS AND PETROPHYSICAL PROPRIETIES ............................................................. 175

TABLE 5.8 CORRELATION TABLE BETWEEN ELASTIC DYNAMIC AND ELASTIC PARAMETERS ................... 181

TABLE 6.1 TESTING SUMMARY TABLE ....................................................................................... 222

TABLE 6.2 SUMMARY TABLE OF THE POROUS PERCENTAGE FOR ALL TYPOLOGIES ............................. 227

TABLE 6.3 SUMMARY TABLE WITH MAIN SOIL CHARACTERISATION PARAMETERS .............................. 231

TABLE 6.4 COEFFICIENT OF PERMEABILITY (K20), INITIAL MOISTURE CONTENT, BULK DENSITY, AND

ANISOTROPY DIRECTION ........................................................................................................... 235

TABLE 6.6 FITTING PARAMETERS FROM SWCC ADJUSTED CURVES FOR ALL MATERIAL TYPES FROM SVFLUX

SOFTWARE USING THE FREDLUND & XING (1994) FITTING CURVE .................................................. 238

xxvi

TABLE 6.7 CIU SHEAR STRENGTH PARAMETERS FOR WHE, WQI/WGI AND WAI CONSIDERING EACH ΒANGLE

RESULTS ........................................................................................................................... 251

TABLE 6.8 FITTING MC EFFECTIVE AND TOTAL STRENGTH PARAMETERS OBTAINED FROM ROCDATA 5.0

(ROCSCIENCE 2021) ............................................................................................................... 252

TABLE 6.9 WEATHERED BIF CHARACTERISTICS SUMMARY TABLE ................................................... 260

TABLE 7.1 SUMMARY OF LABORATORY TESTS ............................................................................ 297

TABLE 7.2 THINS SECTION SUMMARY TABLE FOR ALL BIF (HORTA & COSTA 2016) ......................... 307

TABLE 7.3 AVERAGE VALUE (MEAN) AND STANDARD DEVIATION (SD) SUMMARY TABLE FOR EACH FRESH

ITABIRITE (FQI, FDI AND FAI), PRESENTED BY ΒANGLE GROUPS ......................................................... 313

TABLE 7.4 AVERAGE VALUES (MEAN) AND SD SUMMARY TABLE FOR HHE SHOWN PARAMETERS RESULTS

BY ΒANGLE ........................................................................................................................... 315

TABLE 7.5 SUMMARY TABLE FOR PWQI/ PWGI PRESENTING EVALUATED PARAMETERS SEPARATED BY

ΒANGLE ........................................................................................................................... 316

TABLE 7.6 SUMMARY TABLE FOR HIGHLY TO COMPLETELY WEATHERED TYPES (WHE, WQI, WGI AND

WAI) FOR EACH ΒANGLE ............................................................................................................. 320

TABLE 7.8 ANISOTROPY RATIO (RC) SUMMARY TABLE FOR EACH EVALUATED LITHOTYPE .................... 321

TABLE 7.9 BASIC STATISTICAL SUMMARY TABLE CONSIDERING TOTAL RESULTS FOR EACH FRESH TO SLIGHTLY

WEATHERED LITHOTYPE CONSIDERING AS AN ISOTROPIC MATERIAL .................................................. 323

TABLE 7.10 BASIC STATISTICAL SUMMARY TABLE FOR HHE ........................................................ 324

TABLE 7.11 BASIC STATISTICAL SUMMARY TABLE FOR PARTIALLY WEATHERED LITHOTYPES (PWQI AND

PWGI) ....................................................................................................................... 325

TABLE 7.12 SUMMARY TABLE FOR COMPLETELY WEATHERED LITHOTYPES .................................... 328

TABLE 7.13 SUMMARY TABLE FOR AMPHIBOLITIC LITHOTYPE ...................................................... 330

TABLE 7.14 SUMMARY TABLE FOR DOLOMITIC ITABIRITE COMPLETELY WEATHERING PROFILE .......... 332

TABLE 7.15 SUMMARY TABLE FOR QUARTZITIC ITABIRITE LITHOTYPES .......................................... 334

TABLE 7.16 SUMMARY TABLE FOR HEMATITITE LITHOTYPES ....................................................... 336

TABLE 7.17 SUMMARY TABLE FOR ALL EVALUATED BIF ............................................................. 339

xxvii

LIST OF GRAPHS

GRAPH 4.1 UCS RESULTS FOR EACH ΒANGLE FOR FAI (TOP LEFT), FDI (TOP RIGHT), FQI (BOTTOM LEFT),

AND HHE (BOTTOM RIGHT). BLACK DOTTED LINES PRESENT THE AVERAGE TREND ............................... 86

GRAPH 4.2 P WAVE VELOCITY MEASURES FOR ALL BIF TYPES, IN THE THREE MAIN ΒANGLES. COLOURED LINES

SHOW THE TREND BETWEEN THE MAIN VALUE FOR EACH TYPE USED FOR THE ANISOTROPIC EVALUATION . 87

GRAPH 4.3 UCS TEST WITH BULK DENSITY CORRELATION GRAPH. IN THIS GRAPH, EACH COLOUR

REPRESENTS A BIF TYPE, SYMBOLS REPRESENT THE ΒANGLE AND DOTTED COLOURED CURVES REPRESENT

INDIVIDUAL EXPONENTIAL REGRESSION CURVES WITH THEIR RESPECTIVE EQUATION AND COEFFICIENT OF

DETERMINATION AS PRESENTED IN THE SUBTITLE ............................................................................ 91

GRAPH 4.4 UCS TESTS WITH P WAVE VELOCITY (VP) CORRELATION GRAPH. THE RED DASHED CIRCLE

HIGHLIGHTS FDI NEGATIVE CORRELATION. IN THIS GRAPH, EACH COLOUR REPRESENTS A BIF TYPE, SYMBOLS

REPRESENT THE ΒANGLE AND DOTTED COLOURED CURVES REPRESENT INDIVIDUAL EXPONENTIAL REGRESSION

CURVES WITH THEIR RESPECTIVE EQUATION AND COEFFICIENT OF DETERMINATION AS PRESENTED IN THE

SUBTITLE ......................................................................................................................... 92

GRAPH 4.5 UCS TEST WITH STATIC POISSON’S RATIO CORRELATION GRAPH. IN THIS GRAPH, EACH

COLOUR REPRESENTS A BIF TYPE, SYMBOLS REPRESENT THE ΒANGLE AND DOTTED COLOURED CURVES

REPRESENT INDIVIDUAL EXPONENTIAL REGRESSION CURVES WITH THEIR RESPECTIVE EQUATIONS AND

COEFFICIENT OF DETERMINATION AS PRESENTED IN THE SUBTITLE ..................................................... 93

GRAPH 4.6 UCS TEST WITH STATIC YOUNG’S MODULUS CORRELATION GRAPH. IN THIS GRAPH, EACH

COLOUR REPRESENTS A BIF TYPE, SYMBOLS REPRESENT THE ΒANGLE AND DOTTED COLOURED CURVES REPRESENT

INDIVIDUAL EXPONENTIAL REGRESSION CURVES WITH THEIR RESPECTIVE EQUATIONS AND COEFFICIENT OF

DETERMINATION AS PRESENTED IN THE SUBTITLE ............................................................................. 94

GRAPH 4.7 STATIC YOUNG’S MODULUS WITH P WAVE VELOCITY CORRELATION GRAPH. IN THIS GRAPH,

EACH COLOUR REPRESENTS A BIF TYPE, SYMBOLS REPRESENT THE ΒANGLE AND DOTTED COLOURED CURVES


COEFFICIENT OF DETERMINATION AS PRESENTED IN THE SUBTITLE ....................................................... 95

GRAPH 4.8 BULK DENSITY WITH STATIC YOUNG’S MODULUS CORRELATION GRAPH. IN THIS GRAPH, EACH

COLOUR REPRESENTS A BIF TYPE, SYMBOLS REPRESENT THE ΒANGLE AND DOTTED COLOURED CURVES


COEFFICIENT OF DETERMINATION AS PRESENTED IN THE SUBTITLE ..................................................... 96

GRAPH 4.9 P WAVE VELOCITY WITH BULK DENSITY CORRELATION GRAPH. IN THIS GRAPH, EACH COLOUR

REPRESENTS A BIF TYPE, SYMBOLS REPRESENT THE ΒANGLE AND DOTTED COLOURED CURVES REPRESENT

INDIVIDUAL EXPONENTIAL REGRESSION CURVES WITH THEIR RESPECTIVE EQUATIONS AND COEFFICIENT OF

DETERMINATION AS PRESENTED IN THE SUBTITLE ............................................................................ 97

xxvii

GRAPH 7.1 A (TOP LEFT) AND C (BOTTOM LEFT), REGRESSION STRENGTH (BEST FIT) CURVES IN Σ1 Σ3

STRESS SPACE FOR AMPHIBOLITIC ITABIRITE FOR HB ADJUSTED BEST FIT CURVE FOR FAI, PWGI AND WGI. B

(TOP RIGHT) AND D (BOTTOM RIGHT), STRENGTH BEST FIT CURVE FOR AMPHIBOLITIC ITABIRITE FOR MC

ADJUSTED LINEAR REGRESSION CURVE FOR FAI, PWGI AND WGI ................................................... 330

GRAPH 7.2 A (TOP LEFT) AND B (TOP RIGHT), HOEK–BROWN LINEAR REGRESSION (BEST FIT) LINES IN Σ1

Σ3 STRESS SPACE FOR FDI AND WAI, RESPECTIVELY. C (BOTTOM LEFT) AND D (BOTTOM RIGHT),

MOHR–COULOMB ADJUSTED FOR FDI AND WAI, RESPECTIVELY .................................................... 332

GRAPH 7.3 A (TOP LEFT) AND B (TOP RIGHT), REPRESENT HB A REGRESSION (BEST FIT) CURVES IN Σ1 Σ3

STRESS SPACE FOR FQI AND PWQI AND WQI; C (BOTTOM LEFT) AND D (BOTTOM RIGHT), SHOW MC

ADJUSTED FOR FQI, PWQI AND WAI ........................................................................................ 334

GRAPH 7.4 A (TOP LEFT) AND B (TOP RIGHT), SHOW THE HB REGRESSION (BEST FIT) CURVES IN Σ1 Σ3

STRESS SPACE FOR HHE AND WHE; C (BOTTOM LEFT) AND D (BOTTOM RIGHT), ARE THE MC ADJUSTED

LINEAR REGRESSION CURVES FOR HHE AND WHE ........................................................................ 336

GRAPH 7.5 A (TOP) A REGRESSION (BEST FIT) CURVES IN Σ1 Σ3 STRESS SPACE FOR THE ROCK-LIKE BIF WITH

HB ADJUSTED REGRESSION CURVE FOR HHE, FAI, FDI, FQI, PWQI, AND PWGI. PRESENTING THE THREE

DIFFERENT GROUPING FROM Σ3MAX, RANGING FROM: (1) SAPROLITE – COMPLETELY WEATHERED; (2)

SAPROROCK – MODERATELY WEATHERED, (3) BEDROCK – FRESH BIF; B (BOTTOM) LINEAR REGRESSION FOR

SOIL-LIKE BIF WITH MC ADJUSTED LINEAR REGRESSION CURVE FOR SAPROLITE TO RESIDUAL SOIL WHE,

WAI, WGI AND WQI ............................................................................................................. 338

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LIST OF SYMBOLS

P Load at failure

μw Porewater pressure

q' Deviator stress

p' Mean effective stress

uw Pore air pressure

(σfuw)f Effective normal stress on the failure plane at failure

ꞇff Shear stress on the failure plane at failure

c’ Effective cohesion

ɸ’ Effective friction angle

ɸb Unsaturated friction angle

τf Shear strength

(ua-uw) Matric suction at failure

ua Pore air pressure

(σ-uw) Effective normal stress on the failure plane at failure

e Base of natural logarithm

θs Saturated volumetric water content

θ Volumetric water content

ψ Matric suction (total soil suction)

a Fitting parameter related to the air entry value of the soil

n Fitting parameter related to the maximum slope of the curve

m Fitting parameter related to the curvature of the slope

ψr Fitting parameter related to the residual suction of the soil

QtLower Lower inner fence

Qtupper Upper inner fence

1Qt First quartile

3Qt Third quartile

Rc Anisotropy ratio

σc90° Compressive strength value for βangle perpendicular to the planes of anisotropy

xxx

σcmin Lowest compressive strength value obtained

ρb Bulk density

M Mass of the specimen measured prior to testing

V Volume of specimen

kunsat Unsaturated coefficient of permeability

k20 Coefficient of permeability normalised to 20°C

Ks Saturated coefficient of permeability

S Degree of saturation

‘a’, ‘b’, ‘c’ and ‘p’ Constants determined by curve fitting the permeability data

Edyn Young’s modulus

ν Poisson’s ratio

νdyn Dynamic Poisson’s ratio

Gdyn Shearing modulus

Kdyn Incompressibility modulus

VP Compressional ‘P’ wave velocity

Vs Shear ‘S’ wave velocity

π Salt content of the water

Vp0° Velocity of P waves (propagation parallel to the planes of anisotropy)

Vp90° Velocity of P waves (propagation perpendicular to the planes of anisotropy)

σcd UCS of a sample with diameter

σc50 UCS of a 50 mm diameter sample

d Samples diameter

t Thickness of the sample

βangles Anisotropy directions

Max Maximum values

Min Minimum values

σt Tensile strength

Øb Visual total porosity

Ø Laboratory total porosity

ξax Axial deformation

ξrad Radial deformation

xxxi

ACKNOWLEDGEMENTS

The author thanks thesis supervisor Phil Dight and co-supervisors Eduardo Marques and

Ken Mercer for their support and orientation during this long journey. Also, thanks to Vale S.A.

for their permission to present this thesis and sponsorship of the research. The author’s

appreciation extends to the ACG team in the name of Christine Neskudla, Josephine Ruddle,

Garth Doig and Stefania Woodward for the amazing support and for making me feel part of the

ACG family, and to former ACG staff member Ariel Hsieh for the support and discussions.

Appreciation is extended for all ‘giants’, Paulo Franca, Rene Viel, Peter Stacey, John Read,

Waldyr Oliveira, Flavio Afonso Oxy, Vitor Suckau, Cesar Grandchamp, Rodrigo Figueiredo, Fabio

Magalhães and Sergio Brito (in memoriam) for their contributions in my career and thesis

reviews, and to Laura Horta and Pedro Henrique Alves for precious evaluations and support

during thesis development.

xxxii

DEDICATION

This thesis is dedicated to my mother (in memoriam) for her endless love, sacrifice, prayers, and

support.

Also, to my beloved father and mother in law for their endless love, support, prayers, and

encouragement.

To Mariana, Luiza and Julia for their deep love, emotional support, and sacrifice. I promise to

reward you all very soon!

“If I have seen further, it is by standing upon the shoulders of giants” Isaac Newton, 1675

xxxiii

AUTHORSHIP DECLARATION: CO-AUTHORED PUBLICATIONS

This thesis contains work that has been published and work prepared for publication.

Details of the work: An overview of the weathering process and preliminary density and UCS correlations for fresh itabirites in Vale mines on the western side of the Iron Quadrangle, Brazil This paper presents a review of the geological, geotechnical, and weathering characteristics of the itabirites, and early results for the fresh BIF evaluations from the PhD thesis research. The results showed that to evaluate the bulk density and unconfined compressive strength (UCS) correlation, not only the porosity but also the geological features should be considered, as they are responsible for data dispersion. However, the presented data suggested a trend that provides the confidence to develop advanced studies and data population increment to establish a proper correlation to support a reliable UCS index method. Location in thesis: Appendix I Student contribution to work: 70%

Co-author signatures and dates:

Phil Dight Date: June 25, 2021

Ken Mercer Date: June 21, 2021

Eduardo Marques Date: June 21, 2021

xxxiv

Details of the work: Weathered banded iron formations in Vale iron ore mines on the western side of the Iron Quadrangle, Brazil: weak hematitite and weathered argillaceous itabirite geotechnical characteristics and implications of matric suction effects on slope stability This paper presents a review of the geological and weathering characteristics and related geomechanical and strength assessments of two important lithotypes commonly associated with slope failure mechanisms in Vale mines: weathered argillaceous itabirite (WAI) and weak hematitite (WHE). These studies are based on early research results from the PhD thesis. The results show that minor changes in bulk density, rock fabric, and the percentage of clay minerals lead to distinct geomechanical characteristics in terms of plasticity, intact rock strength and mainly matric suction effects. These changes influence both the shear strength and intact rock behaviour and consequently the overall slope stability as shown in a preliminary insight into the open pit slope design for short and long-term considerations. Location in thesis: Appendix II Student contribution to work: 70% Co-author signatures and dates:

Phil Dight Date: June 25, 2021

Ken Mercer Date: June 21, 2021

Eduardo Marques Date: June 21, 2021

Student signature: Date: June 21, 2021

I, Phil Dight certify that the student statements regarding their contribution to each of the works listed above are correct.

Coordinating supervisor signature: Date: June 25, 2021

1

CHAPTER 1. INTRODUCTION

1.1 PROBLEM STATEMENT

In recent decades, a booming worldwide iron ore market has pressed iron ore mines to increase

their production by developing innovative technologies, opening new mines, and expanding

existing mines, not only in width but mainly in depth, to meet this high demand.

In this period, iron ore mines located in the Iron Quadrangle, Brazil were especially affected by

environmental and social pressures to reduce expansion of the mining footprint. The expansion

was based on technological development and increase of depth in existing mines. With pit

depths going beyond 500 m, the former pit designs, based on soil-like behaviour for friable

material, are no longer valid.

These iron ore mines are developed in banded iron formations (BIF) deposits, originally

described by Dorr (1969) as a high-grade ore, called hematite (most recently denominated as

hematitite) and a low-grade ore, called itabirite. The original mineralogical composition of

non-iron bands, consisting of dolomitic, amphibolitic and quartzitic layers are used to

differentiate itabirite types. In addition, those lithotypes are the hardest (fresh) BIF seen in Iron

Quadrangle iron ore mines that, under multiple geological process (mainly weathering), resulted

in multiple rock types with low strength.

Brazilian iron ore mines exhibit weathering profiles that can reach over 400 m in depth. This

means that for shallow mines (less than 400 m depth) slopes are composed mainly of residual

soil to moderately weathered rocks with a minor presence of fresh rocks and in deeper mines

(more than 400 m deep) the slopes are composed of a range of residual soils to fresh rocks. For

geotechnical purposes, this depth division is appropriate for a geomechanical approach and

these mines must be considered as saprolite deep deposits of BIF where the typical weathering

profile characteristic must be established and considered.

For shallow mines, where the rocks exhibit soil mechanics behaviour, the slope design approach

and the identification of possible failure mechanisms is largely based on applying classical soil

mechanics principals as well as historical trial-and-error experience. While this approach has

been largely satisfactory, there are nevertheless several key geotechnical issues that are still not

well understood, and there have been continued instances of large slope failures which have

resulted in significant disruptions to the mines.

For the proposed deeper mines, the increase in fresh and hard rock at the toes of the slopes

presents new challenges for subsequent geotechnical designs and slope failure assessments,

2

such as the application of rock mechanics, definition of rock mass and related discontinuities

important for failure mechanism identification.

Based on these facts the research will address the main aspects, described below, to determine

the boundary between residual soil, saprolite, saprorock and bedrock for BIF weathering profiles

to establish the best practices for slope design analysis and evaluations of failure mechanisms

at different weathering profile horizons.

Bedrock horizon – hard and fresh rock geotechnical behaviour characterisation

With the deepening of iron ore mines, there will be an increase in the percentage of hard and

fresh rocks exposed (bedrock), which poses new challenges to geotechnical teams to develop

slope designs and evaluate failure mechanisms using rock mechanics principles, breaking some

paradigms used over the last decades of operation in weak rocks. As such, there will be limited

precedence to follow and few numbers of failure mechanism studies on these hard rock masses

that could provide previous experiences.

For these hard rocks’ masses, it is crucial to ensure a good evaluation of intact rock strength and

discontinuities in order to define the rock mass strength and understand deformation behaviour

for slopes composed of a quite complex arrangement of different rock strength and elastic

materials.

The BIF heterogeneity and associated anisotropic effects on intact rock proprieties poses an

extra challenge, due to the heterogeneity produced by typical itabirites compositional

metamorphic banding that could induce significant changes on intact rock strength and elastic

parameters which can even determine failure mechanisms.

Saprorock horizon– partially weathered, a mix of weak and hard rock geotechnical behaviour

characterisation

As a transition between fresh to completely weathered rocks, these moderately to highly

weathered types can reach hundreds of metres of thickness and behave as a rock or soil

depending on intact rock strength and stress level. This ambiguous behaviour affects not only

the choice of laboratorial tests but also the approach necessary to characterise and evaluate

failure mechanism which involves the appropriate use of rock or soil mechanics principles.

It is also important to assess the geotechnical parameters for partially weathered rocks, defined

as saprorock and configuring a mix of hard and weak material.

Saprolite horizon – weak and completely weathered to residual soil geotechnical behaviour

characterisation

For completely weathered rocks to residual soil, these low strength rocks with soil-like behaviour

are the result of alteration of the original fresh rock imposed in part of the hypogene and

3

supergene iron enrichment process but mainly by weathering and could be defined as typical

saprolite as show in this research.

In shallow mines, the approach in dealing with weak rocks and slope design has largely been

based on adopting soil mechanics principles and past experiences. Studies by Costa (2009), Sá

(2010) and Martin and Stacey (2018) suggest that some failure mechanisms have specific

characteristics for weak leached materials, and that the application of classical soil mechanics

principles does not enable failure mechanisms to be fully understood, while rock mechanics

concepts are also not fully applicable. It is therefore apparent that an appropriate geotechnical

approach is required to address these rock types.

Intact rock and shear strength parameters for these designs are normally based on a limited

number of soil laboratory tests (such as triaxial and direct shear) or derived from adapted

geomechanical classifications. While this approach has been mostly successful, it appears they

may be conservative. In spite of that, there have been examples of large slope failures not

captured by these design approaches that have had significant negative impacts for mine

production and in some cases resulted in fatalities.

It is also recognised that failure mechanisms in weak rock slopes are controlled mainly by intact

rock shear strength and anisotropic effects, as well as secondary factors such as structural

control and water content. The situation is particularly challenging when considering the

stratigraphic position and/or tectonic settings of these weak and weathered rocks, which are

very often located at the toe of high slopes, where the stress is concentrated. To address this

issue, historically geotechnical engineers leave a ‘buttress’ at the toe of the wall, resulting in a

loss of reserves.

Additionally, the lack of knowledge of pore pressure effects (in situ and laboratory tests) and the

behaviour of partially saturated or unsaturated conditions for these weak materials increases

the challenge as the porewater pressure influence (positive or negative) is not properly

understood and considered in slope stability analyses.

Generally, in iron ore mines, the slope stability analyses are performed using saturated

parameters, although the slope may remain unsaturated, which could represent a conservative

approach or using unsaturated parameters that could represent a risky approach. For both

cases, the result leads to a difference between site conditions and the analytical results.

1.2 RESEARCH OBJECTIVES

The geological focus of this research is to evaluate how the weathering and other iron

enrichment processes (supergene and hypogene) have affected the geomechanical properties

of BIF for all materials found in a complete weathering profile. To reach this aim, a geological

4

and geotechnical approach was used to determine the weathering profile horizons and zones,

providing reliable intact rock strength and elastic parameter based on site investigation and

laboratory tests, establishing empirical correlations (graphs and/or equations) between these

parameters, petrophysical and geological features, and finally addressing the consequent

importance of these processes on the slope stability for iron ore open pit mines

To address these goals, three primary objectives were established as summarised below:

Objective 1 – Evaluate, for all BIF types, geological and petrophysical macrofeatures and

microfeatures associated with geomechanical properties considering the anisotropy and

heterogeneity characteristics

• Define the most important geological, petrophysical features, and geotechnical

parameters responsible for the geomechanical behaviour of each weathering profile

horizon.

• Define the correlations between geological features and petrographic characteristics

(mineralogy and fabric) with the geomechanical parameters for each individual BIF

weathering profile horizons.

Objective 2 – Evaluate, for all BIF types, the intact rock strength and elastic parameters

considering the anisotropy and heterogeneity characteristics

• Define the most appropriate laboratory test type for each weathering horizon defining

remarks, restrictions, and results variation.

• Define how BIF intact rock and shear strength, and elastic parameters (dynamic and

static) changes from different BIF types at multiple weathering horizons.

• Define the strength anisotropy ratio for each BIF type and weathering horizon.

• Define reliable empirical correlations (graphs and/or equations), considering the most

important petrophysical and geomechanical parameters, for each BIF type and

weathering horizon.

• Evaluate the saturated and unsaturated behaviour considering the porewater pressure

(matric suction) effects for BIF soil types.

Objective 3 – Evaluate the complete BIF weathering profile characteristics, setting the

boundaries, establishing a geological and geomechanical characterisation

• Define the typical BIF weathering profile and show how the weathering affects each

BIF type resulting in different weathering profiles.

5

• Define how and in which way the BIF intact rock strength and elastic parameters

increases in depth and the weathering profile correlations.

• Define the main geotechnical parameters used to correlate and characterise a

weathering horizon.

• Evaluate the influence of the anisotropy and heterogeneity on the complete

weathering profile.

• Establish how the weathering affects BIF geological and geomechanical properties on

different weathering horizons.

Figure 1.1 shows the general objectives flow chart that summarises the main research aims and

its connections.

1Figure 1.1 Research aims interconnection

1.3 RESEARCH LIMITATION

Brazilian BIF are the focus of this research and to evaluate variations of the complete weathering

profile it will consider a total of 10 different types, defined as follows.

Three compositional fresh itabirites lithotypes: fresh quartzitic itabirite (FQI), fresh amphibolitic

itabirite (FAI) and fresh dolomitic itabirite (FDI), considered as low-grade proto-ore, and the

hard-high-grade ore, defined as hard hematitite (HHE).

Two moderately to highly weathered typologies result from the weathering process on those

three itabirites, defined as partially weathered quartzitic itabirite (PWQI) and partially

weathered goethitic itabirite (PWGI).

Finally, three completely weathered typologies denominated as completely weathered

quartzitic itabirite (WQI), completely weathered goethitic itabirite (WGI), completely weathered

Objective 1

Objective 2

Objective 3 • Site investigatiosn

(macro-scale)• Petrographic evaluations

(micro-scale)

Geological and geomechanical characterisation for all BIF types

• Laboratory tests• Intact rock strength and elastic

parameters (dynamic and static)• Empirical correlations (graphs

and equations)

Laboratory geotechnical and petrophysical characterisation for all BIF types

• Weathering profile (saprolite) characterisation.

• Boundaries definitions• Geological and geomechanical

correlations

Complete BIF weathering profile characterisation

6

argillaceous itabirite (WAI) defined as poor itabirites, and the weak hematitite (WHE) defined as

rich friable ore.

Those rocks represent more than 60% of the final slope exposed rocks (another 40% is composed

of country rocks) and more than 90% of BIF type expositions. The semi compact hematitite, was

not evaluated in this thesis owing to the reduced geotechnical importance and spatial

distribution in iron ore mines.

An important limitation is referred to the genesis of the weak material and correlated loss of

strength during geological events and time. This research will concentrate on physical and

chemical changes posed by the weathering process, considering this event the most important

process responsible for reducing the intact rock shear strength of the BIF as described by Ribeiro

(2002a and 2003). However, there are other geological processes such as supergene and

hypogene iron enrichment, tectonic events and metamorphism that could change the rock

strength. With this simplification, it was possible to establish a direct correlation between intact

rock characteristics and strength variations at depth with distinct levels of weathering

throughout the weathering profile.

The research will also focus on the typical BIF heterogeneity attributed to the compositional

metamorphic banding or foliation, and its anisotropic effects in the intact rock strength. For this

evaluation, discontinuities and rock mas strength parameters were not considered, even though

considering these to be crucial for the complete understanding of slope stability behaviour for

residual soil or saprolite that contain relict structures and discontinuities in certain areas.

In addition, discontinuity characterisation and correlation with rock mass strength are not

considered in this research, which focuses on intact rock strength. Nevertheless, the importance

of the structural characteristics for slope stability behaviour, for bedrock and saprorock analyses

is recognised.

The studied mines are part of Vale’s south ferrous division and include 15 mines located in the

centre of Minas Gerais state, Brazil, as shown in Figure 1.2. The mines are: Águas Claras (MAC),

Mutuca (MUT), Mar Azul (MAZ), Capão Xavier (CPX), Tamanduá (TAM), Capitão do Mato (CMT),

Abóboras (ABO), Galinheiro (GAL), Sapecado (SAP), Pico (PIC), Córrego do Feijão (CFJ), Jangada

(JGD), João Pereira (JPE), Alto Bandeira (BAN) and Fábrica (FAB). The mines which have been

studied are located in Iron Quadrangle western low strain domain and green schist metamorphic

zone (Rosière et al. 1993). Evaluations in different tectonic settings and different iron deposits

must be made considering these geological influences.

7

2 Figure 1.2 Studied mines localisation, stratigraphic column, and Iron Quadrangle

geological settings (modified from Morgan et al. 2013)

1.4 THESIS CONTRIBUTIONS

Firstly, this research will provide valuable information about the role of weathering on BIF

heterogeneity and anisotropy behaviour, and a complete geological and geotechnical

characterisation for the main BIF types on the complete weathering profile.

Secondly, it will produce a reliable elastic (dynamic and static) and intact rock strength

parameters database and empirical relationships between petrophysical and geomechanical

parameters defining correlation equations able to define the behaviour of such materials in large

open pits.

The research will bring a better understanding of the weathering profile, defining boundaries

and characteristics. The approach used in this research aims to better understand the intact rock

strength characteristics based on increment of geotechnical laboratory tests, establishing

correlations between chemical (e.g. iron content), and physical (e.g. bulk density) variables with

geotechnical index proprieties (e.g. strength and elastic parameters). Proper correlations and

resulting equations from fitting correlation curves could be used to predict geotechnical

characteristics supporting slope stability evaluations.

Due to the similarity recognised of BIF types, the methodology and the results obtained in this

thesis can be applied for similar rocks in other BIF deposits such as Carajás (Brazil), Pilbara

(Australia) or Sishen (South Africa) with care.

SDB

CH

SM

BD

8

Finally, the thesis expected to contribute to optimise current slope failure mechanism

evaluation, final slope design, and slope safety practices, improving operational productivity,

thereby ensuring necessary safety operations and stability in iron ore open pits.

1.5 THESIS ORGANISATION

This thesis is organised in accordance with The University of Western Australia’s Doctor of

Philosophy rules for the content and format of a thesis presented as a series of papers. It is

organised into nine chapters, which following this introductory Chapter 1, where the problem

statement is presented, the main objectives and expected results, thesis limitations and its

organisation are presented.

Chapter 2 presents a brief summary of the most relevant literature overview, separated by topic.

To reduce the repetition, this chapter presents a general literature review of each topic

discussed in this thesis. The specific and detailed used literature review are presented separately

in each chapter. Chapter 3 presents the site investigations, laboratory tests, and methodology

necessary to develop the thesis.

The following four chapters constitute the main portion of the thesis and present the results,

discussions, and conclusions from unpublished manuscripts. Each chapter stands alone and

covers one or more topics of the research objectives (Section 1.2) and were written to allow the

audience to have a complete understanding of the topics without the necessity to read the other

chapters. This configuration could lead to repetition, especially in the first part of each chapter.

For this reason, some editorial adjustments were made to improve consistency and reduce

repetition, mainly in Chapter 2 (Literature Review) and Chapter 3 (Methodology). Additionally,

thanks to editorial purposes, some chapters could be split into more than one article due to

elevate number of words and size of the figures and graphs contained.

Additionally, the two published papers are presented in the appendices section, also in order to

reduce the repetition. Appendix I (Costa et al. 2015), is titled ‘An overview of the weathering

process and preliminary bulk density and UCS correlations for fresh itabirites in Vale mines on

the western side of the Iron Quadrangle, Brazil’. It is a peer reviewed paper presented and

published in 2015 in Iron Ore, AusIMM and CSIRO conference proceedings – Perth, Australia,

focusing on the geological and geotechnical setting of fresh BIF and weathering profile, thesis

early results regarding the samples grouping, the techniques used to define the outliers, and the

correlation between iron content, bulk density and UCS tests for fresh itabirites and hard

hematitite.

Appendix II (Costa et al. 2015), is titled ‘Weathered banded iron formations in Vale iron ore

mines on the western side of the Iron Quadrangle, Brazil: weak hematitite and weathered

9

argillaceous itabirite geotechnical characteristics and implications of matric suction effects on

slope stability’. It is a peer reviewed paper presented and published in 2015 in the Slope Stability

2015, International Symposium proceedings – SS03, Cape Town, South Africa, focusing on the

geological and geotechnical setting of weak hematitites and completely weathered argillaceous

itabirites, thesis early results regarding the weak rock saturated and unsaturated soil behaviour,

porewater pressure effects and a slope stability case study evaluation considering this influence.

Chapter 4 is denominated ‘Intact rock strength characteristics and elastic static properties of

fresh Brazilian banded iron formations’. This chapter presents the first unpublished manuscript.

It presents the geological and geotechnical setting of fresh BIF, final considerations about the

relationship between physical proprieties, intact rock strength parameters and static elastic

modulus, and defines the influence of anisotropy ratio based on intact rock strength and

established empirical correlations equations for the most important parameters recognised.

Chapter 5 is denominated ‘Petrophysical characteristics and elastic dynamic properties of fresh

to moderately weathered Brazilian banded iron formations’. This chapter presents the second

unpublished manuscript. It presents final considerations about the relationship between intact

rock strength, petrophysical proprieties, VP and Vs, and dynamic elastic modulus, and the effects

of anisotropy ratio on these rocks based on wave propagation.

Chapter 6 is denominated ‘Weak rock behaviour of highly to completely weathered Brazilian

banded iron formations’. This chapter presents the third unpublished manuscript. It presents

the BIF weak rock and soil-like types behaviour for saturated and unsaturated conditions,

defines the influence of anisotropy ratio on these soil types considering the intact rock strength,

and presents the shear strength parameters for all types and porewater pressure effects.

Chapter 7 is denominated ‘Weathering profile, intact rock strength and elastic characteristics of

Brazilian banded iron formations’. This chapter presents the fourth unpublished manuscript. It

presents the geological and geotechnical setting from fresh to completely weathered BIF

defining the weathering profiles’ main characteristics and boundary between the weathering

horizons, final considerations about the relationship between petrophysical proprieties, intact

rock strength parameters and elastic modulus, and defines the influence of anisotropy ratio on

these rocks considering the intact rock strength for all weathering profiles.

Chapter 8 concludes the thesis by presenting the main discussions and findings reached from

the research development. Finally, Chapter 9 suggests the next steps and future research

advance.

10

11

CHAPTER 2. LITERATURE REVIEW

2.1 REGIONAL GEOLOGICAL SETTINGS

The focus area is located at the western part of the Iron Quadrangle on the southern border of

São Francisco Craton. The mines are situated on Moeda (SM) and Don Bosco (SDB) Synclines and

Curral Homocline range (CH) (Figure 1.1).

The structure is delineated by a roughly quadrangular arrangement, with Paleoproterozoic BIF

of the Minas Supergroup, as proposed by Dorr (1969). This supergroup is composed of hundreds

of metres of iron-rich metamorphic rocks belonging to the Itabira group/Cauê Formation. The

Minas Supergroup comprises, from the bottom to the top: Caraça, Itabira, Piracicaba and Sabará

groups; a sequence of psammitic and pelitic rocks, also defined by Dorr (1969); all of which are

overlain by the Itacolomi group. Below this sequence are the Archean greenstone belt terrains

of the Rio das Velhas Supergroup and domes of Archean and Proterozoic crystalline rocks

(Machado & Carneiro 1992, Machado et al. 1989, and Noce 1995).

The Cauê Formation (Itabira Group), which hosts the BIF, is a marine chemical sequence 350 m

thick, dated 2.4 ± 0.19 Gyr by Babinski et al. (1995).

The regional structure is the result of two major deformational super-positional events, as

proposed by Chemale Jr et al. (1994). The first event produced the nucleation of regional

synclines in the uplift of the gneissic domes during the Transamazonian Orogenesis (2.1–2 Gyr),

and the second is related to an east–west verging thrust fault belt of Pan African/Brazilian age

(0.8–0.6 Gyr) described by Marshak & Alkmim (1989). This last event deformed the earlier

structures and was mainly responsible for the deformational gradient. Hertz (1978) described

an eastward increase in the metamorphic grade from green schist to lower amphibolite facies

following the deformational gradient.

The mines which have been studied are located in the western low strain domain and green

schist metamorphic zone, formed in the Transamazonian event. The main trend of the synclines

is north–south, but the structure has also been deformed around Bação dome (BD). It is also

interconnected with Dom Bosco Syncline to the south and partially truncated by Engenho Fault.

To the north, it is continuous with the Curral Homocline. At the junction of these two regional

structures a northwest-verging asymmetric anticline and an interference saddle due to the

refolding were developed (Rosière et al. 1993).

The Moeda Syncline has been partially affected by the younger Brazilian tectonic event, mainly

on the eastern limb, with local development of ductile brittle to brittle shear zones. Several

strike–slip faults cut across the structure, dividing it into several segments.

12

The Serra do Curral hills represent the overturned south-eastern limb of a truncated

northwest-verging syncline–anticline couple, which was highly deformed and rotated by the

right–lateral movement of the northeast–southwest trending inclined ramp of a reginal thrust

fault as described by Chemale Jr et al. (1994). In this segment, the inverted northwestward limb

of a syncline is also truncated near the contact of the Minas Supergroup with the underlying Rio

das Velhas Supergroup by shear zones related to the thrust.

2.2 BIF GEOLOGICAL SETTINGS

Brazilian BIF are Paleoproterozoic, metamorphic and heterogeneous banded rocks presenting a

millimetre to centimetre rhythmic alternation (banding) of iron minerals (hematite, martite and

magnetite), and non-iron minerals (quartz, dolomite, and amphibolite). Initially termed itabirites

as defined by Dorr (1969), these iron deposits are classified as Superior Type according to Gross

(1980).

The origin of itabirites and associated high-grade orebodies (hematitite) remains controversial,

and several works have been produced on this topic, as largely discussed in Spier et al. (2003).

For genesis of the friable orebodies, some authors agree on a supergene process and residual

itabirite enrichment, with leaching of gangue minerals by surface waters. For these rich

orebodies, Spier (2005) and Spier et al. (2006) suggest that the weathering and the

mineralisation period occurred between 61.5 ± 1.2 Myr to 14.2 ± 0.8 Myr, reaching the peak

process in 51 Myr. This dating indicates a tertiary mineralisation, and after this period the further

weathering may not have substantially affected the weathering profile to provide new iron

enrichment phases.

For this type of iron deposit Dorr (1969) defined two main primary lithologies: hematite or

hematitite, the high-grade ore (Fe ≥ 62%), and the low-grade ore itabirite (30% < Fe < 62%), with

three compositional lithotypes: quartzitic, dolomitic and amphibolitic. The tectonic,

metamorphic, and weathering have changed these proto-ores in different ways, resulting in

multiple sets of iron ore variants (typologies), as described in the following sections.

As a metamorphic (green schist grade) rock, BIF present a visible heterogeneity that may induce

a transversal anisotropy as described by Ramamurthy et al. (1993). According to Appendix I, this

transversal anisotropy is defined by mineral composition (banding), orientation (alignment of

minerals), total porosity, and bulk density induced by the original composition or weathering

alterations for each type. This variation may have been a function of the original sedimentary

bedding, the tectonic setting, metamorphic grade, hydrothermal or supergene processes, and

had suffered influence from recent weathering. Further, the superposition of these processes

13

causes partial or total mineralogical and textural changes mainly imposing intact rock strength

reduction.

Regardless of the controversy about the supergene (groundwater leaching or weathering) or

hydrothermal (hydrothermal water leaching) genesis for these large, rich and heterogeneous

iron ore deposits, in this research supergene concentration and subsequent weathering are

considered to be the main events responsible for the chemical and physical changes that induce

reduction of the original itabirite strength, creating the deep soil and weak rock profiles that are

often seen in iron ore mines.

According to the local terminology, the BIF lithotypes are defined by non-iron mineral and iron

contents, as well as rock strength. Itabirites are reclassified by the main gangue mineral found

in three main lithotypes: quartzitic, dolomitic and amphibolitic, and itabirites are also

sub-divided based on Vales’ laboratory crusher test that is directly associated to the rock

hardness or rock strength. The crusher test, which simulates the industrial process, consists of

crushing a known sample weight to less than 31.5 mm, then sieving it at 6.35 mm. This results

in three main designations: hard (more than 50% above 6.35 mm); medium (50% to 25% above

6.35 mm); and weak (less than 25% above 6.35 mm). This test has been largely used as a guide

to identify both the rock strength and the weathering grade, resulting in the classifications of

hard (fresh to a slightly weathered), medium (moderately to highly weathered) and weak

(completely weathered to residual soil). This classification system is also used in association with

field intact rock strengths as presented in the ISRM (1981) tables and suggested by Costa (2009)

and Cruz (2017). In this way field strength characterisation tables and crusher tests association

are used to support a limited number of UCS tests (Costa el al. 2015).

Other geomechanical classifications for Brazilian BIF, have been proposed by several authors

such as Araujo et al. (2014), Castro et al. (2013), Zenóbio (2000) and Zenóbio & Zuquete (2004).

These authors have adapted traditional geomechanical classifications using, for example,

specific field procedures and tests, calibrated with laboratory tests, and applied by several

authors (Zenóbio, 2000; Zenóbio & Zuquete, 2004; Castro et al. 2013; Araujo et al. 2014; Martin

& Stacey 2018).

2.2.1 Itabirites and hematitites geotechnical and geological settings

Typically, BIFs present a small mineralogical variety: hematite, martite, magnetite, specularite,

goethite and ochreous goethite are, in this order, the most important iron minerals. Quartz, iron

dolomite, gibbsite and kaolinite (weathering minerals), are the main present gangue minerals,

and talc, chlorite, and pyrolusite are the main accessory minerals. Several studies of Rosière et

al. (1993, 1996 and 2001), Varajão et al. (1997, 2002), Pires (1995) and Lagoeiro (1998) describe

14

the mineralogical and textural correlation with geological association for several Iron

Quadrangle mines.

A general geological and geotechnical characteristics can be described as follows:

Hard hematitite

The genesis of massive hard hematitite (HHE) ore is a subject of controversy but in local deposits

it has been postulated that hard-massive ore bodies are the result of hypogene or tectonical

iron remobilisation on fold axes, lineation intersections or the result of being originally

concentrated on rich sedimentary bedding (less expressive bodies). Other bodies are correlated

to discrete shear zones, presenting strong heterogeneity due to millimetre tectonic foliation or

from contact metamorphism from intrusive rocks. Each domain presents a typical texture

defining the following typologies: brecciated, massive, bedding or foliated, as defined by Varajão

et al. (2001).

It is a dark grey metallic homogeneous rock with the highest-grade ore composed of granular

martite and microplates of hematite as the main iron mineral followed by magnetite, quartz,

and goethite. A typical dark metallic colour is observed for massive types and more opaque

bands are observed for banding types.

HHE presents the highest intact rock strength and the weathering degree vary in accordance to

ISRM (1981), from W0 to W1 at surface or in very fractured deep zones.

As suggested by Varajão et al. (2001) at Capitão do Mato mine, the total porosity can reach 11%

for slightly weathered HHE, whereas for fresh HHE this could be less than 2.5%. Grain size vary

from 10 μm to 30 μm for hematite and martite granular crystals and are equal to 1 μm for

hematite micro plates. Also, it is suggested that primary micropores vary from Å to 1 μm and

the most important secondary porosity are associated to martite crystals varying from Å to 5 μm.

A medium typology is also described between hard and weak hematitite as a mix of these two

types. However, the geotechnical importance and spatial distribution are very low.

Weak hematitite

The weak hematitite (WHE), is the most common and main high-grade supergene ore type. It

consists of thin (millimetre to centimetre) opaque/dark grey colour bands of hematite and

martite, with low resistance (friable) with high total porosity alternated by more consistent

layers of dark metallic bands of hematite and martite with higher relative strength and lower

total porosity. Total porosity can reach 25% to 30%, according to Costa et al. (2009), or even

higher, according to Ribeiro (2003). This lithotype represents a small heterogeneity, but a

sensitive anisotropy as defined by Costa (2009). In most cases, it represents the tectonic banding

but can also preserve the original bedding.

15

These lithotypes are associated with synclines and can be found in the lower surfaces above

itabirites and can also occur in the shear zones and brittle failures presenting specularite layers,

as presented in Costa (2009). Authigenic breccia is very common and can represent a load

deformation structure associated with the collapse. This collapse is imposed by the leaching

process and respective reduction in the itabirite’s volumes as exposed by Ribeiro (2003).

Grain size and shape of minerals vary according to tectonical settings. The larger hematite

crystals are granuloblastic, the smaller are micro plates, and specularite presents lepidoblastic

texture. Mineral size varies from 0.005 mm to 0.5 mm as presented in Rosière (2005). Natural

moisture content is closely linked to the rain season, presenting an average of 15%.

Fresh quartzitic itabirite

The fresh quartzitic itabirite (FQI), is the typical banded iron formation and more common

itabirite found in the Iron Quadrangle. Non-iron band was totally metamorphosed to quartz

(originally chert crystal), and the iron band is composed of hematite, martite and martitised

magnetite.

The micro texture of the quartz layers is granuloblastic to lepidogranuloblastic. These crystals

are euhedral due to the metamorphic level and their sizes range from 10 µm to 120 µm, and the

hematite layers present tabular and granular shapes with grain sizes between 6 µm to 80 µm.

Presenting a very low total porosity (<5%) and moisture content is around 10% as presented by

Santos (2007) concentrated at secondary porosity.

These rocks are more commonly found at great depths but can also be observed in surface is less

deformed and unfractured zones where it is possible to observe sedimentary structures and the

primary bending. In accordance with ISRM (1981), the weathering grade varies from W0 to W1.

Moderately weathered quartzitic itabirite

This moderately to highly weathered type is defined as partially weathered quartzitic itabirite

(PWQI), presenting a mineralogical composition similar to the FQI, except for the extensive

presence of goethite. The quartz bands present higher void ratio and, in some instances, the

quartz bands can be disaggregated by differential leaching reaching 40% of total porosity as

suggested by Ribeiro (2003). The leaching intensification determines an increase in void ratio,

iron content and the percentage of goethite and ochreous goethite. However, intact rock

strength and grain cohesion decreases, especially in quartz bands.

16

This lithotype represents a typical weathering process over previous FQI with weathering grade

as suggested by ISRM (1981) as W2 and W3 demonstrating a weathering increasing with

reduction of intact rock strength.

Weathered quartzitic Itabirite

This describes the completely weathered quartzitic itabirite (WQI). With the increase in the

weathering process, the total porosity also increases, and bulk density decreases due to silica

leaching. For this weak rock, the original mineralogy remains with addition of goethite and

gibbsite resulting from the weathering mineral alteration. However, there is an important

increase in iron content driven by silica leaching. Macroscopically, these rocks present as friable,

dark metallic grey colour band of hematite and martite, and white to yellow friable quartz bands

with a small amount of goethite. The quartz bands present highly leaching layers (friable), where

total porosity easily reaches 40%.

Fresh amphibolitic itabirite

The fresh amphibolitic itabirite (FAI), is constituted of hematite, martite bands alternating with

quartz, and amphibole (grunerite, tremolite, actonolithe and others) bands. General band

textures are lepidogranuloblastic to granuloblastic and the crystal size is 30 µm on average. The

original mineralogy is preserved just in high depths where a typical brown-yellow colour is

present. Usually, due to the effective weathering process, the amphibole minerals change to

fibrous goethite.

It is important to note that it is difficult to obtain fresh FAI which has not been influenced by

some mineral degradation especially associated with amphiboles oxidation. Amphibolite

minerals are observed only in very deep zones. In this research, FAI is considered fresh if it fits

in the Vale crush test as a hard material and is considered as slightly weathered material even if

the original amphiboles have changed to goethite with weathering grade varying from W0 to

W2 in accordance with ISRM (1981).

Moderately weathered goethitic itabirite

The moderately to highly weathered type is defined as the partially weathered goethitic itabirite

(PWGI), represents the W3 and W4 weathering level according to ISRM (1981) tables, and result

from the weathering from the FAI with significant intact rock strength reduction. The colour

becomes yellow and it is noted that there is an important increase in total porosity and oxidation

of iron ore minerals generating goethite and ochreous goethite.

17

Weathered goethite itabirite

The totally weathered goethitic itabirite (WGI), is characterised by the red and yellow colour

caused by the high goethite and ochreous goethite content presented at matrix and as cement.

They represent the totally weathered materials from the sound itabirites, quarzitic and

amphibolitic, and are considered the most terrigenous lithotypes. They are present close to the

surface over WQI and FAI or FQI but can also be found at depth in open fractures and shear

zones.

Fresh dolomitic itabirite

The fresh dolomitic itabirite (FDI), represents the symbolical Iron Quadrangle BIF, with folded

millimetre to centimetre pink and or white iron dolomite (responsible for the pink colour) and

lower percentage of iron carbonates with crystal size varying from 5 µm to 20 µm and quartz

bands, interbedded with dark grey iron bands constituted by tabular hematite, martite and

martitised magnetite bands. It presents a very low total porosity (<5%), and the moisture

content is around 5% concentrated at secondary porosity. The most important accessory

minerals are sericite and chlorite.

Weathered argillaceous itabirite

The completely weathered argillaceous itabirite (WAI), is characterised by a dark brown colour

determined by high goethite and clay mineral content such as gibbsite and kaolinite. Spier (2005)

argues that they are formed by an effective leaching process over dolomitic itabirites. Typical

banding is millimetre to centimetre and thick composed of very fine crystals of martite hematite

and goethite, and bands of clay minerals. Some manganese minerals are common, such as

pyrolusite and cryptomelanite. Total porosity is lower than 15%.

The main visual characteristics of all previously described lithotypes and the UCS values normally

obtained for each group are presented in Figure 2.1. In this figure the green dotted square

indicates the fresh and hard types, the blue dotted square indicates the moderately weathered

types, and the red dotted square indicates the completely weathered and weak types.

18

3 Figure 2.1 Summary for all studied BIF types, showing in the green dotted square the high

UCS values, in blue the intermediate, and red the low UCS values (after Martin

& Stacey 2018)

2.3 WEATHERING PROFILE SETTINGS

Deep open pits are becoming increasingly common in the iron ore mining industry and many of

these pits are being developed in parts of the world where deep weathering processes have

taken place, producing deep weathering profiles and ore deposits defined as saprolites. These

deposits are the result of chemical and physical changes mainly responsible for density and

strength reduction, and porous increase from the parent fresh rocks inducing a stress unloading

and an increase on hydraulic conductivity. According to Blight (1997), the factors having an

influence on the strength reduction and behaviour of residual soils are mineral contacts (surface

of grains contacts), relict structures and discontinuities, anisotropy (fabric of the parent rock),

partial saturation and void ratio.

According to studies conducted by Little (1969), Deere & Patton (1971), and Anon (1981),

residual tropical soil profiles can be simplified in four basic horizons from higher deeps to surface

as: bedrock, saprorock, saprolite and residual soil as shown in Figure 2.2. These materials can

Hard hematitite

HHE

Medium hematitite

MHE

19

also be divided into six zones, based on the degree of weathering in accordance to ISRM (1981)

suggestions, as also illustrated, ranging from a continuous gradation from the fresh rock, slightly

weathered rock through moderately weathered rock, highly weathered material, to completely

weathered and finally residual soil.

4 Figure 2.2 Schematic diagram of typical tropical residual soil profile presenting the

horizons on the left side and zones on the right side (modified from Deere &

Patton 1971)

Each zone or horizon presents different behaviour and specific methods to describe in their field

and in laboratory characteristics. Intense research has been done for bedrock considering rock

mass, discontinuities and applying rock mechanical approaches. Likewise, for residual soils, soil

mechanical approaches are applied. However, very few works are concentrated at saprolite and

saprorock horizons.

For the residual soil horizon, the set of residual soil and completely weathered rocks (zones) are

dependent on climate, parent rock, topography, drainage and age (Townsend, 1985) and may

exhibit strain softening behaviour when loaded, which causes soil strength to reduce as a

function of strain after the peak strength has been reached. Additionally, to understand some

features it must be considered the following characteristics: thickness, horizons, or profiles

(stratification), and composition (parent rock).

For these reasons, it is important to define the geometry of weathering profile in terms of

horizontal and vertical thickness and distribution. This geometry generally runs parallel to the

ground surface; however, the anisotropy provided by structures that compose discontinuities,

especially the vertical discontinuities, can significantly influence this parallelism.

20

According to Lao (2013) various attempts have been made since Vargas (1953) to devise systems

for the description or classification of a completely weathered profile. A number of studies have

been carried out by researchers like Little (1969), Fookes et al. (1971), Fookes (1997), Dearman

et al. (1978), Irfan & Dearman (1978), and Anon (1981) to investigate the weathering profiles for

a descriptive degree of weathering.

Recently, with an increase in the interest in weak rock lithologies, a considerable amount of

research has been done to investigate the behaviour of completely to highly weathered rocks

mainly for mining operations, such as the recent guidelines published by Martin & Stacey (2017)

that provide a residual soil (soil-like rock) overview.

Evaluating BIF weathering profiles, due to the effectivity of the weathering process in tropical

climates induced by high pluviometry index and the topography featured, the BIF weathering

profile can often extend for more than 200 m and can reach over 400 m in depth. In addition,

the plateau morphology, syncline and anticline geological configuration and the presence of a

favourable structural control, defined by the high banding angles and extensional fractures, can

facilitate the superficial and ground water penetration and circulation, increasing the

weathering profile even more.

Weathering as a continuum and multiple conditioning processes generates a range of different

materials for each compositional BIF; from fresh to completely weathered rock. This weathering

profile presents specific and complex characteristics and boundaries between different

weathered horizons and zones.

Studies by Morris (2002 and 2002a) and Taylor et al. (2001) argue that the supergene process

can reduce the thickness of the BIF bodies by 32% to 40%, and increase the total porosity from

6% to 30%, as described by Mourão (2007). Based on that, weathering can produce an effective

softening from the leaching, and the remaining iron bands present a high total porosity, that

could be cemented by secondary iron oxides (recrystallised hematite or goethite). At more

evolved profiles, the resulting material presents weak strength, and iron enrichment promotes

an increase in bulk density due to iron minerals reconcentration. But in some week materials,

the iron oxides work as cement, improving the strength.

Additionally, studies by Ramanaidou (1989 and 2009), Ribeiro & Carvalho (2002), and Ribeiro

(2003) suggest that during the first leaching processes there have been no volumetric changes

and no quartz to goethite substitution. Progressively, the volumetric changes start to leach the

gangues minerals (more effective on dolomites, but also in quartz bands) and replace quartz to

21

goethite. This porosity could be completely cemented by goethite or iron hydroxide in superficial

levels, creating a metric hard crust called canga.

As presented by Spier et al. (2003), the superficial water is the main physical and chemical agent

responsible for effective dissolution and leaching of carbonates and siliceous minerals, and for

oxidation and hydration of iron-based minerals.

2.3.1 Failure criteria evaluation for the BIF’s weathering profiles

In the Iron Quadrangle iron ore mines, due to the predominance of weak rocks and soil material,

the Mohr–Coulomb failure criterion (MC) has often been used to define the strength parameters

required for slope stability analyses, based on direct shear and triaxial shear tests. For soil

materials, the Mohr–Coulomb criterion is the simplest and most widely used constitutive model

for determining the normal and shear stresses at failure on a loaded friction material. Bai et al.

(2010) argue that MC has been widely used in rock and soil mechanics and has good resolution

for materials that fail in the elastic range and under low strain plasticity.

However, with increasing quantities of hard rocks in deeper mines it has become necessary to

perform specific tests for these materials and, due to the popularity of the Hoek–Brown failure

criterion (HB), the unconfined compression test (UCS) has become the most important rock

strength test applicable for hard rocks. In the absence of a suitable number of these tests,

geomechanical field classification and empirical correlations with petrophysical characteristics

were used to provide input for slope stability analyses.

In Barton & Quadros (2015) it is indicated that the HB was initially proposed for the

determination of intact rock strength and rock mass strength under isotropic conditions.

Nevertheless, in recent years, with some adjustments, this criterion has been also used for the

determination of the anisotropic strengths of rocks. To use this criterion for prediction of

strength in anisotropic, intact rocks, a careful definition of the input parameters (uniaxial

compressive strength, ‘σci’, material constants ‘mi’ and ‘s’) is necessary. Moreover, the

determination of minimum and maximum intact rock strength variation due to rock anisotropy

is important in the selection of strength values for the rocks.

According to Saiang et al. (2014) there are fundamental differences between the two models or

criteria that are rather poorly understood or less well appreciated. First and foremost is the fact

that MC alone is a classic constitutive model, while HB is a failure criterion. This means that the

HB model cannot generally relate stress and strain in the same general way as the MC model.

For instance, MC is not suitable for more bright materials with tensile strength and implies a

22

linear envelope while HB is non-linear and, therefore, closer to the real behaviour. However, it

does not address ductile behaviour, so both have advantages and disadvantages.

Thus, plasticity results from the two models cannot be expected to be the same, as is often

mistakenly assumed. The next significant difference between HB and MC is the assumption

regarding the yield and deformation characteristics of the intact rock. HB assumes that the intact

rock is characterised by an elastic-brittle-plastic behaviour, while MC assumes that it is

characterised by an elastic-perfectly plastic behaviour.

Lin et al. (2014) presents an appropriate process that involves balancing the areas above and

below the MC plot. The HB envelope diagram is a curve, while the MC diagram is a straight line

(depending on the level of applied stress), as shown in Figure 2.3, and is divided into three parts

which are marked as Regions 1, 2, and 3, respectively. At normal stress in Region 1 or Region 3,

the equivalent MC strength parameter will overestimate the shear strength compared with HB

curve. When most of the normal stress concentrates in Region 2, the strength parameter may

be slightly underestimated.

The models will give similar observations up to the point of yield or failure (Region 2). However,

HB will show much larger plastic straining than the MC for the same constant stress levels

beyond yield. For Lin et al. (2014), since HB does not relate stress and strain in the same general

way as MC, the accuracy of the plasticity after the yield is therefore questionable.

5 Figure 2.3 The three regions for the Hoek–Brown strength curve and the equivalent

Mohr–Coulomb strength line (Lin et al. 2014)

23

The use of MC failure envelope is established for weak completely weathered types and

laboratory test results have been used to define the intact rock strength as well as the shear

stress, as presented in Martin & Stacey (2018). For hard BIF, the HB strength curve is more

appropriate for brittle intact rock observed in these types. However, for moderately weathered

ones, the best criterion must be carefully evaluated as these rocks are a mixture of material that

exhibit soil and/or rock strength characteristics.

2.3.2 BIF weathering profile determination and implications for slope stability

For iron ore mines, the deep weathering profiles can present a mix of fresh, partially weathered,

and completely weathered material. The use of classical rock or soil mechanics must be carefully

applied to not under- or over-estimate geomechanical behaviour or misunderstand failure

mechanisms.

In shallow mines, the approach to dealing with weak rocks and slope design has largely been

based on adopting soil mechanics principles and past experiences. However, recent slope

failures and studies by Innocentini (2003), Costa (2009) and Sá (2010) suggest that some failure

mechanisms have specific characteristics for weak leached materials, and that the application

of classical soil mechanics principles does not enable failure mechanisms to be fully understood,

while rock mechanics concepts are also not fully applicable.

In those rocks, shallow, shear failures at the batter or multi batter scale are the most common

failure mechanisms. Large-scale slope failures are rare and more complex, composed of

translational and planar surfaces, the last ones controlled by unfavourable anisotropic planes

with reduced shear strength. Further, secondary influences such as high stress concentrations

at the toe of slopes, water content and discontinuities may also act as important contributing

factors.

The current approach used to undertake slope stability analysis in these types of weak rocks is

based on limit equilibrium analysis using the Mohr–Coulomb shear strength criterion and, in

some special cases, supported by numerical models. The shear strength and elastic parameters

for these designs are normally based on a limited number of intact rock laboratory tests which

are mainly saturated triaxial and direct shear test and supported by adapted rock mass

classifications. While this approach has been mostly successful, it would appear from experience

that they are conservative. Despite that, there have been examples of large slope failures not

captured by these design approaches that have had significant negative impacts on mine

production.

24

The situation is particularly challenging when considering the stratigraphic position and/or

tectonic settings of the weak rocks that are very often located at the toe of high slopes (more

than 400 m depth), where the stress is concentrated. To address this issue, Vale geotechnical

engineers have historically designed a buttress of ore to be left at the toe of the wall, resulting

in a loss of ore reserves, as suggested in Costa et al. (2009).

In addition to classical soil mechanics laboratory testing, to support these materials’ complete

characterisation, other parameters are evaluated such as: total porosity, bulk density, particle

size distribution curves, mineralogy, and anisotropy have been considered in relation to slope

stability behaviour. Describing and correlating these parameters remains a key challenge to the

authors due to the difficulties associated with sample acquisition, testing, scale effect and

sample representativeness.

An important characteristic generally neglected for BIF due to the high permeability and low

plasticity observed on those materials is the matric suction effect on slope stability. However, a

minor presence of clay minerals can influence the slope stability, as presented by Lu and Likos

(2004) which argue that the matric suction contributes to the cohesion and increases the shear

strength of unsaturated soils. Studies by Grgic et al. (2005) on iron formations suggested that an

improvement on the strength and cohesion of partially saturated oolithic iron ore can occur due

to the negative porewater pressure effects.

It has been noted from work outside the mining industry that the shear strength of a soil with

negative porewater pressure can increase the stability of slopes, especially those having shallow

but steep surfaces as supported by Fredlund et al. (1978). So far, matric suction effects have not

been considered in Vale open pit slope stability evaluations where effective stress parameters

are obtained by conventional saturated laboratory testing. This approach seems to be adequate

for high pluviometric index areas where long-term slope stability is required. However, it tends

to be too conservative when applied to the design of short-term slopes above the water level.

The use of unsaturated soil mechanics is therefore necessary to correct the balance between

safety and economical slope designing.

In addition, for Iron Quadrangle mines, studies by Soares (2008) and Ventura & Bacellar (2012)

for country rocks showed the importance of suction effects on iron ore mines’ slope stability

analysis, especially for shallow surfaces.

Contrasting, from fresh and hard BIF, for future proposed deepest open pit mines, there will be

an increase in the percentage of hard and fresh rock exposed, which poses new challenges to

the geotechnical team. They may need to develop slope design criteria and evaluate possible

25

failure mechanisms in hard rock by using classical rock mechanics principles, breaking some

paradigms established over recent decades operating mainly in weak rocks.

For these hard rocks, it is crucial to ensure a good evaluation of intact rock strength, as well as

geotechnical characteristics of discontinuities to understand slope deformation behaviour for

quite a complex arrangement of different rock mass strength materials.

It is also important to determine geotechnical parameters for partially weathered rocks

(a mixture of hard and weak rocks) and to develop a method to better define and map the

boundaries along the weathering continuum.

2.4 ROCK MECHANICS APPROACHES FOR FRESH TO MODERATELY WEATHERED BIF

Slope stability analysis for material with high-value of intact rock strength (fresh to slightly

weathered BIF) is controlled by the discontinuities shear strength and rock mass strength. For

partially weathered itabirites, the reduction in intact rock strength imposed by the weathering

can change this setting, and slope stability analyses are controlled by intact rock strength and

possible associated heterogeneity. It remains unknown where in the weathering horizon of the

fresh rock (bedrock) is reduced to saprorock, and to such an extent that it reaches these strength

reductions, changing the slope stability control.

For the bedrock, UCS testing is largely used, as a direct parameter, to determine rock strength

and indirect tests also applied as: point load test (PLT), Schmidt hammer test and tactile visual

or qualitative estimation based on characteristic tables and geomechanical classifications.

Many past experiences of geomechanical classification have been proposed for iron ore mines

and associated country rocks, even with some attempts from Vale geotechnical and consultant

teams. These experiences suggest that in many cases the use of geomechanical classification has

proven to be more than adequate and reliable to correlate with intact rock strength based on

minimum UCS test results. However, these simple estimation methods must have an adequate

laboratory test calibration and field strength estimation to fully meet complex slope stability

analysis.

The establishment of a standard characteristic table and rock mass classification for the Iron

Quadrangle’s BIF has been proposed by several authors (Zenobio, 2000; Zenobio & Zuquete,

2004; Castro et al., 2013; and Araujo et al., 2014). These authors generally adapt classical

geomechanical classifications using, for example, specific field procedures and tests, calibrated

with laboratory tests results.

26

For fresh rock in Vale mines, the adapted strength field characterisation tables, crush tests and

a limited number of intact mass UCS tests have been used to determine the intact rock strength

and guide slope stability evaluation. Although this approach has been satisfactory, the increase

of fresh rock in deeper mines demands a higher level of confidence and a more reliable

database.

This approach is largely used in different rock types, and historically, index proprieties

interrelationships with rock strength have been established worldwide for several rock types.

Gupta and Rao (1998 and 2001) and Irfan & Dearman (1978) mainly focused on granitic rocks

established correlations between the physical proprieties of the rocks to the weathering process

for geotechnical evaluations.

Specifically, for BIF a similar access was established by Aylmer et al. (1978) evaluating the bulk

density, iron grade and total porosity for Mount Tom Price iron ore mine and concluded that the

iron grade and the bulk density have a good correlation; however, the accuracy was largely

affected by high and variable total porosity.

Also, according to Box and Reid (1976) for iron ore formation from Cockatoo Island, true specific

gravity could be expressed as a function of iron content; however, due to the complexity and

multiple factors the same influence could not be established for total porosity. Thomson (1963)

concluded that for iron ore samples from Australia, a theoretical hematite-quartz curve can be

used for bulk density definition and can present an approximate iron content calculation. For

Sishen South iron deposits, Nel (2007) established that total porosity is directly correlated to the

dry and bulk density providing a reliable calculation index. These works are based on the same

correlation and presented similar findings for different worldwide BIF deposits.

In Brazil, studies of Ribeiro (2014), Santos et al. (2005) and Santos (2007) evaluating the

association between bulk density and iron content for Vales iron mines, concluded that there is

a linear positive correlation between iron content and bulk density. They also presented that

weathering process has an important influence on this correlation and iron content dispersion

is imposed by total porosity. The most significant type of total porosity for slope stability and

engineering applications is the interconnected porosity or effective porosity, due to its relevance

on strength and capacity of transmitting fluids through connected pores. In rock engineering

porosity is an important physical parameter for evaluation of water storage capacity,

identification of weathering profiles and correlations with strength.

However, as proposed by Saroglou et al. (2004), mineral composition, fabric, grain size, degree

of weathering, and anisotropy also influence the strength and deformation of intact rocks.

27

The correlation between UCS and physical properties such as bulk density (𝜌𝜌b), P wave velocity

and elastic properties has been largely used. However, a smaller number of publications

explored the influence of rock banding on strength characteristics of BIF. The study from

Gonçalves et al. (2012), concludes that at slower strain rate the matrix-controlled rheology takes

over and strength of BIF is not dependent on fabric anisotropy or on iron oxide content. The

banded sample becomes as weak as pure and homogeneous hematite aggregate. It also

indicates that iron oxide content and strain rate are controlling factors on BIF.

Another approach used to in this thesis is the evaluation of the dynamic modulus, using

compressional (P) and shear (S) wave velocity that has been used by many authors as a

non-destructive method for physical characterisation of intact rock strength. As shown by Turk

& Dearman, (1986), and Karpuz & Pasamehmetoglu (1997) such experiments are used for

determination of rock weathering degree, fluid saturation, presence of discontinuities and as

input data in geotechnical models and rock engineering applications. P and S wave velocities

have important correlation with other physical and strength properties of rocks such as total

porosity, bulk density, and UCS test results.

Dynamic elastic parameters of rock are well described parameters obtained throughout

empirical relationship among P and S wave velocities and 𝜌𝜌b of samples described by elasticity

theory. From theoretical relationships, P and S wave velocities are also used to calculate dynamic

elastic modulus (Edyn), dynamic shear modulus (Gdyn), dynamic bulk modulus (Kdyn) and dynamic

Poison’s ratio (νdyn), according to the equations presented by Bourbié et al. (1987), Sheriff

(1991), and Soares (1992). The application of these properties is limited to its relationship with

the static deformation modulus. This relationship can only be verified by empirical relations once

there is a huge variation among different rock types. Additionally, dynamic laboratory

measurements (acoustic) can be used to help calibrate the mechanical properties calculated

from the petrophysical model. In the absence of laboratory data, most of these results must fit

within known ranges, depending on lithology (Crain & Holgate 2014). After proposing correct

relationships, dynamic elastic modulus can be used as crucial information for geomechanical

models in mining applications such as slope stability and slope monitoring.

2.5 SOIL MECHANICS APPROACHES FOR COMPLETELY WEATHERED BIF

Soil lithotypes in mining operations are evaluated based on soil mechanics approaches and the

shear strength is obtained performing saturated soil laboratory tests due to the test velocity and

the possibility to determine effective and total strength and obtain MC parameters normally

28

used on slope stability analysis. MC failure criterion and the effective stress concept were

originally defined by Terzaghi (1936) as per Equation 2.1.

τff = c'+(σf uw)f tanφ' 1(2.1)

Where τff is the shear stress on the failure plane at failure; c' is the effective cohesion, which is

the shear strength intercept when the effective normal stress is equal to zero; (σ f uw f ) is defined

as effective normal stress on the failure plane at failure, σff is the total normal stress on the

failure plane at failure; uwf , defined as porewater pressure at failure; φ' the effective angle of

internal friction.

Defined as MC criterion the envelope illustrated at Figure 2.4 represents shear stress and

effective normal stress on the failure plane at failure where the effective stress acting normal to

the plane on which the shear strength is mobilised. The porewater pressure acts equally on all

planes for isotropic soils, and the shear stress described by the failure envelope indicates the

shear strength for each effective normal stress. The failure envelope is obtained by plotting a

line tangent to a series of Mohr circles representing failure conditions. The slope of the line gives

the effective angle of internal friction φ', derived from inter-granular contact and its intercept

on the ordinate is called the effective cohesion c', derived from the inter-particle forces.

Cohesion is sensitive to water, porewater pressure, and chemistry and is mobilised under small

strains and decreases as strain increases, whereas the friction is not developed to its maximum

value until significant amounts of strain have occurred (Maail et al. 2004).

6 Figure 2.4 Mohr–Coulomb failure envelope for a saturated soil

It is noted that a straight line defines this intercept. However, for low effective stresses,

mobilised strength is in fact lower than that given by the straight-line envelope

29

(MC overestimates the strength on this region) and the true failure envelope is the one that is

tangential to the lower circle defining the true cohesion present in most residual soils and

therefore can be relied on even under low-stress conditions (Fourie & Haines 2007).

Another important soil characteristic that must be evaluated is the dependence on strain, which

can show distinct peak and residual strengths where the resistance reduces progressively with

increasing strain or decreases directly to the residual value from the peak value (ductile or fragile

behaviour). This behaviour will be closely associated with soil characteristics such as: mineral

and particle size constituents (clay, silt, sand, or gravel); void ratio, mainly for sand and gravel;

over-consolidation, for clays; and water and air pore pressure. These characteristics will control

the peak and post peak to residual behaviour. It is likely that the transition from peak to residual,

and how much strain occurs during this transition, will be a key factor to be identified even for

partially saturated soil where strain softening behaviour is not expected. These soils must show

a negative porewater pressure due to the voids being filled partially by water and air generating

an apparent cohesion.

For unsaturated or partially saturated slopes, the analyses for soil or soil-like materials in mining

operations are performed using effective stress parameters from saturated laboratory tests. For

slope stability analysis the water effects are considered just during the analyses as water table

(gravel or sand) or positive pore pressure (silt and clay). This is mainly due to porewater pressure

which is generally assumed to dissipate for gravel and sandy soils, leading to more conservative

evaluations. Thus, total stress parameters are used in very specific evaluations (e.g. saturated

long-term slopes stability).

The existence of negative pore pressure on the partially saturated or unsaturated materials for

low-permeability soils, gives an additional cohesion (apparent cohesion) generally disregarded

by the analyses. This analyses misconception mainly for mining evaluation is attributed to the

belief that fast-superficial drainage and soil with high permeability did not retain the partial

saturation and the apparent cohesion for a long period of time; the partially saturated zone for

high permeability soils will represent a few metres above the water table. Additionally, the use

of laboratory tests for partially saturated or unsaturated soils and negative pore pressure

definitions are not trivial.

As presented by Lao (2013) this negative porewater pressure also defined by total suction

(matric suction plus osmotic suction), between soil particles, water and air occurs when the

degree of saturation falls below about 85%, increasing effective stress, and therefore the

available shear strength. When soil becomes partially saturated. Fredlund et al. (1979) proposed

30

Equation 2.2 for unsaturated soil in which two independent stress state variables, the net

normal stress, (σ − ua) and the matric suction, (ua− uw) are used.

τf= c'+(ua − uw )tanφ b + (σ − u )tanφ' 2(2.2)

Where τf is the shear strength, c' is effective cohesion intercept, ua is pore air pressure, uw is

porewater pressure, φ b is the angle of cohesion intercept increase with increasing suction, σ is

the total stress and φ ' is the effective angle of shear resistance. This shear strength equation

defines a planar surface called the extended Mohr–Coulomb failure envelope, as shown in

Figure 2.5 (Gan et al. 1988).

7 Figure 2.5 Extended Mohr–Coulomb failure envelope for unsaturated soils (Gan &

Fredlund et al. 1988)

Positive or zero pore pressures result in matric suction effects which can explain why some soils’

slopes are stable even with angles that are steeper than would be predicted by Equation 2.1,

and why many falls happened during heavy rainfall, even when a rise in the watertable does not

occur. These falls occur with an increase of degree of saturation without necessarily reaching

100% resulting only in a decrease or loss of the additional strength component generated by

matric suction.

Rahardjo et al. (2014) developed an axis translation technique from apparatus capable of

controlling and measuring pore air and porewater pressure enabling the measurement of shear

strength for unsaturated condition, plotting, and interpreting pore pressure effects on total

material strength at an extended three-dimensional MC envelope. Procedures and apparatus

31

used in this technique are described in Fredlund at al. (2012). Using this technique according to

Fredlund et al. (2014) the failure envelope intersects shear strength versus matric suction planes

at a total cohesion, and when this can be obtained for different matric suction, values are

defined an extend MC failure envelope giving φ b angle. This angle was defined by Lee et al. (2005)

as equal to φ' for matric suction less than or equal to the air entry value (AEV), or where φ b is

lower than φ' for matric suction and higher than AEV.

To define the shear strength equation for unsaturated soils one possible approach is to use

estimation equations based on saturated shear strength parameters and soil classification

proprieties as supported by Vanapalli et al. (1996), Fredlund et al. (1996), Oberg & Allfors (1997),

Khalili & Khabbaz (1998), Bao et al. (1998) and Goh et al. (2010). Fitting equations were

described by Fredlund et al. (1978 and 1996).

32

33

CHAPTER 3. METHODOLOGY

The main aims of this research will be discussed using the following three different approaches:

• Use of Vale’s geological and geotechnical database to evaluate and correlate

geomechanical properties of the BIF to understand intact rock strength variation in

response to the weathering.

• Development of a laboratory testing programme, focusing on intact rock

characteristics to determine petrophysical, intact rock strength and elastic parameters

using different approaches for hard and weak rocks in order to complement

information not available or insufficient on the Vale´s database.

• Petrographic thin sections analysis to assess the rock microcharacteristics and its

correlation to weathering changes. Ultimately, this approach will correlate

microgeological with macrogeological and geotechnical features.

With these approaches, it is expected to best correlate weathering profile horizons and zones

with strength and elastic parameters, thus promoting a more complete and comprehensive

understanding of geotechnical behaviour of rock weathering variables and its influence at open

pit excavation. From that, pit design can be optimised so that maximum ore can be recovered,

with the required level of stability and associated level of safety.

In order to achieve proposed objectives, a three-phase methodology was used.

• Phase one: Carried out during the years 2013 and 2014, a field investigation was

undertaken, and several samples collected from outcrops and from core drills for all

BIF lithotypes. Geological and geotechnical information were logged, based on ISRM

(1981) suggestions, and petrophysical characteristics were described (e.g. qualitative

anisotropy and banding) to support the analysis of laboratory test results. Outcrops

and samples were photographed for additional visual information such as anisotropy

plans, banding, type of BIF and discontinuities. A literature review was also performed

and evaluated for problem statement, and evaluation of the Vale geotechnical and

geological database was also conducted.

• Phase two: Carried out during the years 2014 to 2016, focused on the description of

petrographic thin sections, to evaluate the rock mineralogy, fabric, total porosity, and

to provide information on a microscopic scale to be compared with field

macrocharacteristics described in the first stage, to provide geological and

34

geotechnical field work validation. Finally, all laboratory tests for soil and rock

materials were run in Brazilian and Australian laboratories.

• Phase three: Carried out during the years 2015 to 2021, included the evaluation of all

laboratory tests, results used and complied to determine the intact rock properties

together with the correlations with geological characteristics and other geotechnical

parameters of each lithotype obtained during the previous phases. This phase includes

data interpretation, evaluation of the results, draw conclusions and PhD dissertation

development were determined.

3.1 FIELDWORK AND SAMPLING

From the fifteen studied mines the sampling campaign covered different weathering horizons

and degree levels considering all studied BIF lithotypes and typologies. Geological and structural

information was collected, and the geotechnical information described was based on the ISRM

(1981) suggestion tables, and all samples and outcrops were photographed and used to support

the visual information for laboratory tests and physical parameter recognition (e.g. anisotropy

and banding).

Data from previous laboratory test reports available in Vale’s database and from laboratory tests

specifically performed in Brazilian and Australian laboratories were used. The use of Vale’s

previous laboratory test reports was considered just when all geological and geotechnical

information were available.

A total of 21 block samples were shipped to the petrophysical laboratory of the Campina Grande

University at Paraiba state, Brazil to test porosity, permeability, and P and S wave velocity for

fresh to moderately weathered BIF. A total of 10 block samples were shipped to Geocontrole Br

Sondagens laboratory at Nova Lima, Minas Gerais state, Brazil to perform geotechnical soil and

rock tests.

To Australia, more than 500 kg of drill cores and block samples were shipped to The Australian

Centre of Geomechanics/Department of Civil, Environmental and Mining Engineering, The

University of Western Australia and E-precision Laboratory both located in Perth, Western

Australia, for rock and soil geotechnical tests.

3.2 DATABASE CONSISTENCY APPROACHS AND DEFINITIONS

Laboratory test results were evaluated as graphs and tables to verify the correlation between

the geotechnical parameters and to define the level of correlation. From those graphs,

35

regression curve was determined and resulted in an empirical equation (linear or not linear

correlations) that represents the variables’ behaviour.

To better evaluate the dataset, reduce the dispersion and support the variable correlations,

basic statistical concepts and criteria of coefficient of determination was used.

For all test results, the following were defined: the arithmetic mean (x) or average, which is

defined as the central tendency and is the sum of a collection of numbers divided by the number

of total collections; and the standard deviation (SD), used to quantify the amount of variance or

dispersion of the dataset (Ross 1993). Extreme outlier results were removed according to the

Box plot statistical methodology (Whitaker et al. 2013). This technique identifies the mild

outlier’s values from the quartiles (Qt) determination, based on the Equation 3.1.

Values below the lower inner fence (QtLower):

QtLower = 1Qt - 1.5(3Qt - 1Qt) (3.1)

and values above the upper inner fence (Qtupper) as Equation 3.2:

Qtupper = 3Qt + 1.5(3Qt - 1Qt) (3.2)

where 1Qt is the first quartile and 3Qt is the third quartile.

The 3Qt evaluates database dispersion around a central data leaving 75% of data below the sum

and is defined by Equation 3.3:

3Qt = × + 1.5.IQR (3.3)

The first or inferior quartile (1Qt) evaluates database dispersion around a central data leaving

25% of data below the sum and is defined by the Equation 3.4.

1Qt = × - 1.5.IQR (3.4)

Interquartile ranges (IQR) measure how spread out from a central data the values are and these

form what are called outliers and are defined by Equation 3.5:

IQR = 3Qt - 1Qt (3.5)

To evaluate the relationship for two sets of data which are strongly linked together and to be

able to measure and determine the relationship between two variables, an adequate coefficient

of determination (R2), which describes the strength and the direction of the variable’s

correlations, must be determined. In addition, these correlations could be expressed by linear

or nonlinear regression methods due to the nature of evaluating variables. The correlation

36

coefficient has been calculated by using the ‘best fitting’ approach using automatic procedures

through several attempts on the dataset.

For Butel et al. (2014), the quality of the correlation is determined by the values of the R2, the

size of the dataset, and the visual fit of the regression curve. An adjusted curve is defined as the

curve that best evaluates the dispersion of the dataset and defines the adjusted equation that

gives the proportion of the variance in the independent variables. In other words, the R2 is a

measure of how well a regression curve matches a set of data for an evaluated dataset. It could

also be interpreted as the adjusted model of observed effects between two dependent

variables. The strength of the R2 is defined as 0 to 0.29 (little if any correlation), 0.3 to 0.49 (low),

0.5 to 0.69 (moderate), 0.7 to 0.89 (high) and 0.9 to 1.0 (very high correlation) as presented by

Asuero et al. (2006).

During correlation evaluation analysis, several adjusted fitting curves (e.g. linear, exponential,

and potential) were tested. The exponential adjustment curve proved to be the most suitable

for the available dataset, even when some correlation proves to be better for linear or potential

regressions (e.g. the correlation ratios obtained were not significantly different from the

exponential values).

Due to the material characteristics and the lack of references for the studied rock types, a

correlation ratio equal or superior to 0.50 (moderate) was considered adequate.

Previously to the basic statistical validation the non-typical hematitite or itabirite samples were

evaluated, following the characteristics presented in Appendices I and II, in which some

geological features such as intense folding, presence of specularite levels, filling material with

different weathered levels, quartz, and calcite veins and other, that do not represent typical BIF

intact rock characteristic material were eliminated. It is important to note that some of these

geological features are observed on these lithotypes and are generally subordinate to the

anisotropy effects. However, these features induce changes on intact rock values and do not

represent the general media scale model.

For BIF, which presents metamorphic heterogeneity in a determinate scale of observation, it is

necessary to determine the ratio of anisotropy to evaluate the anisotropy degree. Estimating

the variation of intact rock strength due to the anisotropy effect allows the differentiation of

spurious test results induced by rock intrinsic characteristics resulting from the anisotropy

effects which can lead to misleading results and increasing variance. In the present study, the

variation of UCS due to anisotropy was considered to determine the degree of strength

anisotropy (RC), as first proposed by Singh et al. (1989), which defines the Rc as the variation in

37

compressive strength (measured in uniaxial and triaxial tests) depending on the angle between

the direction of the load applied to the tested samples, and the plan that defines the direction

of the anisotropy.

In order to evaluate the influence of metamorphic banding and define anisotropy in intact rock

strength for all tests, the anisotropy βangle (beta angle), as described by Jaeger (1960), was

considered, by testing different angles between banding and the loading direction, varying from

0° to 90°. However, due to the reduced number of valid results for some typologies, results were

grouped in three main βangles ranges: for loading parallel to banding (β0°), all tests results from

0° < β ≤ 30° were considered; for direction of loading oblique to banding (β45°), results from

30° < β ≤ 60° were considered; and for loading perpendicular to banding (β90°), results were

grouped from 60° < β ≤ 90°.

The anisotropy ratio was defined, as presented in Singh et al. (1989), as the ratio between the

maximum compressive strength, normally obtained at β = 90°, divided by the minimum value

obtained. It is defined in Equation 3.6 as σc90°, the compressive strength value for βangle

perpendicular to the planes of anisotropy, and σcmin, the lowest compressive strength value

obtained. The range and classification of the Rc is established by Equation 3.6 is presented in

Figure 3.1, as well as the diagram representing the βangle definition by McLamore & Gray (1967).

𝑅𝑅𝑅𝑅 = 𝜎𝜎𝜎𝜎 90°

𝜎𝜎𝜎𝜎 𝑚𝑚𝑚𝑚𝑚𝑚 (3.6)

where:

Rc = degree of anisotropic or anisotropy ratio.

σc90° = compressive strength value for βangle perpendicular to the planes of anisotropy.

σcmin = lowest compressive strength value obtained.

8 Figure 3.1 Classification based on anisotropic ratio, Singh et al. (1989) and βangle

definition after McLamore & Gray (1967)

Also, to determine and cross-check the anisotropic effects in BIF, a second method to determine

the anisotropy degree, based on P wave velocities measures, was used. For this method,

38

Saroglou & Tsiambaos (2007) proposed a similar equation to determine the velocity anisotropy

index (IVp) given by the ratio presented in Equation 3.7.

𝐼𝐼𝐼𝐼𝐼𝐼 = 𝑉𝑉𝑉𝑉0°𝑉𝑉𝑉𝑉 90°

(3.7)

where:

Vp0° = the maximum velocity of P waves (propagation parallel to the planes of

anisotropy).

Vp90° = propagation is perpendicular to the anisotropy plane.

Figure 3.2 shows the classification of anisotropy according to these authors, based on the values

of the indexes defined.

9 Figure 3.2 P wave velocity anisotropic index, modified from Perucho et al. (2014)

3.3 LABORATORY TESTS

3.3.1 Sampling validation and laboratory test grouping

Considering dispersion induced by the presence of imperceptible discontinuity and not typical

BIF geological features (mentioned in Section 3.2), the samples (pre-test) and failure surface

(post-test) were evaluated using photography to identify and characterise the main sample

geological features and the type of failure surfaces determination.

The first step in the laboratory test programme was to define the appropriate geotechnical tests

for each weathering horizon and zone. Due to the high-strength variations noted in the

weathering profile and the difficulty in preparing samples with high-strength heterogeneity,

mainly for moderately weathered types, the rock strength estimation table from Martin &

Stacey (2018) was used to define a hypothetical strength limit for the three main groups of BIF

considered in this research (fresh, moderately and completely weathered) in a direct association

with the weathering horizons (bedrock, saprorock, saprolite and residual soil) and associated

with the types of laboratory tests results, as shown in Figure 3.3.

For hard rocks (bedrock), limited by green dotted lines, with high strength and fresh to slightly

weathering degree unconfined compressive strength (UCS), triaxial Hoek cell (TRI-HB) and

indirect unconfined tensile strength (UTS) were used. For weak rocks (saprolite horizon to

39

residual soil), limited by red dotted line, with low to very low strengths and highly to completely

weathering degree single stage consolidated undrained triaxial compression test (CIU), and

direct shear test (DST) were used. For moderate strength (saprorock to saprolite horizons),

limited by blue dotted lines, where materials exhibit variable strength behaviour, tests were

selected in accordance with the predominant strength characteristic in each sample (soil for

saprolite or rock for saprorock). For all samples, physical tests were carried out to determine the

bulk density and visual total porosity.

10 Figure 3.3 Weathering grade and estimation of the rock strength table plotted for the

three main groups. The applied laboratory tests for each strength level are

grouped by the green dotted square for hard rocks, a blue dotted square for

moderate rock strength and the red dotted square for weak rock and soil-like

material strength (after Martin & Stacey 2018)

Often large variations in values of the strength of rock samples are attributed to sampling errors,

accuracy of sample preparation or testing procedures. However, high variance also results from

the intrinsic characteristics of the rock samples or negligence during sample selection and

grouping. In order to reduce variability due to geological effects and defects, the sampling

validation approach presented in Appendices I and II, which suggests a sample grouping to

reduce the variance and provides better lithotype characterisation, was used. This approach was

proposed to check the samples before carrying out the tests and considers:

• Samples were grouped by the level of weathering based on ISRM (1981), geological

characteristics (e.g. banding features), transverse isotropy direction (βangle) and bulk

density.

40

• For fresh rocks, P wave velocity (Vp) measurements were used to discard inadequate

samples (inclusions or fractured).

• Itabirites samples that did not show typical banding were not evaluated, thereby

heterogeneity was included and evaluated, and the scale effect related to the

compositional band thickness was qualitatively evaluated.

• Extreme outlier results were removed according to the Box plot statistical

methodology, in accordance with Whitaker et al. (2013) and Ross (1993). This

technique identifies the mild outlier’s values from the quartiles (Qt) determination,

based on the criteria described in Section 3.2.

These approaches aimed to evaluate all samples and identify possible drawbacks that may

influence results increasing dispersion.

Soil and rock materials, present different geomechanical characteristics and parameters, and

are determined by different methodologies, laboratory test and standards. For this reason, they

were presented separately.

3.3.2 Rock mechanics tests

All rock laboratory test results are presented in Appendices III, IV, and V. The rock tests

performed and used in this thesis are listed and described below. All tests were undertaken in

different anisotropic directions (βangle) as presented in Section 3.2.

Unconfined compressive strength test

According to Hoek (1977), among all strength properties of rocks, the UCS is the most used

strength parameter by industry and academia. Moreover, it is a test with high variance induced

by sample preparation, imperfections or associated intrinsic characteristics such as

heterogeneities, anisotropy, no visible discontinuities, and other defects or effects. To reduce

the dispersion caused by geological features and defects, sampling validation presented in

Section 3.3.1 was applied, which suggests sample grouping by similar geological characteristics

such as banding type, bulk density, and presence of typical BIF geological features.

At the Australian laboratory, UCS tests were performed in accordance with ASTM D 2938-95

(ASTM 2002). For all tests, samples (cylindrical plugs) were obtained from original 77.8 mm

diamond drill core subsampled in 20 mm diameter with lengths varying from 40 mm to 50 mm

and prepared accordingly to ASTM D4543-01 (ASTM 2001).

41

For better axial and lateral strains, simultaneous measurements with a minimum of one and

maximum of three diametric pairs of strain gauges were used. The tests were subjected to a

constant loading rate of 7 MPa/min or 9 MPa/min. The average strain was calculated by taking

the average of all strain readings. The modulus was calculated by the moving average of mean

strain.

The Brazilian laboratory tests were performed according to ASTM D 2938-95 (ASTM 2002). The

cylindrical samples were obtained from drilling rock blocks with 76 mm diameter with lengths

varying from 110 mm to 205 mm. All samples were prepared according to ASTM D4543-01

(ASTM 2001). Double dial indicator gauges obtained the axial and lateral measurements, and

the strain was used to calculate by taking the average in each direction. It is noted that dial

indicators have presented less efficiency on strain measurement compared with the strain

gauges used in Australian laboratories.

As different UCS sample sizes were used, results were normalised for D50m, using the Hoek

& Brown (1980) suggestion.

Triaxial Hoek cell test

Initially developed by Bishop & Henkel (1962), the triaxial test is one of the most common

methods for determination of rock strength for a long range of tension. The test consists of

application of hydrostatic and axial load in a cylindrical rock sample confined in the triaxial

Hoek–Franklin cell until the failure.

Triaxial compressive testing was carried out using Hoek–Franklin cells for samples with 20 mm

diameter according to ASTM D2664-04 (ASTM 2004) for the Australian laboratory and for

samples of 76 mm diameter for the Brazilian laboratory using the same standard. Applied

confining pressures were 1 MPa, 5 MPa, 15 MPa and 20 MPa.

Unconfined tensile strength test – Brazilian test

Developed to measure tensile strength for brittle rocks as originally proposed by Carneiro

(1947), this simple and cheap test is a well-known indirect test used to determine tensile

strength.

As suggested by Claesson & Bohloli (2001), tensile strength of rock is among the most important

parameters influencing rock deformability. To calculate the tensile strength from indirect tensile

(Brazilian) tests, one must know the principal tensile stress, in particular at the rock disc centre,

where it initiates. Anisotropy is an important feature that influences the Brazilian test strength

results (Tianshou et al. 2018).

42

These tests were carried out on 50 mm diameter drill core samples, prepared according to ASTM

D4543-01 (ASTM 2001) and were tested according to ASTM D3967-08 (ASTM 2008)

recommendations for the Brazilian and Australian laboratories.

Total porosity tests and visual total porosity

Total porosity is a fundamental physical property for the solution of various geological problems

and affects many other parameters such as wave velocity, permeability, and strength (Lin et al.

2015), and Bourbié & Coussy (1987) define porosity as the ratio between the pore volume and

the total volume of a known body.

There are qualitative and quantitative definitions of porosity. Quantitative measurements of

porosity are related to the pore and total volume of a rock sample; and it can be total or effective

porosity (also known as apparent porosity). Total porosity is related to the total volume of void

spaces, connected, or not connected to one another. Effective porosity, as defined by Gibb et

al. (1984), is related to interconnected void spaces. This thesis focuses on the description of

effective and total porosity through nitrogen gas injection and analysis of visual total porosity

(Øb) through optical microscopy and image analysis using petrographic thin sections.

For the laboratorial total porosity (Ø), according Lima & Costa (2016), the effective porosity and

permeability were measured on each sample as a function of effective pressure using an

automated poro-permeameter, Ultraporoperm500®, with a matrix cup following fundamentals

established by Boyle’s law. The instrument measures porosity in the range 0.1% to 40% (±0.1%)

using Boyle’ s law and Klinkenberg-corrected gas permeability in the range 0.001 mm to 5,000

mm (±2%), using the transient pulse-decay method (Han et al. 2011a and 2011b).

For each sample, porosity was measured six times and the mean value, and the standard

deviation reported. The poro-permeameter injects nitrogen gas into the sample and, by

application of Boyle’s Law, it measures the grain volume. The difference between total volume

and grain volume corresponds to the pore volume. Effective porosity was determined by the

ratio between the pore volume and total volume. Once the sample was dried, it was assumed

that the sample solid mass corresponds to the total mass previously measured.

For total porosity evaluated through gas porosimeter testing, which quantifies the

interconnected voids in a sample and the effective porosity, the results are significantly different

from the visual total porosity obtained from qualitative thin sections evaluation due to test

methods.

43

Bulk density test

Bulk density was defined as the mass (expressed in grams) of a given unit of volume (cm3) of a

substance at 4°C, also named specific gravity (Teixeira 2003). Bulk density (𝜌𝜌b) is the most direct

and easy-to-measure indicator of changes in compactness and level of alteration of a sample

Van-den Akker et al. (2005).

Bulk density is also an important physical property which has good correlation with the majority

of index properties of rock, mainly with P wave velocity and porosity. For this study, 𝜌𝜌b for rock

types was obtained through measurement of volume for each sample using a digital calliper,

and mass using a precise digital scale. This way 𝜌𝜌b is given by the ratio between the mass of the

sample and its total volume.

Bulk density was determined according to AS 1289.6.4.1 (AS 2016) for the Australian laboratory

and ABNT NBR 6508 (ABNT 1984) for Brazilian laboratories.

Additionally, using the Ultraporoperm 500 ® it was also possible to obtain the grain density given

by the ratio between total mass of the sample and grain volume. Once porosity and grain density

were obtained, bulk density was estimated, neglecting density of air, which fills samples’ voids.

Compressional and shear wave velocity experiments (bender elements test)

Primary sonic P wave velocity and secondary sonic S wave velocity experiments were performed

according to procedures established by ASTM D2845-08 (ASTM 2008) which calculates velocity

propagation of compressive waves (Vp) and shear waves (Vs), by measuring its time of transit

through the axial length of the sample. Once the length of the sample is known, as well as the

time of transit of elastic waves, velocities are obtained from the ratio of these parameters.

Wave velocity experiments were performed in Brazilian and Australian laboratories, according

to the same procedures established by ASTM D 28435-08 (ASTM 2008). The AutoLab500®

apparatus (Figure 3.4) was used, which allows performing experiments under controlled

conditions of confining pressures, pore pressure, temperature, and fluid saturation. For this

study, in order to keep the same standard as other laboratory tests, the pore pressure,

temperature and confining pressure were kept under atmospheric levels (Lima & Costa 2016).

To obtain the wave velocity, the sample’s length was measured using a precise digital calliper,

then it was placed in a confining chamber where a pair of piezoelectric transducers generate

compressional shear waves (perpendicular to each other) which travel through the rock sample.

The mechanical wave was then converted into an electric signal, transmitted to an amplifier and

oscilloscope, and finally translated into time of transit through a computer interface. Once the

44

time of transit of the elastic waves and length are known, velocities were obtained from the

ratio of these parameters (Bourbié and Coussy 1987).

Weathered rocks (partially to residual soils) have lower sonic coupling when compared to fresh

lithologies and attenuation of Vp increases with weathering (Barton, 2007). The best sonic

coupling necessary to have reliable Vp and Vs results were obtained for fresh lithotypes. For

partially weathered materials, the coupling was not effective in some of samples due to higher

total porosity.

11 Figure 3.4 The mechanical and electronic apparatus (AutoLab-500®) used for P and S

wave velocities and effective porosity tests (Lima & Costa 2016)

3.3.3 Soil mechanics tests

All soil laboratory test results are presented in Appendices VI, VII, and VIII. The soil tests

performed and used in this thesis are listed and described below. All tests were undertaken in

different anisotropic directions (βangle) as presented in Section 3.2.

In situ and laboratory bulk density test

For the Brazilian laboratory, bulk density (𝜌𝜌b) of soil experiments in the laboratory was carried

out in accordance with ABNT NBR 16867 2020 (ABNT 2020), and for the Australian laboratory,

according to AS 1289.5.1.1-2017 (AS 2017). Additionally, results presented as Vale´s in situ 𝜌𝜌b

experiments were obtained by sand filled in accordance with NBR 7185/2016 (ABNT 2016) which

is applied to soils of any granulation that can be excavated with hand tools, and the pore spaces

need to be small enough not to be penetrated by the sand used during the experiment. Tested

45

material needs to be sufficiently cohesive and strong to avoid deformation during the

experiment.

Natural moisture content test

Natural moisture content of samples was based on removing moisture from samples by

oven-drying until its weight remains constant. Moisture content (%) was calculated from the

sample weight before and after drying. Experiments performed in Australia followed Standards

Association of Australia AS 1289.2.1.1 - 2005 (AS 2005) and NBR 6508/84 (ABNT 1984) for the

Brazilian laboratories.

In situ soil permeability test

In situ soil permeability tests were carried out to determine permeability coefficients of residual

soils and completely weathered rocks. Such experiments were carried on according to ABGE n°

04/1996 (ABGE 1996).

Constant and falling head permeability test

Constant head permeability test (Ks) was used for permeable soils with ks > 10-4 cm/s and the

falling head test for less permeable soils (k < 10-4 cm/s). Tests were performed according to

AS 1289.6.7.1 2001 (AS 2001) for constant head and AS 1289 6.7.2. 2001 (AS 2001a) for falling

head for the Australian laboratories and NBR 14545- 2000 (ABNT 2000) for falling head and NBR

13292 – 1995 (ABNT 1995) for constant head for the Brazilian laboratories.

Particle size distribution and hydrometer tests

Particle size distribution (PSD) is widely used for the estimation of soil hydraulic properties such

as water retention curve (Arya & Paris 1981). Particle size analysis involves the measurement of

mass fractions of gravel, sand, silt and clay and results are basic information for soil classification

systems such as the Unified Soil Classification System (USCS) ASTM D-2847-00 (ASTM 2000).

The hydrometer method is another common procedure for measuring mass fractions of a very

fine soil sample. The weight percentages of sand, silt, and clay are calculated from the density

of an aqueous soil suspension measured by a hydrometer. As suggested by Ashworth et al.

(2001), there are many versions of the procedure, differing in the type of dispersing solution,

the volume of the suspension, the time of settling before taking hydrometer readings, or in the

method of correcting the raw readings.

46

PSD analysis was performed according to AS 1289.3.6.1 -2009 (AS 2009) and the hydrometer

method follows AS 1289.3.6.3.-2003 (AS 2003) for Australian laboratories. For tests performed

at the Brazilian laboratory NBR 7217/1987 (ABNT 1987) was followed.

Atterberg limits tests

The Atterberg limit tests are defined as the water contents corresponding to changes in the

behaviour of fine grain soils (silts and clays). The tests are conducted on remoulded material and

are basically for classification purposes and assessing potential engineering behaviour (Martin

& Stacey 2018).

The limits of consistency, namely plastic limit, and liquid limit are well known as soil Atterberg

limits. Plastic limit is the boundary between semi-solid and plastic state, and liquid limit

separates plastic state from liquid state (Campbell 2001).

Information obtained by Atterberg limits was initially developed to describe transported soils.

However, for residual soils and weathered materials such as weathered itabirites, a method

using Fourier analysis to address the influence of mixing materials in Atterberg limits was

summarised by Martin and Stacey 2018.

Tests were performed according to ASTM 4318 – 2010 (ASTM 2010) for the Australian laboratory

and NBR 7180/84 (ABNT 1984) for the Brazilian laboratory.

Single stage consolidated undrained triaxial compressing test

The triaxial test is considered the standard test for shear strength of soils and it is the most

common and versatile shear strength test. The soil sample is placed in a confining chamber and

axial stress is applied until the failure of the sample.

For rock-like soils types, single stage, isotropic consolidated undrained tests (CIU) were

undertaken to determine the undrained strength (undrained condition) for a known initial

effective stress and a sample assumed fully saturated while monitoring the pore pressure during

shear. This does not allow drainage at all phases of the test, and is the most common test used

for weak, porous, and granular (permeable) materials. Results are presented as total or effective

stress and associated porewater pressure are determined.

Australian and Brazilian laboratory triaxial tests were undertaken according to ASTM

D4767-957181 - 11 (ASTM 1995).

47

Single stage unsaturated direct shear test

The direct shear test was developed in order to describe shear strength of a soil, rock or from

discontinuities of an intact rock or soil (Bardet 1997).

The undrained condition direct shear test (UDS) is conducted when porewater is unable to drain

out of the soil, and the rate of loading is much quicker than the rate at which the porewater is

able to drain out of the soil. As a result, most of the external loading is taken by the porewater,

resulting in an increase in porewater pressure. The tendency of soil to change volume is

suppressed during undrained loading.

Undrained shear strength analyses are presented in this thesis applying the Fredlund et al.

(1978) shear strength equation. Effective shear strength parameters of the soil, along with the

initial matric suction and results from unconfined and confined compression tests are required

for the analysis. The procedure focuses on the use of conventional soil testing techniques to

provide a measure of the angle φb.

Changes in matric suction due to an applied total isotropic pressure can be computed from a

knowledge of the initial conditions of the soil, using a marching forward technique. This

procedure is detailed in Fredlund & Rahardjo (1993). This technique is not required if the matric

suction condition is measured.

During undrained axial compression, the matric suction can increase, decrease, or remain

constant depending upon the pore pressure parameter of the soil.

Tests were performed at the Australian and Brazilian laboratory according to AS 1289.6.2.2

– 1998 (AS 1988) using modified apparatus as presented in Figure 3.5.

48

12 Figure 3.5 The schematic modified suction controlled direct shear test apparatus for

testing the shear strength of unsaturated soils, from Gan et al. (1988)

Soil-water characteristic curve

The soil-water characteristic curve (SWCC), has become an important relationship to determine

when applying unsaturated soil mechanics in engineering practice (Fredlund and Houston,

2013). This research focused on the observation of SWCC for weathered BIF samples and

experiments were performed according to ASTM D6836 - 02 (ASTM 2003).

Dry and wet curves produced must reach the moisture content minimum of 0.5% and matric

suction of 1,000 kPa or 1,500 kPa.

3.3.4 Petrographic thin sections

Several petrographic thin sections were prepared and described to evaluate

microcharacteristics such as mineralogy, fabric and visual total porosity (Øb) providing useful

information able to compare with field work and laboratory samples’ macrocharacteristics such

as banding, anisotropy, bulk density, and others.

From thin sections evaluation it was possible to define the total porosity (primary and

secondary) and evaluate the arrangement of grains (fabric), interconnection of pores and

microporosity.

For petrographic description, Raith et al. (2012) guidelines were followed and the evaluations

were performed under a polarising petrographic microscope using cornoscopic and orthoscopic

https://en.wikipedia.org/wiki/Polarization_%28waves%29

https://en.wikipedia.org/wiki/Petrographic_microscope

49

illumination modes to evaluate opaque minerals (e.g. iron minerals) and translucid minerals

(e.g. quartz).

Petrographic thin sections descriptions were performed according to anisotropy direction

(perpendicular and parallel to banding) for a better evaluation of the grain and pore size, shape,

texture, distribution, and percentage for fresh and partially weathered BIFs. For completely

weathered lithotypes (WHE, WAI, WQI and WGI) results from Costa (2006) and Vale internal

reports from Zaparolli (2004) were used.

A total of 33 thin sections were prepared at The University of Western Australia’s School of Earth

Science and results were presented by Horta & Costa (2016). Another 13 thin sections from

Costa (2006) were revaluated and considered for this research.

3.4 SOFTWARE USED

Rocdata 5.0 (Rocscience 2021) was the database and processing software used to provide

analysis for strength parameters obtained through laboratorial experiments. This software is

based on Hoek & Brown (1998), Hoek (1990 and 1994) and Hoek et al. (2002) which perform

calculations generating the failure envelopes and strength parameters. Failure envelopes were

plotted in both shear-normal and principal stress space.

Slide 5.0 (Rocscience 2014) was the software used from limit equilibrium analyses and

determination of the safety factor on slope stability analyses performed in the case study

presented in Appendix II.

The experimental data for unsaturated parameters were obtained from the SoilFlux SoilVision

software (SoilVision System LTD. 2009).

50

51

CHAPTER 4. INTACT ROCK STRENGTH CHARACTERISTICS AND ELASTIC

STATIC PROPERTIES OF FRESH BRAZILIAN BANDED IRON FORMATIONS

This chapter presents the first unpublished manuscript.

ABSTRACT

In recent decades, a worldwide iron ore boom has fuelled a rapid increase in pit depths of

Brazilian iron ore mines, especially at mines located in the Iron Quadrangle. This has been due to

environmental restrictions and urban pressure on the lateral expansion of mines. For this reason,

the friable material has been mined and the presence of fresh banded iron formations on final

slopes has become more plentiful, and failure mechanisms and slope instability controlled by

discontinuities and hard rock behaviour have become more common. To support this new

challenge, in mines where instabilities were mainly composed of hard rock materials, rock

mechanics approaches became necessary. These approaches must be based on reliable

geological, structural, and geotechnical investigations supported by laboratory test databases.

In this context, an extensive geological and geotechnical field investigation and laboratory

testing have been conducted to determine the intact rock strength, elastic parameters and

geomechanical behaviour of brittle, hard and fresh banded iron formations (BIF), locally defined

and divided into four main types: fresh quartzitic, amphibolitic and dolomitic itabirites and hard

hematitite.

Laboratory tests undertaken to support the studies include uniaxial compressive strength (UCS),

P wave velocity and bulk density used to define strength and deformability parameters and

establish correlations with Young’s modulus and Poisson’s ratio. The interrelationships were

based on regression curves and the respective coefficients of determination was established to

support research statements. An extra challenge to determine these parameters and correlations

is created by the anisotropy observed for these rock type.

Geological and geomechanical field evaluations, and petrographic thin sections were used to

establish correlations between microtexture, mineralogical composition and macro

characteristics observed on mine slopes. The study focused on the itabirites which exhibit typical

compositional metamorphic banding (heterogeneity), hematitite with no clear heterogeneity

identified, and other geological features to define anisotropy ratio and other characteristics in

BIF.

52

Empirical correlations and adjusted regression curve equations between rock strength, elastic

parameters, and physical properties for different types of fresh BIF were identified. Such

equations can be used to estimate geotechnical parameters, establishing, and identifying

patterns of geomechanical behaviour of the BIF deposits.

Using such approaches, hard hematitite, quartzitic, and amphibolitic itabirites exhibited the

higher intact rock strengths with similar mean values, followed by the dolomitic itabirite with

lower-strength values. P wave velocity is higher for hard hematitite and relatively lower for

itabirites, in some instances following the bulk density reduction. However, it is influenced by the

heterogeneity defined by the main non-iron mineral.

For all BIF types, minor changes in bulk density induced by the heterogeneity, the mineralogical

composition and rock fabric are responsible for variations in the rock mass strengths and elastic

parameters. Anisotropic effects induced by typical banding are not significant for hard

hematitites (isotropic material) and represent a low influence for fresh quartzitic and

amphibolitic itabirites (low anisotropy degree), and a slightly higher anisotropic effect is

observed for fresh dolomitic itabirites (FDI) (low to medium anisotropy degree).

Additionally, qualitative evaluations of the scale effect related to the thickness of the

compositional metamorphic banding (heterogeneity) proved to be an important characteristic

that controls the intact rock strength and other parameters as P wave velocity, and bulk density.

Relative empirical correlations were obtained mainly for fresh quartzitic and amphibolitic

itabirites, between UCS, P wave velocity, bulk density, and elastic parameters providing a reliable

tool to intercorrelate different parameters, assist where there is a lack of laboratory test

information, and establishing the intact rock and elastic behaviour of fresh BIF.

4.1 INTRODUCTION

In recent decades, a booming world iron ore market has pressed local and global iron ore mines

to increase their production by developing innovative technologies, opening new mines, and

expanding existing mines, not only in width but mainly in depth, to meet this high demand.

In this period, iron ore mines located in the Iron Quadrangle, Brazil were especially affected by

environmental and social pressures to reduce expansion of the mining footprint. The expansion

was based on technological development and increase of depth in existing mines. With pit

depths going beyond 500 m, the former pit designs, based on soil like behaviour for friable

material, are no longer valid.

53

These iron ore mines are developed in banded iron formations (BIF) deposits, originally

described by Dorr (1969) as a high-grade ore, called hematite (most recently denominated as

hematitite) and a low-grade ore, called itabirite. The original mineralogical composition of non-

iron bands, consisting of dolomitic, amphibolitic and quartzitic layers are used to differentiate

itabirite types. In addition, those lithotypes are the hardest (fresh) BIF seen in Iron Quadrangle

iron ore mines.

For the deeper mines, the pit slopes are characterised by a full range of residual soil fresh rocks,

such that both soil and rock mechanics principles must be applied to represent these material

behaviours. Over the next decades, it is expected that the mines increase in depth, up to 700 m.

In this future scenario, hard rocks will be responsible for more than 70% of the final slope. In the

forthcoming mines, the complete understanding of intact rock, and rock mass strength,

discontinuity characteristics and strength, and in situ stress will form the basis required for any

pit slope design. Currently, there is a lack of geomechanical information of these materials, and

this is the main reason for studying these lithotypes.

Previous studies show, for these hard rocks, even belonging to the same rock type, a large

scattering in strength and elastic parameters. It is crucial to ensure a proper evaluation of intact

rock strength and elastic parameters, as well as geological and geotechnical characteristics to

understand the behaviour of different types and the influence of anisotropy and heterogeneity

on slope stability analyses and design. In this study, the focus was on intact rock characterisation

and thus an investigation of the discontinuities was not considered, even though it has been

recognised by the authors as an important aspect of the analyses of slope stability and failure

mechanisms.

Based on geological and geotechnical field investigation, thin sections evaluation, P wave

velocity propagation, unconfined compressive strength (UCS) and bulk density (𝜌𝜌b) tests, this

study proposes empirical correlations curves and equations between rock strength, static elastic

parameters (Young´s modulus and Poisson’s ratio) and petrophysical parameters for fresh BIF.

4.2 OBJECTIVES AND APPROACHES

Many studies have been undertaken to determine and establish relationships among

petrophysical and intact rock strength for several rock types. From these, only a few studies have

been focused on BIF. This research presents petrophysical, intact rock strength and elastic static

properties results for several Brazilian BIF, rock types occurring at the Iron Quadrangle, Brazil,

with the aim of establishing correlations by the regression technique (linear or nonlinear). The

degree of association between these variables was measured by the coefficient of

54

determination in an attempt to derive reliable empirical approaches for a better comprehension

of geomechanical and anisotropic behaviour, and to provide basic statistical correlations.

The aims of this research are to define, for the studied fresh BIF types, intact rock strength and

elastic characteristics, with values based on laboratory tests, geological and geomechanical field

characteristics and intrinsic correlations. The aims are summarised below:

• Evaluate and compare for different types of BIF present the intact rock strength, elastic

parameters, and petrophysical properties.

• Evaluate the influence of the anisotropy and heterogeneity, defined by the

metamorphic banding, for the different types of BIF.

• Establish a correlation between petrophysical properties, geological (macro and micro)

properties and intact rock strength and elastic parameters.

The research approach focuses on intact rock properties, assessing the effect of physical

heterogeneity and anisotropy to establish a practical estimate of properties and correlations

that could replace the lack of rock strength and elastic parameters. These correlations, to be

valuable, must consider mechanical and elastic rock properties as a function of petrophysical

characteristics such as mineralogical composition, thin section total porosity, bulk density, and

fabric of each lithotype.

Based on a considerable number of UCS tests, P wave velocity, bulk density, and correlated

geological/geotechnical (macro and micro) and rock fabric characteristics, it is expected for

these types of rocks to be able to determine strength and elastic parameters and will ultimately

help to optimise pit slope designs and promote a better understanding of potential failure

mechanisms and, in turn, lead to a reduced risk of slope failure by allowing an improvement of

operational productivity and safety of iron ore mines. These results are also useful for crusher

hardness definition, optimised design of blasting operations, digging equipment capability, and

other mining operation and planning applications.

To address this work, a PhD research project was undertaken by the lead author at the Australian

Centre for Geomechanics, School of Civil, Environmental and Mining Engineering, The University

of Western Australia, sponsored by Vale S.A. The main aim of the thesis was to investigate the

complete weathering profile characteristics, from fresh and hard to weak residual soil of BIF

from several Brazilian iron ore mine sites.

This chapter summarises the key geological and geotechnical characteristics of four fresh and

hard BIF types based on site investigations and petrographic thin sections description.

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/anisotropy

55

Subsequently, it presents the results of several laboratory tests, including 𝜌𝜌b, UCS, and P wave

velocity. Young’s modulus and Poisson’s ratio values were also defined. All results were

presented and discussed, and correlations were proposed between petrophysical

characteristics, strength, and elastic parameters, considering the effects of rock anisotropy. The

chapter concludes by presenting correlation plots and discussions of fresh BIF geomechanical

behaviour.

4.3 GEOLOGICAL AND GEOTECHNICAL SETTINGS

The studied mines are part of Vale’s South Ferrous Division and comprise fifteen mines located

on the western part of the Iron Quadrangle, in the southern border of the São Francisco Craton,

Minas Gerais, Brazil. The mines are placed on the Moeda and Don Bosco Synclines and Curral

Homocline ranges: Águas Claras (MAC), Mutuca (MUT), Mar Azul (MAZ), Capão Xavier (CPX),

Tamanduá (TAM), Capitão do Mato (CMT), Abóboras (ABO), Galinheiro (GAL), Sapecado (SAP),

Pico (PIC), Córrego do Feijão (CFJ), Jangada (JGD), João Pereira (JPE), Alto Bandeira (BAN) and

Fábrica (FAB). The locations of these fifteen mines are shown in Figure 4.1.

13 Figure 4.1 Mine locations (red circles) and Iron Quadrangle geological settings (modified

from Morgan et al. 2013)

4.3.1 Regional geological settings

The Iron Quadrangle is delineated by a roughly quadrangular arrangement, with

Paleoproterozoic BIF of the Minas Supergroup, as proposed by Dorr (1969), originally called

itabirites, as defined by Dorr (1969). These iron deposits are classified as superior type,

according to Gross (1980). It is composed of hundreds of metres of iron ore rich metamorphic

heterogeneous rocks belonging to the Itabira Group/Cauê Formation. The Minas Supergroup

56

comprises, from bottom to top, Caraça, Itabira, Piracicaba and Sabará groups and a sequence of

psammitic pelitic rocks, all being superimposed by the Itacolomi Group (Dorr 1969).

Below that sequence is the Archean greenstone terrains of the Rio das Velhas Supergroup and

domes of Archean and Proterozoic crystalline rocks (Machado & Carneiro 1992; Machado et al.

1989; Noce 1995).

According to Chemale Jr et al. (1994), the regional structure is the result of two main

deformational superimposed events. The first produced the nucleation of regional synclines in

the uplift of the gneissic domes during the Transamazonian Orogenesis (2.1–2 Gyr), and the

second is related to an east–west verging thrust fault belt of Pan African/Brazilian age


structures and was responsible for the east–west deformational gradient.

Hertz (1978) described an eastward increase in the metamorphic grade and followed the

deformational gradient from greenschist to lower amphibolite’s facies.

The Cauê Formation (Itabira Group) hosting the BIF is a 350 m thick marine chemical sequence,

dated 2.4 ± 0.19 Gyr (Babinski et al. 1995).

The Iron Quadrangle BIF are Paleoproterozoic, metamorphic and heterogeneous banded rocks,

presenting a millimetre to centimetre rhythmic alternation banding of iron minerals (hematite,

martite and magnetite), and non-iron minerals (quartz, dolomite, and amphibolite). Dorr (1969)

defined, for this type of iron deposit, two main lithologies: hematite, recently redescribed by

Selmi et al. (2009) as hematitite, the high-grade ore (Fe ≥ 62%); and the low-grade ore, the

itabirite (30% < Fe < 62%), with three compositional types or lithotypes, according to the

presence and relative abundance of gangue minerals: quartzitic, dolomitic and amphibolitic (all

defined as proto-ore). Tectonic, metamorphic, and weathering processes have changed it in

different ways, resulting in multiple sets of iron ore lithotypes.

The studied mines are in the western low metamorphic grade (greenschist) and low strain

domain, nucleated in the Transamazonian event. The main trend of the synclines is north–south,

but the structure has also been deformed around the Bação dome. In the south, it is

interconnected with the Dom Bosco Syncline and is partially truncated by the Engenho Fault

(FAB, JPE and BAN). To the north, the Moeda Syncline is continuous with the Serra do Curral

(Rosière et al. 1993) (Figure 4.1).

The Moeda Syncline (MUT, MAZ, CPX, TAM, CMT, ABO, GAL, PIC, and SAP mines) has been

partially affected by the younger Brazilian tectonic cycle and mainly in the eastern limb with the

57

local development of ductile–brittle to brittle shear zones that cut all lithologies or are

subparallel to the bedding planes. Several strike–slip faults cut across the structure dividing it

into several segments.

The Serra do Curral (MAC, JGD and CFJ) represents the overturned southeastern limb of a

truncated northwest verging syncline–anticline couple highly strained and rotated by the right

lateral movement of the northeast–southwest trending oblique ramp of thrust fault as

supported by Chemale Jr et al. (1994). In this segment, the northwestward inverted limb of a

syncline is truncated near to the contact of the Minas Supergroup in the underlying Rio das

Velhas Supergroup by shear zones related to the thrust.

4.3.2 BIF geological and geotechnical settings

Compositional metamorphic banding is the most typical itabirite characteristic and is considered

to define a heterogeneity. This variation could have been controlled and disturbed by the

original sedimentary bedding, tectonic setting, metamorphic grade, hypogene or supergene

processes and weathering, as the superposition of these processes causes partial or total

mineralogical and textural changes. Studies of BIF conducted by Rosière et al. (2001) for the

western side of the Iron Quadrangle indicate a very low internal deformation by plastic flow of

the minerals with the ductile deformation accommodated mostly by buckling and flexural gliding

with local development of a spaced cleavage. A continuous schistosity and an anastomosing

foliation occur, related to discontinuously developed shear zones. Brecciated zones are usually

related to sites of local water pressure build-up, during the percolation of hydrothermal fluids.

Not considering the controversy about the supergene (groundwater leaching) or hypogene

(hydrothermal water leaching) genesis for these large, rich, and weak Iron Quadrangle ore

deposits, this study is focused on the three main hard and fresh itabirites proto-ore, and hard

hematitite, produced either tectonically or by a hydrothermal event. This variation is

conditioned by the original composition of sediments, the intensity of deformation, and the

degree of metamorphism and hydrothermal alteration imposed. Tectonic structures produced

a large variety of textural features due to the development of discontinuity planes, and

microstructures resulting from different tectonic types of itabirite.

Hematitite and itabirites are locally subdivided by using rock strength characteristics (hardness),

rated in technological crusher laboratory tests, used to simulate the industrial process for better

control of mining operations, which are also used for geotechnical purposes as a rock strength

index. This test consists of crushing a sample to less than 31.5 mm and passing it through the

58

6.35 mm sieve, resulting in three main different lithotypes: hard (more than 50% above

6.35 mm); medium (50% to 25% above 6.35 mm) and weak (less than 25% above 6.35 mm).

Vale’s crusher classification has been used as a guide to identify weathering level and rock

strength index (hardness), where typology is defined as hard, representing fresh to slightly

weathered material; medium, representing moderately to highly weathered material; and weak,

representing completely weathered to residual soil. It could also be associated with the field

intact rock strength as hard, representing extremely hard to medium-hard material; medium, as

medium-hard to medium-soft; and weak, as medium-soft to extremely soft material, according

to ISRM (1981) tables.

Typically, BIF present a poor mineralogical variety: hematite, martite, magnetite, specularite,

goethite and ochreous goethite are, respectively, the most important iron minerals. Quartz, iron

dolomites (ankerite and siderite), gibbsite and kaolinite (weathering minerals), are the main

gangue minerals; and talc, chlorite and pyrolusite are the main accessory minerals. Several

studies described the mineralogical and fabric correlation with geological association for Iron

Quadrangle mines, including Rosière et al. (1993, 1996 and 2001), Lagoeiro (1998), and Pires

(1995).

The main geological and geotechnical characteristics of the lithotypes studied are outlined as

follows:

Hard hematitite

The genesis of massive hard hematitite (HHE) (Figure 4.2A) is a subject of controversy; in local

deposits it is postulated that hard-massive ore bodies (Figure 4.2B) result from hypogene or

tectonically iron remobilisation on fold axes or intersection plans and contact metamorphism

from intrusive dikes, or an initial concentration of iron rich sedimentary bedding. Minor bodies

are correlated with discrete shear zones and are strongly anisotropic due to millimetric tectonic

foliation defined by the presence of specularite. Each domain presents a typical fabric defined

by Varajão et al. (2002) as the following typologies:

• Massive, without banding or foliation, constituted by granoblastic hematite/martite.

• Banded, presenting centimetric bands, constituted by tabular hematite/martite with

casual specularite.

• Foliated, presenting a thin foliation, constituted by tabular hematite/martite with a

wide presence of specularite.

59

Hard hematitite is recognised as exhibiting the highest intact rock strength values within the

group of BIF rocks. It is characterised as a dark grey metallic homogeneous rock, and is the

highest-grade ore formed. It is formed by granular martite and microplates of hematite as the

main iron mineral, followed by magnetite, quartz, and goethite. A typical dark metallic colour is

observed for massive types, while more opaque bands are observed for banded types in which

a higher percentage of porous material is concentrated.

As suggested by Varajão et al. (2002), at Capitão do Mato Mine, the total lenses of porosity can

reach 11% for slightly weathered HHE, whereas for fresh HHE this could be less than 2.5%.

Crystal size varies from 10 μm to 30 μm for hematite and martite granular crystals and is equal

to 1 μm for microplates of hematite. Also, it is suggested that primary micropores vary from 1 Å

to 1 μm. The most important secondary porosity is associated with martite crystals, varying from

1 Å to 5 μm. The average density is 4.4 t/m3 for iron content higher than 64%, while the natural

moisture content presents an average of 1% (Santos 2007).

Figure 4.2A shows a microphotograph of granular martite (large light grey crystals) and tabular

hematite (small light grey crystals), and large pores (in black) and, in Figure 4.2B, an outcrop of

fractured, massive HHE.

(A) (B)

14 Figure 4.2 A (left), HHE at microscope view with granular crystals of hematite (light grey)

and smaller crystals of hematite microplates (light grey) (Horta & Costa 2016).

B (right), outcrop of fractured HHE at Capitão do Mato Mine


Fresh quartzitic itabirite (FQI) is the typical BIF and most common itabirite, as originally

described. The heterogeneity is defined by the alternation of a non-iron band that was totally

60

metamorphosed to quartz (originally chert), and the iron band composed of hematite, martite

and martitised magnetite.

Banding can be folded and the microtexture of the quartz layers is granoblastic to

lepidogranoblastic. According to Rosière (2005), some crystals can be euhedral due to the

metamorphic level, with sizes ranging from 10 µm to 120 µm. Hematite layers present tabular

and granular shapes, with crystal sizes between 6 µm to 80 µm (Figure 4.3A). There is a very low

visual total porosity (less than 5%) concentrated at fractures (secondary porosity), with a

moisture content less than 10% and a 𝜌𝜌b = 3.1 (±0.29) t/m3, as presented by Santos (2007).

These rocks exhibit a high intact rock strength, especially for fresh materials (W1), and are

considered to be the most important rock bridges at the toe of the proposed final walls. Some

initial weathering (W2) is identified, principally on fractures or banding contacts, as shown in

Figure 4.3B.

(A) (B)

15 Figure 4.3 A (left), shows microphotography of FQI presenting typical quartz banding

(write crystals), granular to tabular hematite (light grey) and porous (black)

(Horta & Costa 2016). B (right), typical outcrop presenting fractures in

Tamanduá Mine


The fresh amphibolite itabirite (FAI) presents a heterogeneity defined by layers of hematite,

martite and goethite alternating with quartz, goethite, and rare amphiboles (grunerite,

tremolite, and actinolite) bands (Figure 4.4A). General band textures are lepidogranoblastic to

granoblastic and on average the crystal size is 30 µm. The original mineralogy with amphibole is

preserved only at depth where a typical brownish-yellow colour is present (Rosière 2005).

Quartz

Hematite band

500 μm

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It exhibits a very low effective porosity (less than 5%), while moisture content is 10% and 𝜌𝜌b =

2.8 (±0.46) t/m3, as presented by Santos (2007).

Due to weathering, the amphibolic minerals easily change to fibrous goethite and, due to the

deep weathering profile, it is usually difficult to obtain ‘fresh FAI’ (with amphibole) at the surface

or shallow depths that has not been influenced by some mineral degradation. For this reason,

in this chapter, the so-called ’fresh FAI’ will also consider the slightly weathered FAI (W2), even

in the absence of amphibolitic minerals and rare W1 samples.

The intact rock strength of the FAI is equivalent to other itabirites and is still strong when

classified as W2 where some weathering and discoloration (dark yellow or brown) can be

observed in fractures and banding layers (Figure 4.4B).

(A) (B)

16 Figure 4.4 A (left), shows microphotography of a FAI highlighting the presence of fibrous

goethite an amphibolite acicular old crystal (dark fibre minerals) immerse on

quartz bands (Horta & Costa 2016). B (right), shows a typical slope of folded

and fractured FAI at Jangada Mine


As presented by Rosière (2005), the heterogeneity is defined by the millimetric to centimetric

pink or white iron dolomite and lower percentages of iron carbonates and quartz bands. These

layers occur interbedded with dark grey iron bands comprised of tabular hematite, martite, and

martitised magnetite bands. The most important accessory minerals are sericite, talc, and

chlorite. The non-iron minerals vary in size from 2 µm to 15 µm, and the iron bands present

crystal size varying from 5 µm to 20 µm (Figure 4.5A).

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The fresh dolomitic itabirite (FDI) presents a very low effective porosity (less than 5%), the

moisture content is 5% concentrated at a fracture porosity (secondary porosity) and the 𝜌𝜌b =

3.2 (±0.36) t/m3, as presented by Santos (2007).

It is also a strong rock and occurs as very fresh material (Figure 4.5B), typically W1. Slightly

weathered materials are defined by chlorite or talc levels, presenting discontinuities for more

weathered levels.

(A) (B)

17 Figure 4.5 A (left), shows a microphotography of FDI highlighting the typical banding of

iron dolomite and quartz (blue light colour), and ferroan dolomite and tabular

hematite bands (dark grey) (Horta & Costa 2016). B (right), typical hand

sample of folded FDI

4.3.3 Intact rock strength parameters, anisotropy and petrophysical properties

correlations

Unconfined compressive strength

Due to the popularity of the Hoek & Brown criterion, the UCS test became the rock parameter

most used to provide the first evaluations in slope stability. However, in Vale operations, due to

the predominance of weak materials with soil characteristics, historically Mohr–Coulomb

parameters have been used to characterise existing materials and for slope stability evaluations.

For this reason, the UCS testing was disregarded by Vale for many years, with adapted strength

field characterisation tables and crusher tests associated with a limited number of UCS testing

being used to support slope stability evaluations.

Barton & Quadros (2015) argued that the Hoek & Brown failure criterion was initially proposed

for the determination of intact rock strength and rock mass strength in isotropic conditions.

Nevertheless, it has been proposed to determine the strength of anisotropic rocks. To use the

63

criterion in this way, careful parameter selection is necessary, especially for the UCS (σci) and the

material constants (mi) and (s). Moreover, determining the minimum and maximum intact rock

strengths due to rock anisotropy is very important when selecting the characteristic values of

the intact rock.

It is recognised that slope stability in hard and fresh rocks is controlled mostly by kinematic

freedom and the shear strength of discontinuities. However, intact rock strength parameters

are fundamental to characterise and evaluate slope stability. Thus, UCS is ideally obtained from

laboratory tests, but can also be obtained by correlation index tests (e.g. point load, P wave

velocity propagation or Schmidt tests). In some cases, bulk density or other physical parameters

can be used as a simple measurement to estimate the UCS (Marques et al. 2010).

Petrophysical properties and correlations

It is recognised that other physical characteristics such as porosity, bulk density, mineralogical

composition, crystal size, and fabric are directly responsible for variations in the strength of

intact rock. However, few studies of the interrelationship between intact rock strength and

petrophysical parameters have been established worldwide for BIF.

In this regard, conclusions from Thomson (1963) for Australian iron ores have suggested a

theoretical hematite–quartz curve used to determine the 𝜌𝜌b and appropriate iron content

approximation. Correlations presented by Aylmer et al. (1978) have associated 𝜌𝜌b, iron grade

and porosity for Mount Tom Price iron ore. Studies by Box & Reid (1976) for the iron ore

formation at Cockatoo Island attempted to correlate true specific gravity with iron content and

the influence of porosity. Finally, studies by Nel (2007), developed for Sishen iron deposits in

South Africa, developed a direct correlation between porosity and dry bulk density, providing a

reliable calculation index.

Static and dynamic elastic properties and correlations

Classical rock mechanics provides various techniques for determining the elastic parameters,

the most widely employed being stress-strain curves obtained from destructive compressive and

tensile rock tests. Most recently, a start has been made on using non-destructive tests based on

acoustic wave velocity propagation to obtain such parameters.

Timoshenko (1983) defines strain as a change in geometry, orientation, volume, and position

induced by either a load or unload of stress in a certain period of time (Dantas 2018). For each

rock type (homogeneous or heterogeneous) and load (constant or variable) applied it is possible

to write the constitutive stress-strain laws resulting in various deformability parameters. Such

64

parameters are derived directly in situ or through laboratory testing, and indirectly from rock

mass classification using correlations. Laboratory tests are categorised as static, when samples

are submitted to loading or unloading that induces different equilibrium stresses, and dynamic

elastic parameters are obtained in response to a cyclic, rapid energy source such as an acoustic

wave emission.

Using specific laboratory testing, Kearey et al. (2009) showed that the elastic modulus can be

obtained indirectly by calculating acoustic emission velocity (mechanical seismic waves) within

a rock specimen resulting from particle vibrations triggered by an energy source. It differs from

the elastic modulus and material density and can be divided into two types: compressive waves

or P waves, which move through the propagation direction, and shearing waves or S waves,

which move perpendicularly to the propagation path.

For isotropic materials, the seismic pulse is perpendicular to the wavefront, and wave

propagation velocity is defined by the rate at which the acoustic pulse travels through the

material.

Kearey et al. (2009) also explain that wave velocity is a function of the mineralogical

composition, texture, porosity, and the presence of fluids. These factors severely affect the

P wave, whereas the S wave did not propagate through discontinuities or empty spaces. Several

characteristics of P and S wave velocity for common lithotypes are listed below:

• Felsic igneous rocks have lower speeds relative to mafic rocks.

• The velocity reduces as the number of discontinuities (fractures) increases.

• Porous sedimentary rocks present velocity dispersion associated with the porosity and

density correlation, with higher velocities for high density and low porosity, and lower

velocities for low density and high porosity. It also depends on the cementation grade,

consolidation, and other factors.

• In contrast to those with smaller crystals, lithotypes with the same mineral

composition with larger crystals present higher velocities.

• Higher wave velocity occurs parallel to the anisotropy direction and lower velocity is

observed in the direction perpendicular to anisotropy in metamorphic rocks. These

changes are associated with mineral and porous alignment, and the set of

discontinuities. Lower velocities are controlled by the weakness planes along banding

and schistosity. Otherwise, higher velocities are associated with the strongest crystal

contacts along the anisotropy direction.

65

• For the same reason, the P wave velocity anisotropy decreases with stress and

increases with temperature. The latter effect may be due to the widening of pores and

microcracks during the expansion of the sample due to thermal stress.

As presented in Deere & Miller (1966), it must be assumed that elastic constants determined

using acoustic wave emission, which defines the elastic behaviour, are dependent on a broad

range of variables such as rock type, mineralogical composition, rock texture and structure, grain

size and shape, density, porosity, degree of anisotropy, porewater, confining pressure, moisture

content, stress amplitude, rate and duration of loading, temperature, weathering and alteration

zones, and joint properties and defining the most important variable could be a difficult task.

The use of acoustic wave propagation velocity to determine intact rock strength have been used

for several typologies by a number of researchers including Inoue & Ohome (1981) for weak

rocks. Gaviglio (1989) focused on rock density, and Kaharaman (2001) and Yasar & Endogan

(2004) applied it to carbonate rocks. Based on its close relation to the intact rock properties,

structure, and texture, these methods have been used by major global mining companies such

as Vale S.A. Later studies and subsequent standard rock strength tests have confirmed the

validity of static and dynamic comparisons using non-destructive sonic wave measures

techniques. Furthermore, the low cost, repeatability, and speed of acoustic emission tests are

attractive characteristics.

According to Oyler et al. (2010), for coal mines, the results of regression analyses of the

relationships between P wave velocity and UCS tests are meaningful and are routinely used to

determine the σci of the coal seam.

Acoustic emission can be coupled with laboratory static tests, facilitating constant monitoring in

different directions during the stress level increase. As stated in White (1983), it is possible to

calculate the dynamic elastic constants such as Young’s modulus (Edyn), Poisson’s ratio (νdyn),

shearing modulus (Gdyn) and incompressibility modulus (Kdyn) based on compressional wave

velocity (VP), shear wave velocity (Vs) and the sample’s bulk density (𝜌𝜌b) by the application of the

elasticity theory. Equations 4.1, 4.2 and 4.3 show the relationship between dynamic elastic

constants, 𝜌𝜌b, and elastic wave velocities modified from White (1983):

Dynamic Poisson’s ratio correlations:

𝑣𝑣𝑑𝑑𝑑𝑑𝑚𝑚 =(𝑉𝑉𝑝𝑝

𝑉𝑉𝑠𝑠� )2−2

2��𝑉𝑉𝑝𝑝

𝑉𝑉𝑠𝑠� �2−1�

(4.1)

66

where:

νdyn = Poisson’s ratio.

VP = compressional wave velocity.

Vs = shear wave velocity.

Dynamic Young’s modulus correlations:

𝐸𝐸𝑑𝑑𝑑𝑑𝑚𝑚 = 𝜌𝜌𝑉𝑉𝑠𝑠2(3𝑉𝑉𝑝𝑝2−4𝑉𝑉𝑠𝑠2)(𝑉𝑉𝑝𝑝2−𝑉𝑉𝑠𝑠2)

(4.2)

where:

Edyn = Young’s modulus.

Compressive wave velocity (Vp) correlations:

Vp2 = 𝐸𝐸𝑑𝑑𝑑𝑑𝑑𝑑(1−υ)ρb(1+υ)(1−2υ)

(4.3)

where:

𝜌𝜌b = bulk density.

ν = Poisson’s ratio.

In this chapter, just one basic mechanism of energy transmission by elastic waves was studied:

compressional waves (P), which propagate through the plug’s axial direction in compression. For

this reason, just Equation 4.3 was applied. A complete evaluation is presented in Chapter 5.

Studies, correlating dynamic elastic proprieties with petrophysical and static strength

parameters on BIF, have been developed by Wassermann et al. (2009) for an oolithic iron mine

in Lorraine, France. The study evaluated damage processes on UCS tests using P wave velocity

and found that mechanical behaviour deduced from strain measurements is dilatant for some

samples and non-dilatant for the others, even when elastic properties indicate damage

processes for all samples. Silva (2014), studying Archean schists from Cuiabá mine, Brazil had

measured permeability, total porosity and elastodynamic properties that show the rigidity of

these rocks and were used to define lithostructural constraints. Regarding iron-oxide

mineralisation in the Blötberget mine, Sweden, Maries et al. (2017) obtained several physical

properties from geophysical logging and laboratory measurements to assess the ore-bearing

rocks by increasing density and dynamic elastic modules measured from the full-waveform

sonic.

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For BIF in the Iron Quadrangle, the studies by Pereira (2017) using acoustic emission and other

geophysical techniques on well logging, defined rock strength parameters and discontinuity

characteristics to establish RQD and elastic in situ modules. Dantas (2018), using part of this

research database, evaluated, and correlated dynamic and static proprieties in different

anisotropy directions using statistical approaches. The author has shown a well-adjusted

regressive model for fresh itabirites. In general, the levels of the statistical parameters are

improved for initial anisotropy directions with poorly adjusted regression curves after the

elimination of spurious data.

Bulk density is the most direct and easy-to-measure indicator of changes in compactness and

level of alteration of a sample (Van-den Akker & Soane 2005). It is an essential physical property,

which has good correlation with the majority of index properties of rock, mainly with P wave

porosity. Studies by Drake et al. (1963) and Gardner et al. (1974) presented an empirical

correlation equation for Vp and ρb for limestone and granitic rocks.

BIF heterogeneity and anisotropy

An important geological BIF characteristic easily observed and measured on several scales

varying from a few millimetres to several centimetres is the heterogeneity defined by typical

metamorphic banding. This heterogeneity can result in a type of anisotropy described as

transverse isotropy, in which layers have approximately the same rock properties along the

plane of the band and different rock properties across the band. As suggested by Hudson &

Harrison (2000), geomechanically the important rock properties that can be affected by

anisotropy are deformability modulus, strength, brittleness, permeability, and discontinuity

frequency.

Singh et al. (1989) and Ramamurthy (1993) pointed out that metamorphic rocks are mostly

anisotropic due to the effect of schistosity, cleavage and microcracking. For BIF, anisotropy is

defined by its mineralogical composition (metamorphic banding), mineralogical orientation

(alignment of minerals at the same orientation), the porosity induced by the original

composition or weathering, and different bulk densities in each layer. Additionally, secondary

anisotropy directions, defined by schistosity and foliation, are locally found mainly in hematitites

and in itabirites with some discreet shear zones and fold axes.

The compositional metamorphic banding (heterogeneity) is defined by the alternation of iron

and non-iron layers (typical itabiritic metamorphic banding). It is the most important itabirite

feature that dictates the formation of the intact rock strength, and is controlled by iron or non-

iron layer thickness, porosity, and mineral composition. A simple example is observed in zones

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where the banding contains thicker layers of iron minerals, the itabirite will be denser, with

lower porosity and relative higher strength values.

Ultimately, BIF heterogeneity is responsible for determining the different lithotypes, ranging

from country (waste) rocks such as dolomite, quartzite, and amphibolite (very few or non-iron

bands); they range to high-grade ore (very few or no waste bands) as rich itabirite or hematitite.

This evaluation accords with results presented by Dalstra et al. (2003), that compare the

Hamersley Province iron ore with many global deposits and correlate the proto-ore assemblages

with intermediate iron grades between ore and the host BIF.

For Brazilian BIF, studies by Ribeiro et al. (2014), Santos et al. (2005), and Santos (2007) focused

on evaluating the association between bulk density and iron content for Vale’s iron mines and

concluded that there is a linear positive correlation between the total iron content and the bulk

density. Also, Appendix I presented a first attempt to establish a linear positive correlation

between bulk density, iron content and UCS tests for fresh and hard BIF.

Saroglou & Tsiambaos (2007) found that the engineering behaviour of rock masses is strongly

dependent on the anisotropy present at different scales. In the microscale (intact rock), it

depends on the alignment of the rock crystals (i.e. inherent anisotropy). In the macroscale (rock

masses), with anisotropic structure, it is characterised by distinct bedding or schistosity planes

(structural anisotropy). The inherent strength anisotropy of intact rock, which is the focus of this

chapter, can be determined by the variation of UCS (σci) and characterised by the strength

anisotropy degree (RC), as proposed by Singh et al. (1989), or anisotropic index (IVp) as proposed

by Saragou & Tsiambaros (2007) due to the existence of bedding, foliation and schistosity planes

in both intact rock observed in the macroscale and microscale.

4.4 METHODOLOGY

To evaluate the geological and engineering characteristics of fresh and hard BIF and to achieve

the proposed goals, the research methodology adopted was divided into three phases:

• For the first phase, a field investigation was undertaken, and several samples collected

from outcrops and from core drills for all BIF lithotypes. Geological and geotechnical

information were logged, based on ISRM (1981) suggestions, and physical

characteristics were described (e.g. anisotropy and, banding) to support that analysis

of laboratory test results. Outcrops and samples were photographed for additional

visual information such as anisotropy, banding, type of BIF and discontinuities.

69

• The second phase focused on the description of petrographic thin sections, to evaluate

the rock mineralogy, fabric, total porosity, and to provide information on a microscopic

scale to be compared with field macro characteristics described in the first phase.

• The third phase included the execution and evaluation of all laboratory tests and

results used and complied to determine the intact rock properties together with the

correlations with geological characteristics and geotechnical parameters of each

lithotype obtained during the previous phases. The correlation between anisotropy,

UCS, 𝜌𝜌b, P wave velocity, Poisson’s ratio, and Young’s modulus were established.

Tests results dispersion evaluation

When determining σci, from intact rock strength, it is expected that minor microfractures or

other discontinuities will be present in the tested sample. The difficulty of preparing samples

where there are no visible fractures depends on with the inherent anisotropy of the intact rock,

where the alignment of rock-forming crystals cannot be excluded when determining the intact

rock strength.

Recognised as the most common and widely used strength test for rock mechanics, the UCS test

is also known for its high variances mainly related to sample preparation, imperfections or

geological aspects associated with intrinsic characteristics such as heterogeneities, mineral

veins, anisotropy, microfractures, and other defects or effects. However, high variable results

can also be derived from intrinsic sample characteristics and/or negligence/error during sample

selection and grouping. A sampling validation approach based on grouping by geological

characteristics such as banding type, bulk density, and the presence of typical geological

features, is presented in the Appendices I and II.

The features used to group the samples followed approaches presented in Appendix I as follows:

• Samples were grouped by weathering level W1 and W2 (ISRM 1981a).

• P wave velocity (Vp) measures were used to discard inadequate samples with

inclusions or microfracture.

• As BIF presents a typical 𝜌𝜌b variation, a range was established to separate fresh

itabirites (from 2.7 t/m3 to 3.8 t/m3) from non-iron or waste rock (below 2.7 t/m3) and

hematitite (above 3.8 t/m3), as described by Ribeiro et al. (2014).

• Each sample was photographed before and after testing to identify the main geological

characteristics, failure geometry and modes.

70

• UCS test results were size normalised using the empirical scale effect relationship for

intact strength suggested by Hoek & Brown (1980).

• As the UCS result could present an anisotropy dependency, samples were tested at

several different orientations (βangle) to the plane of anisotropy.

• Geological features such as intense folding, the presence of specularite, matrix

material with a different degree of weathering, and the presence of quartz and calcite

veins and other materials which did not represent the typical itabiritic heterogeneity

were discarded.

• To maintain the same compositional metamorphic banding scale and to avoid the

scaling effects, only samples with centimetre thick layers (i.e. typical banding), were

inspected and evaluated.

Extreme outlier results were removed according to the box plot statistical methodology

(Whitaker et al. 2013). This technique identifies the mild outlier’s values from the quartiles (Qt)

determination, based on Equation 4.4.


QtLower = 1Qt - 1.5(3Qt - 1Qt) (4.4)


Qtupper = 3Qt + 1.5(3Qt - 1Qt) (4.5)




3Qt = × + 1.5.IQR (4.5)


and is defined by the Equation 4.6.

1Qt = × - 1.5.IQR (4.6)

Interquartile ranges (IQR) measure how spread out from a central data the values are. These


IQR = 3Qt - 1Qt (4.7)

71

Anisotropy ratio (degree of strength anisotropy) and heterogeneity evaluation

The anisotropy of a rock is the property which allows it to present, for the same physical

property, different results in different directions and the degree or ratio of anisotropy is used to

quantify how far the rock is from being isotropic. For geotechnical rock characterisation, the

main physical property used to determine the anisotropy is the rock strength. The term

‘heterogeneity’ is used for rock when it is composed of layers or bands (scale related) that are

different from one another that could, or may not present for the same physical properties,

different results in different directions.

For BIF, which presents metamorphic heterogeneity in a determinate scale of observation, it is

necessary to determine the ratio of anisotropy to evaluate the anisotropy degree. Estimating

the variation of intact rock strength due to the anisotropy effect allows the differentiation of

spurious test results induced by rock intrinsic characteristics from the anisotropy effects which

can lead to misleading results and increasing variance. In the present study, the variation of UCS

due to anisotropy was considered to determine the degree of strength anisotropy (RC), as firstly

proposed by Singh et al. (1989) with defines the RC as the variation in compressive strength

(measured in uniaxial and triaxial tests) depending on the angle between the direction of the

load applied to the tested samples and the direction of the anisotropy.

In order to evaluate the influence of metamorphic banding and define anisotropy in intact rock

strength for all tests, the anisotropy β (beta angle) as described by Jaeger (1960) was considered,

by testing different angles between banding and the loading direction, varying from 0° to 90°.

However, due to the reduced number of valid results for some typologies, results were grouped

in three main βangles ranges: for loading parallel to banding (β0°), all tests results from 0° < β ≤ 30°

were considered; for direction of loading oblique to banding (β45°), results from 30° < β ≤ 60°

were considered; and for loading perpendicular to banding (β90°), results were grouped from

60° < β ≤ 90°.

The RC was defined as presented in Singh et al. (1989) as the ratio between the maximum

compressive strength, normally obtained at β = 90°, divided by the minimum value obtained. It

is defined in Equation 4.6 as σc90°, the compressive strength value for βangle perpendicular to the

planes of anisotropy and σcmin, the lowest compressive strength value obtained. The range and

classification of the RC established by Equation 4.8 is presented in Figure 4.6, as well as the

diagram representing the βangle definition by McLamore & Gray (1967).

𝑅𝑅𝑅𝑅 = 𝜎𝜎𝜎𝜎 90°

𝜎𝜎𝜎𝜎 𝑚𝑚𝑚𝑚𝑚𝑚 (4.8)

72

where:

Rc = anisotropic ratio.



18 Figure 4.6 Classification based on anisotropic ratio, Singh et al. (1989) and βangle


To cross-check the anisotropic effects in BIF, a second anisotropy degree determination based

in P wave velocities measures was used. For this method, Saroglou & Tsiambaos (2007) proposed

a similar equation to determine the velocity anisotropy index (IVp) given by the ratio presented

in Equation 4.9.

𝐼𝐼𝐼𝐼𝐼𝐼 = 𝑉𝑉𝑉𝑉0°𝑉𝑉𝑉𝑉 90°

(4.9)

where:

Vp0° = the maximum velocity of P waves (propagation parallel to the planes of

anisotropy).

Vp90° = propagation is perpendicular to the anisotropy plane.



19 Figure 4.7 P wave velocity anisotropic index, modified from Perucho et al. (2014)

Empirical relations between rock Strength and physical properties

73

practical approach in engineering problems. The use of such relations is often the only way to

estimate strength, due to the absence of cores for laboratory tests or even the lack of tests. The

basis for such relationships is the fact that many of the factors that affect rock strength also

affect other physical and mechanical parameters such as wave velocity, elastic module, bulk

density, Poisson’s ratio, and porosity for example.

To evaluate this relationship for two sets of data which are strongly linked together and to be

able to measure and determine the relationship between two variables, an adequate coefficient

of determination, which describes the strength and the direction of the variable’s correlations,

must be determined. In addition, these correlations could be expressed by linear or not linear

regression methods due to the nature of evaluating variables. The correlation coefficient has

been calculated by using the best fitting approach using automatic procedures through several

attempts on the dataset.

For Butel et al. (2014), the quality of the correlation is determined by the values of the coefficient

of determination (R2), the size of the dataset, and the visual fit of the regression curve. An

adjusted curve is defined as the curve that best evaluates the dispersion of the dataset and

defines the adjusted equation that gives the proportion of the variance in the independent

variables. In other words, the R2is a measure of how well a regression curve matches a set of

data for an evaluated dataset. It could also be interpreted as the adjusted model of observed

effects between two dependent variables. The strength of the correlation is defined as 0 to 0.29

(little if any correlation), 0.3 to 0.49 (low), 0.5 to 0.69 (moderate), 0.7 to 0.89 (high) and 0.9 to

1.0 (very high correlation) as presented by Asuero et al. (2006).

During correlation evaluations analysis, several adjusted fitting curves (e.g. linear, exponential,

and potential) were tested. The exponential adjustment curve proved to be the most suitable

for the available dataset and even when some correlation proves to be better for linear or

potential regressions (e.g. the correlation ratio obtained were not significantly different from

the exponential values).

Due to the material characteristics and the lack of references for studied rock types, a

correlation ratio equal or superior to 0.50 (moderate) was considered adequate.

4.4.1 Laboratory tests

For hard, fresh to slightly weathered lithotypes, UCS (σci) and modulus of elasticity (Estat and νstat)

were determined. P wave velocity (Vp) was used to evaluate the integrity of the samples. Bulk

74

density (ρb) results and thin sections descriptions were used to correlate and define geological

and physical properties such as total porosity, anisotropy, mineralogy, and microfabric.

For laboratory tests, selected rock samples from surface outcrops and drill cores were

subsampled, rectified, and prepared according to each standard, as detailed below.

All laboratory tests results are presented at Appendices III and IV.

Uniaxial compressive or unconfined compressive strength tests

The first set of UCS tests (total of 93 samples) was undertaken at an Australian laboratory in

accordance with the ASTM D2938-95 (ASTM 2002) standard procedures. For all analysis,

samples (cylindrical plugs) were obtained from the original 77.8 mm diamond drill core,

subsampled in 20 mm diameter, with lengths varying from 40 mm to 50 mm, prepared according

to ASTM D4543-01 (ASTM 2001). Sub-sampling allowed for evaluation of possible anisotropy not

available when using full size core. For a better axial and lateral strain determination,

simultaneous measurements were undertaken using, in general, two pairs of diametral strain

gauges and up to four axial gauges and four lateral gauges. All samples were subjected to a

constant loading rate of 7 MPa/min or 9 MPa/min.

The second set of UCS tests (total of 62 samples) was undertaken at a Brazilian laboratory. These

tests were also performed in accordance with ASTM D 2938-95 (ASMT 2002). For all tests,

cylindrical samples were obtained from the original diamond drill core of 50 mm to 76 mm

diameter, with lengths varying from 110 mm to 205 mm, prepared in accordance with ASTM

D4543-01 (ASTM 2001). Axial and lateral measurements were obtained by double dial indicator

gauges, and the strain was calculated by taking the average value for each direction. It is noted

that dial indicator gauges were less efficient than strain gauges. For this reason, some bias was

expected for this set of results.

Considering both sets, a total of 155 samples were tested. The average strain was calculated by

taking the average of all strain readings, and Young’s modulus and Poisson’s ratio were

calculated.

The axial (ξax) and radial (ξrad) deformation curves obtained from UCS testing are a function of

the compressive stresses applied and represents the typical deformational behaviour of the rock

in the absence of loading to the ultimate stress (σaxial) or peak stress, providing a good

description of the nonlinear behaviour model of rocky materials. The static elastic coefficient,

or Young’s modulus (Estat), was determined for that stretch of the axial curve (or stress range)

where the rock exhibits elastic behaviour (linear portion of the curve).

75

In this chapter, the tangent modulus (Estat) was determined by the slope of the line that tangents

the regression curve at a given point corresponding to a fixed percentage of the ultimate or peak

stress (𝜎𝜎u), normally 50%. The Poisson's ratio (νstat) which correlates the axial and radial

deformation, was obtained in the stretches of the curve corresponding to the observed limits

for obtaining the deformability moduli.

Due to the difficulty in preparing reliable samples during the extraction process, some types

presented reduced numbers of proper samples, specially FDI at β0°, HHE at β90°, and β45°.This

reduced number of tests could influence evaluations and produce bias in the interpretation.

For all samples prepared and tested with diameter sizes different from 50 mm, the results were

converted to d50mm based on the empirical correlation proposed by Hoek & Brown (1980).

Equation 4.10 shows the referenced methodology where σcd is the UCS of a sample with

diameter, ‘d’; and σC50 is the UCS of a 50 mm diameter sample. It is generally accepted that there

is a significant reduction in strength with increasing specimen size.

𝜎𝜎cd = 𝜎𝜎𝑅𝑅50 � 𝑑𝑑50�0.18

(4.10)

where:

σcd = UCS of a sample with diameter.

σC50 = UCS of a 50 mm diameter sample.

d = samples diameter.

P wave velocity

For the second UCS test set (62 tests), the primary sonic wave velocity (Vp) experiments were

undertaken according to ASTM D2845-08 (ASTM 2018). The experiments were performed using

equipment that calculates the velocity propagation of compressional waves (the P wave), by

measuring its time of transit through the axial length of the sample. Once the length of the

sample is known, as well as the time of transit of elastic waves, velocities are obtained from the

ratio of these parameters. Although P wave velocities were measured in all UCS tests, the lack

of S wave velocity made it impossible to obtain the dynamic elastic parameters.

Bulk density

In this chapter, bulk density was determined for all UCS tests (n = 155) according to the ratio

between the total dry mass and total volume. Bulk density of each specimen is calculated

according to IAS 1289.6.4.1 (AS 2016) for Australian laboratory and ABNT NBR 6508 (ABNT 1989)

for Brazilian laboratory, using Equation 4.11:

76

ρb = M/V (4.11)

where:

ρb = bulk density (kg/m3)

M = mass of the specimen measured prior to testing (kg)

V = volume (m3) of the specimen, calculated from dimensions measured during

sample preparation.

Petrographic thin sections description

Petrographic thin sections were prepared perpendicular and parallel to the bedding plane to

evaluate visual total porosity (Øb), crystal and pore size, crystal shape and distribution,

mineralogy and pore percentage, and fabric. This information was used to confirm the presence

of microscale features already observed on a macroscale, such as heterogeneity, anisotropy,

total porosity, and mineral distribution.

A total of 23 fresh BIF thin sections were prepared and evaluated. The sections were studied

under a research grant, supervised by the lead author, and presented in Horta & Costa (2016).

It should be noted that variations in crystal and pore size and percentage are closely correlated

with the geological settings of the sample site and the evaluation presented here is restricted to

the 23 thin sections from studied sites and reference review available.

4.5 RESULTS

4.5.1 Mineralogical and fabric overview

The mineralogical ensemble, textural characteristic (rock fabric) and visual total porosity are

important information used to define the anisotropic behaviour and geomechanical properties

for the BIF. The analyses for FQI, FDI and FAI were supported using field investigations and were

used in conjunction with microscopic evaluations from Horta & Costa (2016), while for HHE, as

there is a vast reference available related to its fabric and mineralogical composition, reference

the studies by Rosière et al. (1996 and 2001), Varajão et al. (2002) and Spier et al. (2003) were

used.

As suggested by Baars & Rosière (1994) and Rosière et al. (2001), the BIF fabric at the western

side of the Iron Quadrangle can be separated into three major domains: a subhedral to euhedral

crystals domain, with an overall granoblastic fabric, defined as the most common type and

considered to represent post-tectonic partial recrystallisation; a euhedral domain of locally

77

tabular-shaped or very elongated specularite platelets, preferred oriented, defining a secondary

schistosity induced by shearing and a high stress domain; and a locally brecciaed fabric domain,

that can occur induced by tectonical or physical collapse zones.

Such domains reflect distinct micro characteristics that are not always noticed on the

macroscale. They are associated with the main three geological settings that influence the

geomechanical and anisotropic behaviour: a post tectonical domain (weathering) characterised

by the leaching of the boundary contact; tectonical high stress zones describing oriented mineral

assemblage; and brecciated zone (weathering or tectonical) with porous level oriented or not.

Table 4.1 shows summary results for 23 thin sections presenting the name and percentage of

the main constituent minerals, and maximum and minimum crystal size (between brackets).

Also presented the percentage, and maximum and minimum size (between brackets) of pores

for three types of measures. As known, intragranular pores correspond with the percentage of

pores inside crystals that can or cannot be interconnected. Intergranular pores correspond with

porosity between crystals and are generally interconnected. Secondary pores refer to fracture

pore percentage. Total porosity is the sum of the three pore percentages and is considered the

maximum total percentage. For Øb in brackets, the minimum value did not consider the

secondary pores (intact rock) and for maximum value, the secondary pore is included.

Hematitites, as mentioned, were not evaluated because there is vast reference related to its

fabric, mineralogical composition, and crystal size. This information is presented in Table 4.1

obtained from Rosière et al. (1996 and 2001), Varajão et al. (2002) and Spier et al. (2003).

1Table 4.1 Sum

mary table w

ith microscopic inform

ation from thin sections description for fresh itabirites (H

orta & Costa 2016)

Rock

type

Granular

hematite (%

)

Size (min-m

ax)

mm

Tabular

hematite (%

)

Size (min-m

ax)

mm

Specular

hematite (%

)

Size (min-

max) m

m

Goethite (%

)

Size (min-m

ax)

mm

Ochreous

goethite (%)

Size (min-m

ax)

mm

Quartz (%

)

Size (min-

max) m

m

Gibbsite/kaolinite

(%)

Size (min-m

ax)

mm

Amphibolite

pseudomorph

(%)

Size (min-

max) m

m

Carbonate

(%)

Size (min-

max) m

m

Intragranular

pores (%)

Intergranular

pores (%)

Secondary

pores (%)

Visual total

porosity (%)

Size (min-m

ax)

mm

Num

ber of thin

sections

FAI 34

(0.002-1.16) –

– 4

(0.005-0.6) –

43

(0.02-0.8) –

14–

42

5

(0.005-0.08) 5

FDI 25

(0.020-1.5)

7

(0.009-0.08)

1

(0.09-0.2)

2

(0.005-0.08) 10

17

(0.015-

0.4)

– –

32

(0.025-

0.5)

3 3

6

(0.005-0.18) 5

FQI

26

(0.002-1.9)

7

(0.002-0.8) –

1

(0.002-0.15)

2

(0.005-1.5)

36

(0.01-1.7)

5

(0.005-0.03) –

– 7

4 2

11–13

(0.001-0.11) 13

HHE (0.005 -0.015)

(0.001-0.081) (0.05-1.5)

– –

– –

– –

3 5 to 10

0 3 –10

Rosière et al. (1996,

2001), Spier et al.

(2003) and Varajão

et al. (2002)

78

79

In general, the analyses of these thin sections indicated three main characteristics of pervasive

fabric responsible for inducing penetrative planes that could represent heterogeneity and result

in rock anisotropy:

• Band boundary contacts between layers with different mineral compositions, defining

the compositional metamorphic banding, mainly quartz for FQI, FAI, and carbonates

for FDI. This characteristic is not observed for HHE as it is a monomineralic rock.

• Planar or granular minerals oriented in one direction (mineral orientation), defined by

tabular hematite and recrystalised quartz and carbonate crystals.

• Oriented porous layers (pore orientation), defined by intergranular pores oriented

align the metamorphic banding (band boundary and mineral orientation) or along

discontinuities. This characteristic defines the metamorphic banding for HHE.

A summarised description of each lithotype is presented below:


Fresh quartzitic itabirite mineralogy is marked by granular quartz and hematite, tabular

hematite, and goethite. Ochreous goethite, kaolinite, and gibbsite occur as accessory minerals.

The goethite appears as small crystals in quartz rich bands, with anhedral shapes and can also

occur filling pores. Bands of quartz have higher Øb = 20%, mainly resulting from intergranular

pores. In these bands, microplates of hematite occur in larger quantities and the visual total

porosity is slightly smaller.

Bands of hematite present with lower Øb = 7% but the presence of lobed crystals, tabular

hematite crystals larger than 0.80 mm and granular hematites larger than 1.3 mm. In some

bands the Øb can reach 30%. Hematite crystals lower than 1.3 mm were considered part of the

banding itself.

In general, FQI presented higher Øb ranging from 11% to 13%, compared with another fresh BIF.

This difference can be attributed to the presence of slightly weathered degree (W1) thin sections

that could influence the results.


The mineralogy of fresh amphibolitic itabirite is mainly defined by quartz, hematite, goethite as

pseudomorph of amphibolite, and ochreous goethite. The pseudomorphs of amphibolite are

generally altered to goethite and only occur in some thin sections. Goethite usually occurs with

anhedral shape between quartz crystals as a cement, but also in smaller percentages as crystals.

80

Quartz bands are well preserved and large hematite crystals (1.1 mm) were interpreted as being

part of the preserved hematite band. Hematite bands present lower total porosity and the limits

between quartz and hematite bands are not very clear due to the pore distribution and lower

concentration in the boundaries as observed in FQI. Ochreous goethite and gibbsite are also

present, and kaolinite appear as accessory minerals. All the samples present low Øb = 5%.


The mineralogy of fresh dolomitic itabirite is defined by hematite, iron dolomite, quartz, and

goethite. Granular hematite predominates, although its size (0.90 mm) decreases to 0.02 mm

when higher quantities of ankerite and siderite dolomites occur. These minerals occur

interlayered with hematite bands as granoblastic crystals, as well as a cement in the filling spaces

between crystals. Goethite occurs as a cement filling spaces in between crystals of hematite and

iron dolomite (minor). Quartz is present in bands and exhibits oriented crystals and sub-crystals.

FDI presents low Øb = 6%. Bands of hematite also present some percentage of iron dolomites

and are characterised by relatively lower visual total porosity. In this band, hematite occurs as

granular and/or tabular crystals and iron dolomites as a matrix with very fine crystals. Talc,

carbonate and goethite also occur as accessory minerals.

Hard hematitite

As already presented in Section 4.3.2.

4.5.2 BIF, heterogeneity and anisotropy

Based on geological macro and micro features, it is expected that the heterogeneity promoted

by typical compositional metamorphic banding in BIF will be the main characteristic responsible

for an anisotropic behaviour. Analysis conducted during this research showed at least three

different types of either repetitive or discrete surfaces of disruption that could create an

anisotropic intact rock strength for fresh BIF.

Analyses conducted in Section 4.1 found three main characteristics of pervasive fabric that could

be responsible for inducing penetrative planes representing heterogeneity and result in rock

anisotropy: boundary contacts between layers with different mineral composition, defining the

compositional metamorphic banding; planar or granular minerals oriented according to one

direction (mineral orientation); and oriented porous levels (pore orientation).

The heterogeneity observed in itabirites is different from that observed in hematitites. The

differences will be presented in the following sections. Both satisfy the heterogeneity

81

characteristic defined by Amann et al. (2013) in that they visually appear heterogeneous, with

evidence of non-uniform crystal size distribution at the specimen scale, variable mineral grain

composition and total porosity.

Bands boundary contact

For fresh itabirites (quartzitic, dolomitic and amphibolitic), the millimetric to centimetric layers

with different mineralogy (e.g. hematite and quartz or iron dolomite) are defined as

’heterogenous compositional metamorphic banding’.

Thin section evaluations of itabiritic fabric observed higher crystal size variation (0.002 mm to

1.9 mm) for iron bands and smaller differences (0.01 mm to 1.7 mm) for non-iron bands. This

size differences induce high roughness at the boundary as presented in Figure 4.8A.

For fresh hematitite, millimetric to centimetric bands with similar mineral composition

(e.g. granular hematite, martite or microplates of hematite) but different crystal sizes define a

hematitite heterogeneous metamorphic banding. Fresh hematitite fabric in microscope was

described by Varajão et al. (2002), as crystal size varying from 10 μm to 30 μm for hematite and

martite granular crystals, and equal to 1 μm for microplates of hematite. The contact boundary

surface also present roughness, as observed in Figure 4.8B, although less prominently than as

observed in itabirites.

(A) (B)

20 Figure 4.8 A (left), FQI bands of hematite (light grey) with smaller crystal size in contact

with bands of larger crystals of granular quartz (coloured) and a rough contact

between (dashed red line) (Horta & Costa 2016). B (right), HHE contact less

rough (dashed red line) between larger granular hematite (dark band) with

smaller crystals of tabular hematite (light band) (Horta & Costa 2016)

500μm

A B

82

Mineral orientation

The mineral orientation is characterised as a band of pervasive fabric where the bands are

formed mainly by tabular hematite, specularite and granular quartz and/or iron dolomite

according to itabirite typical banding.

For HHE, the orientation is characterised by microplates of hematite (more common), oriented

granular hematite and larger crystals of specularite in high stress zones, representing continuous

homogeneous oriented bands, as shown in Figure 4.9A.

For fresh itabirites, the orientation is defined for the iron and non-iron bands as outlined

according to the compositional metamorphic banding as shown in Figure 4.9B.

(A) (B)

21 Figure 4.9 A (left), typical specularite orientation crystal (Horta & Costa 2016). B (left),

FDI bands of oriented granular iron dolomite with large crystals (light brown)

and bands of tabular hematite mixed with iron dolomite and quartz (dark

brown) (Horta & Costa 2016)

Pores concentration and orientation

Fresh BIF presents, in general, Øb is lower than 10% with pore sizes ranging from 1 μm to 0.2 mm

defining weaker planes easily observed in slightly weathered (W1) typologies, resulting from

leaching of prone minerals (boundary corrosion or mineral dissolution), with reprecipitation of

iron oxides and/or hydroxides, preferentially using initial strong anisotropy surfaces (banding

contact or mineral orientation). These pores’ concentration and orientation can cause

continuous or discontinuous bands of pores defining anisotropic layers that contribute to the

heterogeneity mainly for hematitite, but also for itabirites.

500μm

A B

1000μm

83

Representing the HHE heterogeneity, the following is suggested: a metamorphic banding

defined by bands of iron minerals with larger crystal size variation and higher Øb (up to 10%),

and bands of non-iron minerals with smaller crystal size variation and lower Øb (3%), defining

what is called hematitite heterogeneous metamorphic banding as presented in Figure 4.10A.

Fresh itabirites present a high concentration of pores, especially in the contact within iron and

non-iron bands (band boundary contact), oriented according to the compositional metamorphic

banding. Oriented pores are noted even in layers with no mineral orientation, as shown in Figure

4.10A. For superficial materials, pores can be cemented by goethite, as shown in Figure 4.10B.

(A) (B)

22 Figure 4.10 A (left), pore bands in massive HHE. B (right), FAI thin section with pores along

banding (tabular hematite – light grey, and quartz – white) filled by goethite

(light red) (Horta & Costa 2016)

4.5.3 Intact rock strength anisotropy

UCS and P wave velocity tests were used to evaluate the anisotropic behaviour and define the

intact rock strength and elastic parameters of the BIF. The test results in three different test

directions were used to evaluate the anisotropic effects and define the anisotropic ratio.

To evaluate the effects of heterogeneity (mineral, pore orientation, and contact boundary) on the

anisotropy strength of BIF in different βangles, the equations presented by Singh et al. (1989) were

applied to define the anisotropic ratio (Rc) for UCS tests. For P wave velocities (Vp), equations

proposed by Saroglou & Tsiambaos (2007) were used to define the anisotropy index (IVp).

The statistical evaluation of each type used in the three different directions is presented in Table

4.2. This table presents for each type and anisotropy directions (βangles) the mean, standard

deviation, maximum values (Max), minimum values (Min), values of the first and third quartiles

(1 Qt and 3 Qt), and number of tested samples (n).

0.5 mm

2 Table 4.2 Statistical results of each type is presented for FAI, FDI, HH

E and FQI for the three m

ain βangles

Lithotype Anisotropy

(βangle )

Parameter

Bulk density

(t/m3)

E stat

(GPa)

v stat U

CS

(MPa)


(βangle )

Parameter

Bulk density

(t/m3)

E stat

(GPa)

v stat U

CS

(MPa)

FAI

90°

Mean

3.3 95

0.20 211

FDI

90°

Mean

3.19 115

0.16 174

Standard deviation 0.29

18 0.03

79 Standard deviation

0.31 43

0.03 54

Max

4.13 124

0.24 360

Max

3.58 200

0.18 257

Min

3.00 71

0.14 95

Min

2.75 77

0.14 112

1 Qt

3.13 84

0.17 171

1 Qt

2.94 84

0.15 129

3 Qt

3.37 109

0.22 239

3 Qt

3.41 126

0.17 225

n 12

10 10.00

12 n

10 7

2.00 10

45°

Mean

3.2 82

0.16 138

45°

Mean

3.55 76

0.21 122


25 0.03


0.28 31

0.16 56

Max

3.73 115

0.20 229

Max

4.08 117

0.33 243

Min

2.99 40

0.11 33

Min

3.03 32

0.12 40

1 Qt

3.05 68

0.15 50

1 Qt

3.46 56

0.17 73

3 Qt

3.25 103

0.18 205

3 Qt

3.65 104

0.24 159

n 12

11 8.00

12 n

14 8

8.00 14

0°

Mean

3.33 101

0.23 167

0°

Mean

3.49 97

0.27 104


36 0.16


0.61 20

0.06 73

Max

3.81 190

0.35 330

Max

4.66 118

0.34 262

Min

2.99 52

0.13 37

Min

3.05 73

0.23 41

1 Qt

3.14 76

0.19 92

1 Qt

3.07 82

0.24 72

3 Qt

3.43 113

0.27 247

3 Qt

3.74 115

0.29 102

n 17

16 15.00

17 n

7 5

3.00 7

84


(βangle )

Parameter

Bulk density

(t/m3)

E stat

(GPa)

v stat U

CS

(MPa)


(βangle )

Parameter

Bulk density

(t/m3)

E stat

(GPa)

v stat U

CS

(MPa)

HHE

90°

Mean

4.92 93

0.33 100

FQI

90°

Mean

3.50 91

0.18 246


42 0.04


0.27 41

0.03 102

Max

5.02 120

0.35 155

Max

3.79 122

0.22 350

Min

4.77 31

0.30 78

Min

3.02 20

0.14 82

1 Qt

4.86 83

0.31 82

1 Qt

3.34 99

0.16 217

3 Qt

5.00 120

0.34 101

3 Qt

3.69 114

0.19 330

n 4

4 2

4 n

9 9

9.00 9

45°

Mean

5.06 50

0.28 103

45°

Mean

3.45 84

0.20 158


39 0.05


0.24 40

0.05 87

Max

5.15 90

0.31 139

Max

4.04 131

0.31 335

Min

4.48 11.

0.22 83

Min

2.92 16

0.15 18

1 Qt

5 30

0.26 84

1 Qt

3.27 59

0.17 81

3 Qt

5.1 70

0.31 112

3 Qt

3.60 116

0.23 220

n 4

4 4

4 n

25 20

2.00 25

0°

Mean

5.07 83

0.29 210

0°

Mean

3.35 85

0.19 157


75 0.04


0.27 39

0.16 95

Max

5.21 223

0.32 585

Max

3.78 124

0.,29 315

Min

4.84 6

0.22 51

Min

2.88 16

0.07 41

1 Qt

5.01 32

0.27 104

1 Qt

3.11 64

0.16 61

3 Qt

5.18 90

0.32 292

3 Qt

3.57 114

0.22 222

n 19

15 6

19 n

17 14

14.00 17

85

86

Strength anisotropy ratio

Graph 4.1 shows for studied BIF types the UCS test results for each βangles and Table 4.3

summarises the results used to determine the Rc and classification as proposed by Singh et al.

(1989).

0-1 Graph 4.1 UCS results for each βangle for FAI (top left), FDI (top right), FQI (bottom left),

and HHE (bottom right). Black dotted lines present the average trend

3 Table 4.3 Rc summary table for each studied lithotype in three tested directions, as

classified (Class) by Singh et al. (1989), presenting the number of tested

samples (n)

UCSLithotype (MPa)/ β (°) 0 45 90 Rc n Class

HHE 210 102 100 1 32 Isotropic

FDI 104 122 174 2 51 Low

FQI 157 158 246 1.6 31 Low

FAI 167 138 211 1.5 41 Low

The following traits were noted from Graph 4.1, and Tables 4.2 and 4.3:

• The highest UCS values are found at β90° for FQI, FAI, and FDI. HHE is the exception,

which presented a higher mean value at β0°.

• FDI (Rc = 2) presents the highest anisotropic ratio, classified as the limit between low

and medium. FQI (Rc = 1.6) and FAI (Rc = 1.5) present low anisotropic ratios, and HHE

(Rc = 1) is classified as the limit between isotropic and low.

157 158

246

104

122

174

210

102 100

167

138

2119

FAI

FQI HHE

FDI

87

• The low number of tests (less than five samples), for FDI at β0°, HHE at β90°, and β45°

could represent some bias on obtained results noting low HHE mean values and very

low FDI mean values at β0°.

• Lower UCS values are at β45° for HHE, and FDI.

• FQI presents the highest UCS mean value, followed by HHE, FAI and FDI.

• FQI and FAI present similar UCS mean values, unless for β90°.

• Differences in the UCS results, observed for β0° compared with β90° were about 60% for

FDI, 43% for FQI, 55% for HHE, and 35% for FAI.

P wave velocity, anisotropy index

Graph 4.2 shows the average values obtained for each type and Table 4.4 presents the velocity

anisotropy index (IVp) using the classification proposed by Saroglou & Tsiambaos (2007) for the

three main βangles (0°, 45°, and 90°) as presented before.

0-2 Graph 4.2 P wave velocity measures for all BIF types, in the three main βangles. Coloured

lines show the trend between the main value for each type used for the

anisotropic evaluation

4 Table 4.4 IVp summary table for each studied type in three tested directions, as classified

(Class) by Saroglou & Tsiambaos (2007), presenting the number of tested

samples (n)

Vp Lithotype (m/s)/β (°) 90 45 0 IVp n Class

HHE 6287 7122 7188 1.1 33 Fairly FDI 4414 4776 5946 1.3 42 Fairly FQI 5022 4823 5513 1.1 17 Fairly FAI 5307 5574 6171 1.2 30 Fairly

88

The following traits were noted from Graph 4.2 and Table 4.4:

• The highest Vp values are found at β0° for all types. HHE presents the highest mean

value for all βangles.

• Lower Vp values for all BIF are at β90° except for FQI at β45°

• At β90°, HHE presents highest mean value followed by FAI, FQI and FDI. For β45°, FDI and

FQI presented similar results.

• FAI presents the highest values of Vp from all itabirites, and FDI shows the lowest mean

except for β0°

• Differences in Vp results, were obtained for β0° compared with β90° with about 26% for

FDI, 9% for FQI, 12% for HHE, and 14% for FAI.

• All types present fair degrees of anisotropy. This behaviour can be mainly induced by

the low total porosity and the fact that the pore bands are not continuous, as

previously reported in Section 4.5.2, allowing the P wave pulse to penetrate through

the sample portion not affected by pores, independent from anisotropy direction.

Considering this, the pulse will preferably travel through the portions with no pores

independent from the anisotropic direction or heterogeneity. The small difference

observed in different directions, mainly for itabirites, is associated with the band

mineral composition (heterogeneity). This heterogeneity contributed to the slightly

higher Ivp ratio obtained for FDI with most different mineral composition bands

compared to other itabirites.

Comparing the anisotropic ratio classification obtained for UCS and Vp in Tables 4.3 and 4.4, it is

possible to conclude that FDI is the type with highest anisotropic behaviour (Rc = 2 and IVp = 1.3),

HHE presents the lowest anisotropy degree (Rc = 1 and IVp = 1.1), and FQI (Rc = 1.6 and IVp = 1.1),

and FAI (Rc = 1.5 and IVp = 1.2), presenting similar anisotropy degree for both evaluations. In

general, HHE behaves like an isotropic material, and FAI, FQI and FDI behave similarly to low

anisotropic materials, however, FDI presents a slightly higher anisotropic ratio.

4.5.4 BIF characterisation of geomechanical properties and parameters

Results from Section 4.5.3 established that, in general, BIF are isotropic or have a low anisotropic

ratio. Based on this behaviour, geomechanical properties characterisations were evaluated for

seven different geomechanical parameter correlations (UCS and ρb, UCS and Vp, UCS and νstat,

89

UCS and Estat, Estat and Vp, ρb and Estat, and ρb and Vp). To be able to evaluate minor changes

induced by this low anisotropic ratio, the βangles variations were plotted and presented in graphs.

Table 4.5 summarises all test results, presenting basic statistics for fresh lithotypes HHE, FQI, FDI

and FAI not considering the βangle values (isotropic material) containing for each type the mean,

standard deviation (SD), maximum values (Max), minimum values (Min), values of the first and

third quartiles (1 Qt and 3 Qt), and number of tested samples (n).

5 Table 4.5 Basic statistic summary test results for strength and elastic parameters

evaluated for all fresh BIF

Lithotype Parameter ρb (t/m3) Estat (GPa) vstat UCS (MPa) Vp(m/s)

HHE

Mean 5.02 70 0.29 159 7022 SD 0.13 61 0.04 134 586 Max 5.21 223 0.35 585 7889 Min 4.77 6 0.22 45 5798 1 Qt 4.95 32 0.29 82 6639 3 Qt 5.11 90 0.31 195 7130 n 32 28 12 32 33

FQI

Mean 3.42 86

0.19 173 5019

SD 0.26 39 0.05 96 1178 Max 4.04 131 0.31 350 7265 Min 2.88 16 0.07 18 2211 1 Qt 3.24 60 0.16 81 4010 3 Qt 3.62 115 0.22 236 5552 n 51 43 23 51 42

FDI

Mean 3.42 95

0.22 135 5226

SD 0.41 36 0.07 64 1122 Max 4.66 200 0.34 262 6757 Min 2.75 32 0.12 40 2782 1 Qt 3.10 73 0.17 82 4505 3 Qt 3.59 116 0.24 169 5747 n 31 20 13 31 17

FAI

Mean 3.27 93

0.20 172 5596

SD 0.24 29 0.05 89 438 Max 4.13 190 0.35 360 6467 Min 2.99 40 0.11 33 4732 1 Qt 3.09 76 0.18 100 5271 3 Qt 3.36 108 0.23 228 5566 n 41 37 33 41 30

Table 4.5 shows a high SD for all types and all evaluated parameters except for 𝜌𝜌b and Vp. Since

measures were taken to reduce intrinsic test and sample dispersion normally observed at UCS

90

tests and parameters derived from UCS results (e.g. Estat and νstat), as presented in Section 5.4.1,

the presence of high SD is attributed to the anisotropy effects that, even at low values, cause

disturbances in the means. As outlined in Bewick et al. (2015), for heterogeneous rocks, intact

rock strengths based on UCS test results are often considered regardless of the type of failure

(e.g. shearing, axial splitting), and scattered results with coefficient of variance higher than 25%

are common.

In contrast with other parameters, Vp and ρb presented appropriate SD values. This is attributed

to the nature of the tests presenting an adequate repeatability and minor changes for different

anisotropy directions. This behaviour can also be attributed to the low total porosity and pore

distribution in these BIF types supporting the argument that Vp and ρb are not or slightly

anisotropy dependent.

From Table 4.5 results it is possible to note that:

• Itabirites are define in a group with similar low bulk density: ρbFAI = 3.27 (±0.24) t/m3;

ρbFQI = 3.42 (±0.26) t/m3, and ρbFDI = 3.42 (±0.41) t/m3.

• Hematitite is the type with higher bulk density: ρbHHE = 5.02 (±0.13) t/m3. These ranges

are in accordance with Ribeiro et al. (2014).

• Itabirites also presented low relative P wave velocity: VpFQI = 5,019 (±1,178) m/s,

VpFDI = 5,226 (±1,122) m/s, and VpFAI = 5,596 (±438) m/s.

• Hematitite presented higher P wave velocity: VpHHE= 7,022 (±586) m/s.

• HHE presented higher SD values for UCS tests and can induce bias in the correlations.

• Poisson’s ratio results were obtained from few tests for HHE and FDI, and can also

induce bias even with low SD.

To better evaluate these correlations, in the following sections several graphs are presented

correlating UCS with elastic static parameters and petrophysical properties, and between elastic

static parameters and petrophysical properties. In these graphs, different symbols represent the

test results for each βangle, colours are linked to each lithotype, and dotted coloured curves

represent individual exponential regression curves with their respective equations and

coefficient of determination.

For a better graph correlation, the spurious data were removed from the original dataset

following the basic statistics presented in Tables 4.2 and 4.5.

91

Predicting unconfined compressive strength from bulk density

In this section, the UCS values were correlated with the corresponding ρb of each rock type. The

significance of ρb as a variable in the UCS correlation was assessed using the values obtained

from the R2 and the equation obtained for the regression correlation curve.

This relationship is largely used for rock strength determination, mainly for isotropic and

homogeneous rocks, and generally presents a direct linear or exponential correlation that is

strongly affected by porosity, which is not considered due to the very low total porosity found

for fresh BIF. Graph 4.3 shows the correlation between UCS and ρb.

0-3 Graph 4.3 UCS test with bulk density correlation graph. In this graph, each colour

represents a BIF type, symbols represent the βangle and dotted coloured curves

represent individual exponential regression curves with their respective

equation and coefficient of determination as presented in the subtitle

The following traits were noted from Graph 4.3:

• The distribution of the UCS values allowed the definition of three different clusters: low

(UCS ≤ 150 MPa), moderate (150 MPa < UCS ≤ 300 MPa), and high (UCS > 300 MPa).

• Three clusters have also been defined for bulk densities: low (ρb ≤ 3.3 t/ m3), moderate

(3.3 t/m3 < ρb ≤ 4.6 t/m3) and high (ρb > 4.6 t/m3). The obtained cluster is in accordance

with iron ore content, used to define and separate country rocks from itabirites and

hematitites as evaluated by Santos et al. (2005) and Santos (2007).

92

• FDI presents the higher R2FDI = 0.70, moderate positive correlation, expressed by the

equation UCS = 1.18e1.29ρb, and the higher 𝜌𝜌b values showed the lowest UCS values

results from all BIF.

• FQI presents R2FQI

= 0.56, moderate positive correlation, expressed by the equation UCS

= 0.04e2.37ρb.

• FAI presents R2FAI = 0.50, moderate positive correlation, expressed by the equation UCS

= 0.03e2.52ρb.

• HHE presents R2HHE = 0.50, moderate positive correlation, expressed by the equation

UCS = 3.23E-06e3.43 ρb.

Predicting unconfined compressive strength from p wave velocity

Establishing a UCS and Vp relationship provides empirical equations that indirectly correlate a

destructive, costly, and time-consuming test (UCS) with a non-destructive, cheap, and faster test

(VP), providing a reliable and suitable equation that can be used when a reduced number of UCS

tests is available. Graph 4.4 shows the correlation between UCS and Vp.

0-4 Graph 4.4 UCS tests with P wave velocity (Vp) correlation graph. The red dashed circle

highlights FDI negative correlation. In this graph, each colour represents a BIF

type, symbols represent the βangle and dotted coloured curves represent

individual exponential regression curves with their respective equation and

coefficient of determination as presented in the subtitle

93


• The distribution of the Vp values allowed the definition of three different clusters: low

(Vp ≤ 4,500 m/s), moderate (4,500 m/s < Vp ≤ 6,500 MPa), and high (Vp > 6,500 m/s).

• FQI presents the higher R2FQI = 0.82, high positive correlation, expressed by the

equation UCS = 2.18e0.001Vp.

• HHE presents R2HHE = 0.64, moderate positive correlation, with the equation UCS

= 0.2e0.0009Vp.

• FAI presents R2FAI = 0.56, moderate positive correlation, with the equation UCS

= 0.05e0.001Vp.

• FDI presents R2FDI = - 0.33, low negative correlation, highlighted in Graph 4.4 by a

dashed red circle, is attributed to the presence of samples with high Vp and low UCS

results. This behaviour is promoted by the high Vp typical from iron dolomites that

present low mineral strength.

Predicting unconfined compressive strength from static Poisson’s ratio

The relationship between UCS and νstat can provide empirical correlations between rock stiffness

and strength parameters as shown in Graph 4.5.

0-5 Graph 4.5 UCS test with static Poisson’s ratio correlation graph. In this graph, each colour



equations and coefficient of determination as presented in the subtitle

94

The following traits were noted in Graph 4.5:

• Poisson’s ratio can be categorised into three clusters: low (νstat ≤ 0.150), moderate

(0.150 < νstat ≤ 0.300), and high (νstat > 0.300).

• FQI presents R2FQI = -0.50, moderate negative correlation, expressed by the equation

UCS = 2,051e-13.5νstat.

• FAI presents R2FAI = -0.59, moderate negative correlation, expressed by the equation

UCS = 806e-8.6νstat.

• HHE presents R2HHE = 0.01 and FDI presents R2

FDI = 0.06, the lack of correlation could be

associated with the small dataset (n ≤ 13).

• A negative correlation is expected for this correlation.

Predicting unconfined compressive strength from static Young’s modulus

Using the same approach for the relationship between UCS and static Estat can provide an

empirical correlation between rock stiffness and strength parameters, as presented in Graph

4.6.

0-6 Graph 4.6 UCS test with static Young’s modulus correlation graph. In this graph, each

colour represents a BIF type, symbols represent the βangle and dotted coloured

curves represent individual exponential regression curves with their respective


95


• The Estat can be categorised into three clusters: low (Estat ≤ 50 GPa), moderate (50 GPa

< Estat ≤ 100 GPa), and high (Estat > 100 GPa).

• FQI presents the higher correlation R2FQI = 0.83, high positive correlation, expressed by

the equation UCS = 27.7e0.02Estat.

• FAI presents R2FAI = 0.78, high positive correlation, expressed by the equation UCS

= 25.13e0.02Estat.

• HHE presents R2HHE = 0.37, low positive correlation attributed to the high dispersion

obtained for Estat = 70 GPa (SD = ±61 GPa) and also for UCS = 159 MPa (SD = ±134 MPa).

• FDI presents R2FDI = 0.30, low positive correlation, possibly caused by the presence of

iron dolomite minerals that reduces the intact rock strength.

Predicting static Young’s modulus from P wave velocity

Graph 4.7 shows the correlation between Estat and Vp that could establish a relationship between

dynamic and static parameters.

0-7 Graph 4.7 Static Young’s modulus with P wave velocity correlation graph. In this graph,

each colour represents a BIF type, symbols represent the βangle and dotted

coloured curves represent individual exponential regression curves with their

respective equations and coefficient of determination as presented in the

subtitle

96


• FQI presents the highest value R2FQI = 0.89, high positive correlation, expressed by the

equation Estat = 0.62e0.0009Vp.

• HHE presents R2HHE = 0.69, moderate positive correlation, expressed by the equation Estat

= 0.15e0.001 Vp.

• FAI presents R2FAI = 0.54, moderate positive correlation, expressed by the equation Estat

= 1.26e0.001 Vp.

• FDI presented R2FDI = 0.39, low positive correlation is attributed to the low number of

data and dispersion. The lower correlation is also attributed to the presence of iron

dolomite with low hardness (Mohr scale) and high Vp values.

Predicting bulk density from static Young’s modulus

Graph4.8 shows the correlation between ρb and Estat.

0-8 Graph 4.8 Bulk density with static Young’s modulus correlation graph. In this graph, each

colour represents a BIF type, symbols represent the βangle and dotted coloured

curves represent individual exponential regression curves with their respective



• FAI presents the higher value of R2FAI = 0.80, high positive correlation, expressed by the

equation Estat = 11.6e0.64ρb.

97

• FQI presents R2FQI = 0.59, moderate positive correlation, expressed by the equation Estat

= 0.01e2.66ρb.

• HHE presents R2HHE = 0.26, little correlation attributed to the high standard deviation

for Estat = 70 GPa (SD=± 61 GPa).

• FDI presents R2FDI = 0.07, no correlation attributed to the high standard deviation for

ρb = 3.42 t/m3 (SD = ±0.41 t/m3).

Predicting bulk density from P wave velocity

Graph 4.9 shows the correlation between ρb and Vp where different dot symbols and colours

represent the respective test results for each βangle and lithotype. The dotted coloured curves

represent individual regression curves together with their corresponding equations.

0-9 Graph 4.9 P wave velocity with bulk density correlation graph. In this graph, each colour





• FQI presents the highest R2FQI = 0.55, moderate positive correlation expressed by the

equation Vp = 636e0.62ρb;

• HHE presents R2HHE = 0.52, moderate positive correlation expressed by the equation

Vp = 703e0.46ρb;

98

• FAI presents R2FAI = 0.58, moderate positive correlation expressed by the equation Vp

= 2,42e0.26ρb;

• FDI presents R2FDI = -0.9, very high negative exponential correlation expressed by the

equation Vp = 50,580e-0.68ρb;

• The negative correlation observed is attributed to the presence of iron dolomite with

high Vp values and low mineral strength or hardness defined by the Mohr scale.

Table 4.6 summarises the range obtained for all strength and elastic parameters, and

petrophysical properties for fresh BIF types. To provide a simple evaluation, the results were

separated into three value ranges, each one being associated with a geological and

geomechanical characteristic, with range and correlations likely to be important when

describing the behaviour and characteristics of these rocks.

6 Table 4.6 Evaluated parameters trend summary table for all fresh BIF

Parameters Low Medium High

UCS (MPa) UCS < 150 150 ≤ UCS ≤ 300 UCS > 300

Bulk Density ρb (t/m3)

ρb < 3.3 3.3 ≤ ρb ≤ 4.6 ρb > 4.6

P wave velocity Vp (m/s)

Vp < 4,500 4,500 ≤ Vp ≤ 6,500 Vp > 6,500

Static Young's modulus Estat

(GPa)

Estat < 50 50 ≤ Estat ≤ 100 Estat > 100

Static Poisson's ratio νstat

νstat < 0.150 0.150 ≤ νstat ≤ 0.300 νstat > 0.300

Main geological and

geomechanical characteristics

• Higher relative total porosity; • Lower relative iron content (poor itabirites); • Slightly weathered BIF (W2). •Presence of iron minerals with lower strength

• Typical Itabirites; • Fresh BIF (W1).

• Lower relative total porosity; • Higher relative iron content (rich itabirites and hematitite); • Fresh BIF (W1) with very low total porosity. • Presence of iron minerals with lower strength

Table 4.7 summarises the coefficients of determination and respective correlation equations for

all evaluated correlations (see columns) for each BIF type (see rows). Geological and

geomechanical characteristics are presented to provide a general overview of the fresh BIF

geomechanical behaviour and petrophysical characteristics.

7Table 4.7 Evaluated param

eter correlations summ

ary table for all fresh BIF

Lithotype Correlations

UCS × ρ

b U

CS × Vp

UCS × ν

stat U

CS × Estat

Esta t × V

P E

stat × ρb

VP × ρ

b G

eological and

geomechanical

characteristics

FQI

R2

0.56 (M

oderate) 0.82 (High)

(-)0.50 (M

oderate) 0.3 (Low

) 0.89 (High)

0.59 (M

oderate) 0.55 (M

oderate) •M

ost reliable itabirite forassessing correlations. •G

eological characteristics express typical itabirite geom

echanical behaviour. •The presence of quartz as m

ainnon-iron m

ineral conditions the geom

echanical behaviour.

Equation U

CS = 0.04e2.37ρb

UCS = 2.18e

0.001Vp U

CS = 2,051e-13.5νstat

UC = 27.7e

0.02Estat E

stat = 0.62e

0.0009Vp E

stat = 0.01e2.66ρb

Vp = 636e

0.62ρb

FAI

R2

0.50 (M

oderate) 0.56 (M

oderate) (-) 0.59 (M


0.54 (M


0.58 (M

oderate) •Presence of goethite (am

phibole alternated) induces the predom

inance of slightly w

eathered (W2) and influence in

the geomechanical behaviour.

Equation U

CS = 0.03e2.52ρb

UCS = 0.05e

0.001Vp U

CS = 806e-8.6νstat

UCS = 25.13e

0.02Estat E

stat = 1.26e0.001 Vp

Estat 11.6e

0.64ρb V

p = 2,42e0.26ρb

FDI

R2

0.70 (M

oderate) (-) 0.33 (Low

) 0.06 (N

o correlation) 0.30 (Low

) 0.39 (Low

) 0.07 (N

o correlation) (-) 0.9 (Very high)

•The presence of iron dolomite

minerals, as the m

ain mineral in

the nom iron band, influence the

geomechanical behaviour and

correlations.Equation

UCS = 1.18e

1.29ρb N

ot determined

Not determ

ined N

ot determined

Not determ

ined N

ot determined

Vp = 50,580e

-0.68ρb

HHE R

2 0.50 (M

oderate) 0.64 (M

oderate) 0.01 (N

o correlation) 0.37 (Low

) 0.69 (M

oderate) 0.26 (Little)

0.52 (M

oderate) •Different behaviour fromitabirites. •The high 𝜌𝜌

b is directly associatedto the high U

CS. Equation

UCS = 3.23E-06e

3.43 ρb U

CS = 0.2e0.0009Vp

Not determ

ined N

ot determined

Estat = 0.15e

0.001 Vp N

ot determined

VP = 703e

0.46ρb

Correlations characteristics

•Moderate positive

correlation for all types as total porosityis very low

for fresh and hard BIF.

•FDI negative low

correlation attributedto the high V

P SD and the high V

p values ofiron dolom

ite m

inerals.

•No correlation

obtained for HHE and FDI are attributed the high dispersion and sm

all dataset.

•Low HHE correlation is

attributed to the high E

stat SD.•The low

correlation forFDI is associated to the presence of iron dolom

ite minerals that

reduce the intact rock strength (w

eaker m

ineral).

•FDI low correlation

attributed to the small

dataset, low strength and

to the high Vp observed for iron dolom

ite m

inerals.

•No correlation for

FDI induced by the high ρ

b SD and higherferroan dolom

ite m

ineral 𝜌𝜌b com

pared w

ith quartz.

•FDI very high negative correlations m

ust be associated to the high V

p valuesand low

strength of iron dolom

ite m

inerals.

99

100

The high coefficient of determination obtained for some lithotypes between bulk density,

P wave velocity, Young’s modulus and UCS represents a reliable approach able to define intact

rock strength and elastic parameters and describe mechanical characteristics and

geomechanical behaviour of each lithotype. Even for the low coefficient of determination results

obtained, influenced by the high data dispersion observed at some lithotypes, useful knowledge

that can describe the geomechanical behaviour can be observed.

4.6 DISCUSSION

4.6.1 BIF compositional metamorphic banding heterogeneity and strength

anisotropy

Analyses conducted in this research found three main types of petrophysical characteristics of

pervasive fabric responsible for inducing penetrative planes that can result in anisotropy for fresh

BIF:

• Band boundary, fabric contrast (crystal and pore size) at boundary, and contact

between bands of iron and non-iron minerals.

• Mineral orientation, planar or granular minerals oriented along a single or multiple

directions.

• Pore orientation, oriented levels/bands with pore concentration.

These BIF fabric characteristics consist of persistent and repetitive layers or bands that may

define a heterogeneity wherein the anisotropy ratio and index, depending on the BIF type, varies

from isotropic to low anisotropy ratio.

In this context, HHE is a monomineralic (mainly iron minerals) rock, with metamorphic

heterogeneity defined by interlayered bands of low or high total porosity and different shape

and size of iron minerals. For this reason, it is defined macroscopically as homogeneous due to

the difficult to see porous variation without thin section analyses. In contrast, the itabirites, are

heterogeneous rocks with compositional metamorphic banding defined by the repetition of iron

and non-iron mineral bands. From the geomechanical perspective, both can represent a low

transverse anisotropy as defined by the three fabric characteristics, band boundary, pore

orientation, and mineral orientation.

The low visual total porosity observed in BIF the sections, averaging 5% for fresh (W1), reaching

13% for slightly weathered (W2), is an important petrophysical characteristic conditioned by the

rock fabric and weathering. The pore layers or bands represent a pervasive plane and are used

101

as preferential paths for water percolation, inducing leaching and other weathering effects,

responsible for enlarging the pores, and mineral chemical alteration (e.g. oxidation) reducing

parental intact rock strength. Continuous weathering through the rock matrix or mainly by the

pore layers or discontinuities will change in great depths the parental hard and fresh itabirite to

slightly weathered and, depending on the main non-iron mineral, progress the weathering grade

(in shallower depths) to moderately, highly, completely weathered rock, and finally residual soil,

progressively reducing its intact rock strength.

The heterogeneity defined by the non-iron mineral band and the thickness of this band are

important features that controls the bulk density, iron content and, ultimately, the intact rock

strength of itabirite. For hematitite as mineralogically homogenous type, the metamorphic

banding heterogeneity is defined by the alternation of different porous bands normally

associated with the presence of tabular hematites and granoblastic hematite minerals, with a

minor presence of quartz. Additionally, the low strain and low metamorphic grade (green schist)

observed along the western side of the Iron Quadrangle imposed on the relatively simple

mineralogical assemblages of the BIF a fabric with small mineral size and poor orientation that

did not impose strong anisotropy on the intact rock.

Correlating the mineralogical (fabric) behaviour, evidenced by Sections 4.5.1 and 4.5.2, with the

three main fabric domains suggested by Rosière et al. (2001) for western side of the Iron

Quadrangle into:

• Domain 1: Subhedral to euhedral crystals in an overall granoblastic fabric which is the

most common fabric, associated with contact band boundaries and itabirites

heterogeneity, and considered to represent post-tectonic partial recrystallisation;

• Domain 2: Euhedral, composed of locally tabular-shaped and locally very elongated

specularite platelets that induce a preferred mineral orientation;

• Domain 3: Brecciated fabric domain that occurs locally and can be associated with pore

concentration.

Each of these domains will affect BIF intact rock strength, heterogeneity, and anisotropy in

different ways. Domain 1 is considered the main representative and is in accordance with

geological and geomechanical characteristics evaluated. Although domains 2 and 3 are more

restricted in the studied mine sites, they could represent BIF subtypes (breached and sheared)

not evaluated in this chapter.

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Considering just the band thickness of itabirites when it presents thicker centimetric layers of

non-iron minerals (e.g. iron dolomite or quartz bands), there is a relatively less dense itabirite

sample. Also, the intact rock strength will be lower than other specimens with more iron bands

(thicker iron layers), which increase the iron content, reduce total porosity, and increase the

bulk density, as illustrated schematically in Figure 4.11. In the figure, bulk density (ρb) values

were obtained from correlations presented in Section 4.5. UCSmax represents the maximum UCS

values, P wave velocity (VP), static Young’s module (Estat), and static Poisson’s ratio (νstat) shown

in Table 4.2, and total porosity Øb were obtained from Table 4.1.

23 Figure 4.11 Heterogeneity variation according to the increase of iron content from poor

BIF (iron rich country rocks) to richest BIF (hematitite)

Even considering qualitative evaluations as shown in Figure 4.11, the thickness of the

compositional metamorphic banding for itabirites is an important feature directly controls the

geomechanical characteristics. This characteristic is discussed, and samples are shown in

Appendix I.

In summary, in great depths, the BIF exhibit simple mineralogy assemblage and predominantly

a granoblastic fabric controlled by compositional metamorphic banding, which results in low

variations in shapes and sizes generating a strong frame (skeleton). It is also observed, as shown

by Rosière et al. (2001), that a low metamorphic grade (mainly greenschist) helps to determine

the mineral assemblage, crystal size and shape variability. The reduced visual total porosity (8%

on average), high bulk density, and absent or very low weathering activity in quartz and hematite

crystals, respectively, with weathering stability rate as defined by Goldich (1938), contributed to

the high intact rock strength and low anisotropic behaviour. However, each studied type

𝜌𝜌b > 4.6 t/m3 4.6 t/m3 ≥ 𝜌𝜌b > 3.3 t/m3 𝜌𝜌b < 3.3 t/m3

Øb Øb Øb

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presents individual geomechanical and petrophysical characteristics that represent different

behaviour and results for intact rock strength and deformability.

To assess the existence of this different behaviour, an evaluation of the anisotropy effects

defining the anisotropic ratio using UCS tests and anisotropy index using Vp was presented in

Section 4.3.1. From IVp, all BIF are fairly anisotropic with a slightly higher anisotropy index for FDI

(IVp = 1.3). From the UCS tests (Rc) FDI also presented a low to medium anisotropic ratio (Rc FDI

= 2), while all the other lithotypes were defined as having a low anisotropic ratio. HHE proved to

be the most isotropic in the two techniques used (IVp HHE = 1.1 and Rc HHE = 1).

FDI was defined as the lithotype with higher relative anisotropy (IVp FDI = 1.3 and Rc FDI = 2) this

behaviour is defined by the presence of bands with weaker minerals (iron dolomite), that can

induce strength reduction where these minerals are more concentrated. The bias imposed by

the reduced number of tests at β0° linked to the difficulty to obtain proper samples should also

be considered. The difficulty of preparing proper samples for the different βangle observed for

HHE and FDI is an indirect indication of the lower-strengths of some surfaces that exhibit

elongated minerals such as specularite for HHE and accessory chlorite or talc for FDI.

The intact rock strength of BIF obtained from UCS tests showed a high SD. However, the mean

values are similar with a small range for each type.

4.6.2 Petrophysical, geological and geomechanical properties characterisation and

correlations

The regression curves obtained for the petrophysical and geomechanical property correlations

are summarised and outlined in the following section considering each correlation sets.

UCS and bulk density correlation

As shown in Graph 4.3, all types presented a positive exponential curve with a moderate R2, in

agreement with the references as the increase of intact rock strength increases proportionally

with the bulk density. Clusters with low bulk density and low UCS values are suitable with

itabirite samples with fewer iron bands, and slightly weathered (W2) itabirites and hematitites

due to the slightly high relative total porosity. The R2 increases from FAI and HHE to FQI with

similar values.

FDI shows lower UCS values with mean equal 135 MPa and SD = ±64 MPa (moderate),

representing the lower value from all BIF even presenting low pore content and smaller crystal

size (0.005 mm to 0.87 mm). This lower intact rock strength is attributed to the presence of iron

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dolomite minerals (siderite and ankerite) which present higher ρb = 2.83 t/m3, when compared

with quartz ρb = 2.65 t/m3, and are considered weaker, in absolute Mohs hardness (also

compared with quartz), inducing a relative lower intact rock strength with no considerable bulk

density reduction. Additionally, the presence of low strength cement (calcite) could be I some

cases, responsible for the lower relative UCS mean value obtained.

The anisotropic ratio obtained for FDI (Rc = 2) is closer to medium anisotropy class and this

variation can be in somehow influence in this relationship.

For FQI, the most common itabirite, with the UCS mean equal to 173 MPa and SD = ±96 MPa

and FAI, with UCS mean equal to 172 MPa and SD = ±89 MPa presented the same UCS value. FAI

and FQI presented similar anisotropic ratios and consequentially strength values, interpreted as

being related to their similar mineral constitution, with quartz as the main non-iron mineral.

The HHE presents the higher bulk density but not the highest UCS mean values, although it

presented the highest individual the UCS test results present the mean equal to 159 MPa and

higher SD = ±134 MPa from all BIF, and a higher maximum UCS test result (UCSmax = 585). The

unexpected slightly lower UCS mean value obtained for the samples with higher ρb, lower total

porosity and absence of non-iron mineral bands is explained by the low number of reliable test

results obtained mainly for β90° and β45° that may bias the mean UCS values for this rock type.

This behaviour can locally be attributed to the influence of thin layers of penetrative, low

strength specularite or tabular hematite at shear surfaces, especially for samples with β90° and

β45°. These micro, highly anisotropic surfaces normally can only be observed in thin sections,

which creates challenges during laboratory selections and evaluations only detected by Vp

measure evaluations.

UCS and P wave velocity correlation

Jamshidi et al. (2018), present a summary table proving important correlation equations

between UCS and Vp obtained from several authors. The positive correlations and equations

obtained here are in accordance with the table proposed by these authors. From Graph 4.4,

HHE, FAI and FQI presented an expected positive exponential correlation curve.

P wave velocity is relatively lower for the itabirite group (FQI to FDI and FAI) with lower iron

content and relative lower bulk density and increase for the hematitite group (HHE) with higher

iron content and relative higher bulk density.

The negative correlation obtained for FDI is explained by the presence of iron dolomite mineral

as the main non-iron band mineral that present a high Vp value (±7,500 m/s), close to the

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hematite (Vp = ±7,400m/s) and much higher than quartz (Vp = 6,060 m/s) (Fourmaintraux 1976).

Higher Vp velocities for iron dolomite minerals and lower UCS results caused by this mineral

lower hardness, as already explained, can contribute to this atypical correlation. Thus,

mineralogy highly influences the correlation of UCS and Vp for FDI.

UCS and static Poisson’s ratio correlation

Graph 4.5 shows similar behaviour for FAI and FQI, presenting moderate R2 and expected

negative exponential correlation. It is possible to note that as proposed by Li (2014), if quartz

content increases, the Poisson’ ratio decreases as observed for FAI νstat = 0.2 and FQI νstat = 0.19

(quartz-based mineral at non-iron bands) compared with HHE νstat = 0.29 with n quartz bands

and FDI νstat = 0.22 with iron dolomites as the mains non-iron bands.

Little or no correlation were observed for Poisson’s ratio. It is partially attributed to the low

accuracy associated with the measurement of Poisson’s ratio as this parameter is sensitive to

the position of the strain gauges on the samples. For this feature, inaccuracies could be the cause

of the low correlation obtained as the gauges’ positions were not controlled. Additionally, the

deformation which each rock specimen was exhibited (deep and stress dependent) may

influence on this correlation (also not controlled in this research).

UCS and static Young’s modulus correlation

As shown in Graph 4.6, all BIF presented the expected exponential positive correlation curve.

The unexpected low R2 and low mean value obtained for the HHE Estat = 70 GPa (SD = ±61GPa) is

attributed to the presence of pore layers for W2 samples. The low correlation observed for FDI

is attributed to the presence of iron dolomite minerals, which reduces the intact rock strength

due to the relative lower Mohr scale of hardness (value between brackets) presented by calcite

(3) and dolomite (3.5) compared with quartz (7).

Arslan et al. (2008), present a summary table proving important correlation equations between

Estat and UCS obtained from several authors. The correlation and equations obtained here are in

accordance with table proposed by these authors.

Static Young’s modulus and P wave velocity correlation

As observed in Graph 4.7, all lithotypes presented positive exponential curves in accordance

with expected behaviour. The low correlation obtained for FDI is explained by the presence of

iron dolomite as already explained for UCS with Vp correlation.

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Static Young’s modulus and bulk density correlation

From Graph 4.8, HHE shown higher range and bulk density values, and in contrast, the FDI

presents a small range of bulk density values. The exponential positive correlations obtained for

BIF types are in accordance with expected behaviour and dataset size, as is expected an increase

in Young´s modulus with bulk density increase. The lack of correlation obtained for FDI is

explained by the presence of iron dolomite with higher mineral bulk density compared with

quartz also affecting the correlation; same as verified with other correlations.

P wave velocity and bulk density

Graph 4.9 shows an exponential positive correlations as expected, as Vp increases as ρb increases.

The high negative correlation for FDI is attributed to the high relative Vp of iron dolomite minerals

presented in this type when compared with other itabirites.

The regression curves obtained for the petrophysical and geomechanical property correlations

are summarised and outlined below for each BIF type:

• Hard hematitite

The hard hematitite, as almost monomineralic rock, is constituted mainly of iron oxides and

hydroxides (e.g. hematite and martite) with percentage of pores between 5% and 10%

(Table 4.1). The heterogeneous metamorphic banding is represented by layers with higher total

porosity interlayered with more massive ones, conditioned by the weathering (W2) and mineral

and pores orientation. The Rc and the IVp were isotropic to fairly anisotropic.

Considering anisotropy effects, the UCS varies from 210 MPa to 100 MPa and for the isotropic

mean is 159 MPa. High intact rock strength is mainly defined by the high ρb = 5.02 t/m3, low pore

content and small mineral size (0.001 mm to 0.08 mm). The Vp mean obtained of 7,022 m/s is in

average 30% higher than the other BIF This difference is attributed to the mineral constitute and

higher bulk density of this type.

The most reliable equations, with moderate R2, were obtained for all Vp correlations as presented

in Table 4.7. This pattern is attributed to the higher Vp and ρb obtained for this type compared with

the itabirites group. Estat and νstat proved not to be reliable correlation parameters mainly imposed

by the high SD obtained for UCS results where static elastic parameters are obtained. Since the

anisotropy effects are not important for HHE, the high SD noted could be attributed to the

sample’s intrinsic characteristics mainly by the presence of pores and tabular mineral microlayers

that induces early peak resistance in UCS tests.

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• Fresh quartzitic itabirite

FQI is the most common BIF lithotype, presenting compositional metamorphic banding

conditioned by concentrated quartz bands interplayed with iron bands representing the typical

itabirite heterogeneity. The degree of anisotropy obtained from different parameters (Rc and

IVp) were low and fair, respectively. The total percentage of pores is less than 11% (Table 4.1).

These higher percentage of pores is concentrated in quartz bands or at the contact between iron

and quartz bands inducing some anisotropy parallel to the compositional metamorphic banding.

These pores are generated by weathering (leaching the quartz and recrystallising different iron

ore oxides and hydroxides), mainly in slightly weathered lithotypes (W2). Increasing percentages

of the pore’s layers potentially (increasing the weathering in shallower depths or fractured

zones) generate preferential weathering paths that can increase the anisotropy and change to

moderately weathered lithotypes to completely weathered ones.

The high intact rock strength is mainly defined by the low pore content and higher percentage

of iron bands defined by the typical itabirite heterogeneity as presented in Figure 4.11. The

variation in intact strength is imposed by chances in the thickness of the compositional bands.

As the iron bands present similar crystal sizes for all itabirites, this characteristic seems to not

influence the UCS results as the mean values are very close (inside the SD).

FQI presented the best correlations from all BIF, with moderate to high coefficients of

correlation providing reliable correlation equations as observed in Table 4.7. The presence of

quartz, with lower bulk density and higher Mohr hardness, compared with iron dolomite band

minerals, may have contributed to this high correlation and high relative UCS mean values.

• Fresh amphibolitic itabirite

With high susceptibility to weathering, FAI is mainly observed in deeper mines. Consequently,

slightly weathered FAI (W2) is widely distributed across studied mines and determining its

geomechanical attributes was considered to be more important than determining those of FAI

(W1). The compositional metamorphic banding is conditioned by mineral concentration of

quartz and goethite (as cement) interplayed with iron bands representing the heterogeneity.

The Rc and Ivp obtained was low and fair, respectively. The mean total percentage of pores is 6%

for typical slightly weathered (W2) specimens as presented in Section 4.5.1.

FAI presents similar high intact rock strength, compared with FQI, mainly defined by the low

pore content and higher percentage of iron bands, as discussed in Section 4.6.1. In addition to

the presence of hematite and goethite cementation observed in this type.

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This type also presented moderate to high coefficients of determination providing reliable

correlation equations as observed in Table 4.7. The presence of quartz gives to FAI the same

characteristics observed for the FQI. Additionally, the presence of goethite (Vp = 6,950 m/s) may

induce a higher Vp compared with the FQI.

• Fresh dolomitic itabirite

FDI is the type most prone to weathering (leaching the dolomitic bands). The mean total

percentage of pores is 6% as presented in Table 4.1, and the Rc and the Ivp were low to fair,

respectively. However, this represents the higher values obtained from all types (class superior

limit). The different behaviours observed from other itabirites for the FDI can be partially

attributed to the compositional metamorphic banding inducing a slightly higher anisotropy,

which is in turn attributed to the presence of iron dolomite minerals with lower Mohr hardness,

and higher Vp and bulk density when compared with quartz, mainly at non-iron bands.

FDI presents the lowest UCS mean value from all BIF, even presenting relatively lower total

porosity and high ρb similar to FQI and higher from FAI. The same consideration about

heterogeneity addressed for FQI and FAI could be used for FDI. However, the main

characteristics that impose the different behaviour is the presence of iron dolomite minerals in

non-iron bands. These minerals impose negative correlation curves for Vp correlations attributed

to the high Vp and an intact rock strength reduction. Secondarily, the main cementation consists

generally of calcite (low strength cement) that could locally be responsible for lower relative

UCS values and mean obtained contributed to the reduction in intact rock strength and elastic

parameter.

Compared with other itabirites, the FDI presents the most different behaviour, as presented in

Table 4.7. In general, low to no correlations are observed, except for a moderate correlation for

UCS with ρb and very high negative correlation for Vp with ρb.

4.7 CONCLUSION

It is expecting a natural variation obtained experimentally for intact rocky material (even

considering the absence of microcracks), largely due to the randomness of geological and

eventual experimental errors, which, without a doubt, should manifest themselves even in

samples of the same lithology tested in a common anisotropy direction. However, the results

showed a variation beyond what was initially expected, even with all the criteria adopted to

eliminate outliers, which must be linked to the natural heterogeneity of the tested rocks, a fact

that must be considered in the analyses involving these materials.

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As experienced, geological features induce large dispersions of the results, since they may have

different variations in fabric, affected by rock heterogeneity and early influence of weathering,

inducing small but not irrelevant anisotropy effects, and variations in the petrophysical

properties of the samples (e.g. ρb and Øb). Even so, using several methodologies (samples

separation and statistical methods) it was possible to verify reliable trends of a correlation

mainly for HHE, FAI and FQI.

A field survey provided geological and geotechnical characterisation applied to group and

remove non-suitable laboratory test results from the dataset reducing bias and data variance.

This approach emphasised the role of identifying the geological characteristics that were crucial

to define a proper sample grouping to separate and better represent the three different types

of fresh itabirites. After this initial step, based on laboratory tests, it was possible to evaluate

the intact rock strength and its static deformability parameters. Thin sections added important

information to explain each rock fabric behaviour and correlation with results from laboratory

tests.

Additionally, even with several attempts to reduce data dispersion, the FDI some parameters

showed high SD. This may represent a natural range expected for each property and also be

attributed to dispersions caused by intrinsic geological features, or by the slightly higher

anisotropic effects observed for FDI.

Three main petrophysical characteristics in the rock fabric were found to be responsible for

inducing penetrative planes that could result in low anisotropies for fresh BIF:

• Heterogeneity – the contact between iron and non-iron minerals bands, which define

the itabirites compositional metamorphic banding.

• Mineral orientation – planar or granular minerals oriented along a single or multiple

directions which also define the metamorphic banding.

• Pores concentration and orientation – oriented porous layers defining the hematitite

metamorphic banding.

Due to the high SD observed for UCS results, the importance of transverse anisotropy for intact rock

strength in each type must be considered with some reservation. Anisotropy effects due to the

compositional banding (heterogeneity) for fresh itabirites (FAI and FQI) and pore concentration

banding for hard hematitites cause minor effects on the intact rock strength. However, as FDI

presents low anisotropy but with a range close to the medium anisotropy class and different

correlations from those obtained for other itabirites, the anisotropy must be considered.

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From the research finding, it was possible to group fresh BIF into two similar behaviour types:

• Hematitite group – mostly for hard hematitite (HHE), is defined as the isotropic type,

mineralogically homogeneous but with metamorphic banding heterogeneity is defined

by different percentages of pore bands. This group is characterised as the highest bulk

density, P wave velocity, and high UCS values. For this type, the empirical correlations

between UCS with ρb and P wave velocity, and between Vp with ρb and Estat showed

reliable correlation equations.

• Itabirite group – includes all the three fresh itabirites (FAI, FQI and FDI) that showed

several similarities in their definition and characterisation. However, some

particularities can be used for individualisation. A characteristic of this group is the

lower relative ρb, and UCS values and elastic parameters dispersion deeply associated

with the itabirite heterogeneity (defined by the mineral composition and band

thickness), and the low anisotropic behaviour.

This itabirite group was divided into two subgroups defined by the main mineral present in the

non-iron band:

• Quartz based itabirites (FAI and FQI) – low anisotropic material present higher UCS

values and ρb presented reliable equations for all evaluated correlations.

• Iron dolomite based itabirite (FDI) – Presents Ivp and Rc slightly closer to the medium

anisotropy class. This subgroup showed low ρb and UCS results, and few reliable

equations due to the obtained correlations generally in opposition to the other itabirites.

This behaviour is attributed to the intrinsic characteristics of the iron dolomite minerals

with present higher relative Vp, and lower Mohr hardness scale compared with quartz

inducing a lower intact rock strength.

UCS test results proved to be adequate for distinguishing the intact strength variance, from

homogeneous and almost isotropic hard hematitite to heterogeneous and low anisotropic fresh

itabirites, and effective in distinguishing the changes between different compositional itabirites.

Due to the general low porosity of fresh BIF (Øb<5%), the bulk density is directly correlated to

the iron content and an increase in the intact rock strength is proportional with the increase of

iron bands in the specimen. This proportionality was not deeply evaluated in this research.

However, it is suggested that the correlation between UCS and ρb could be used to infer the UCS

values as presented in Appendix I.

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Moderate (in general) to high and very high coefficients of determination and adjusted regression

curves provided reliable equations for FAI and FQI and made it possible to claim that petrophysical

and elastic properties of the intact rock obtained from dynamic tests, were useful for supporting

the correlations and determining the intact rock strength and deformability parameters of these

types. By contrast, FDI and HHE presented low correlation variations imposed by the intrinsic

characteristics that should be considered when evaluating these types of correlations. These lower

correlations could be related to the Vp being more sensitive to elastic modulus than bulk density

as proposed by Marques et al. (2010). Even the low coefficient of determination observed for some

lithotypes represents useful data for describing geomechanical behaviour.

Findings from the experimental works and basic statistical analyses showed that this study

provided a reliable method of predicting the BIF typology, UCS and elastic parameters and

indirectly establish petrophysical characteristics for ρb, Øb, and Vp.

The field geological and geomechanical investigations, thin sections and laboratory tests

undertaken during the research programme enabled the assessment of the effect of physical

heterogeneity and anisotropy on the geomechanical properties, intact rock strength and elastic

parameters of the BIF and allowed for the development of procedures and equations that will

optimise slope stability evaluations, promote a better understanding of potential slope failure

mechanisms and reduce the risk of slope failure – gains that will help Vale and other iron ore

mines improve the operational productivity and safety of their mines.

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CHAPTER 5. PETROPHYSICAL CHARACTERISTICS AND ELASTIC DYNAMIC

PROPERTIES OF FRESH TO MODERATELY WEATHERED BRAZILIAN BANDED

IRON FORMATIONS

This chapter presents the second unpublished manuscript.

ABSTRACT

Over recent decades, a worldwide iron ore boom has fuelled a rapid increase in pit depths of

Brazilian iron ore mines, especially at mines located in the Iron Quadrangle. This has been due to

environmental restrictions and urban pressure on the lateral expansion of mines. For this reason,

the weak material has been mined and the presence of fresh banded iron formations on final

slopes has become more plentiful, and failure mechanisms and slope instability controlled by

discontinuities and hard rock behaviour have become more common. To support this new

challenge, in mines where instabilities were mainly composed of hard rock materials, rock

mechanics approaches became necessary. These approaches must be based on reliable

geological, structural and geotechnical investigations supported by laboratory test databases.

Strength and elastic properties of rock materials are important parameters used for limit

equilibrium and stress–strain analyses in slope stability and design. It is possible to obtain these

parameters by fast, indirect, and non-destructive dynamic tests supported by empirical

correlation methods, rather than destructive tests that require high-quality cores and

techniques.

An extra challenge to determine these parameters and correlations is posed by the anisotropy

and heterogeneity observed in banded iron formations BIF that can define different parameters

for different anisotropy directions.

Based on geological and geotechnical field investigation, thin sections evaluation, P and S wave

velocity propagation, total porosity and bulk density tests, from a special apparatus (poro

permeameter – AutoLab-500®) this study proposes empirical correlations between dynamic

elastic parameters (Young´s modulus and Poisson’s ratio) and petrophysical parameters (bulk

density and total porosity), for fresh and moderately BIF.

For these proposals, five lithologies have been tested in three different anisotropy planes, defined

by the compositional metamorphic banding (heterogeneity), to provide the anisotropic index and

identify elastic modules and petrophysical correlations.

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Results show that mineral composition, rock fabric, total porosity and bulk density have the

greatest influence on anisotropy index and elastic (dynamic) properties. The evaluated

assemblage of BIF types was grouped by similar behaviour:

• Fresh to slightly weathered itabirites group, with typical high range for wave velocity,

Young´s modulus values and bulk density, and total porosity.

• Fresh hematitite group, with higher wave velocity, Young´s modulus values and bulk

density, and very lower total porosity.

• Moderately weathered itabirites group, with lower wave velocities, Young´s modulus

values and bulk density, and higher total porosity.

The anisotropy index was isotropic for dolomitic itabirite, and fairly anisotropic for hard

hematitite, fresh quartzitic and amphibolitic itabirites, defined by the mineral composition, rock

fabric, and total porosity. For moderately weathered quartzitic and goethitic itabirites the

anisotropy index is higher (moderately anisotropy index) due to the higher total porosity and

lower bulk density as the weathering process increases. However, this behaviour could be

affected by iron oxides or hydroxides remobilisation and cementation, depending on the

weathering level.

The use of correlations graphs to evaluate dynamic elastic characteristics, petrophysical

properties and predict correlation equations to define geomechanical parameters has proved to

be a reliable methodology to estimate the lithotype that is directly associated with these

geomechanical characteristics. Additionally, the weathering horizons (degree) and

heterogeneity were defined as an important feature that controls anisotropy, bulk density,

elastic parameters and, ultimately, the intact rock strength of the BIF.

5.1 INTRODUCTION

Over recent decades, a booming worldwide iron ore market has put pressure on local and global

iron ore mines to increase their production by - innovative technologies, opening new mines,

and expanding existing mines, not only in width but mainly in depth, to meet this high demand.

For this reason, after decades of mining soil-like and weak rock from the deep weathered

profiles and evaluating slope stability analyses based on soil mechanics principles, Brazilian iron

ore mines are now experiencing new challenges related to deep open pits, where fresh and hard

rock predominate.

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These iron ore mines are developed in BIF deposits, originally described by Dorr (1969) as a high-

grade ore, called hematite (most recently denominated hematitite) and a low-grade ore, called

itabirite. The original mineralogical composition of non-iron bands, consisting of dolomitic,

amphibolitic and quartzitic layers are used to differentiate itabirite types. In addition, those

lithotypes are the hardest (fresh) BIF seen in Iron Quadrangle iron ore mines.

These four fresh and hard BIF lithotypes, and two subtypes derived from moderately weathered

alteration, were tested in three different anisotropy planes, defined by the metamorphic

banding (heterogeneity plane), to prove the anisotropic index, identify elastic rock properties,

and establish petrophysical (physical rock properties) correlations.

These BIF are easily recognised in mines and the proper assessment of intact rock strength and

elastic parameters is essential for implementing robust geotechnical models for engineering

purposes and slope stability analyses. In addition, for two moderately weathered itabirites

(quartzitic and goethitic) the intact rock strength reduction imposed by weathering reduces the

original fresh and hard behaviour becoming a mixture of hard and weak material. Characterising

such behaviour will be essential to understand this ambiguous comportment, define the

boundary between soil and rock materials, and apply the proper geomechanical approach for

each case.

For decades, in Brazilian iron ore mines the slope stability analysis and design reviews were

based on limit equilibrium techniques supported by the soil mechanics approach. For deep and

hard rock mines, this approach can be insufficient, and to address this problem rock mechanics

must be considered and more robust computational approaches are necessary where the elastic

parameters (Young's and Poisson’s ratio) are important inputs for these applications.

The acquisition of such data using classical rock tests can be time-consuming, costly and,

sometimes, not feasible due to the difficulty of obtaining reliable representative samples from

the field or due to the difficulty of preparing proper samples for laboratory tests or even to the

rock test apparatus setup difficulties (e.g. position of gauges and anisotropy). Otherwise,

dynamic testing is a reliable, low-cost, fast, and non-destructive analysis used to provide

effective and reliable correlations capable of replacing traditional static mechanical tests, such

as uniaxial compressive strength (UCS). Nevertheless, the calibration and analysis of dynamic

effects also requires a minimum number of static laboratory tests.

Based on geological and geotechnical field investigation, thin sections evaluation, P and S wave

velocities propagation, total porosity and bulk density tests undertaken in a high pressure pore

permeameter (AutoLab-500®) this study proposes empirical correlations curves and equations

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capable of defining dynamic elastic parameters (Young´s modulus and Poisson’s ratio) and

petrophysical parameters (bulk density and total porosity), for fresh and moderately weathered

Brazilian BIF.


Many studies have been undertaken to determine and establish relationships among

petrophysical and dynamic properties for several rock types. From these, only a few studies have

been focused on BIF. This chapter presents petrophysical and dynamic property results for

several Brazilian BIF, with the aim of establishing correlations through the regression technique

(linear or non-linear). The degree of association between these variables was measured by the

coefficient of determination in an attempt to derive reliable empirical approaches for a better

comprehension of geomechanical and anisotropy behaviour, and to provide basic statistical

correlations.

The main aim of this chapter is to evaluate elastic intact rock and petrophysical properties based

on laboratory tests and geological/geotechnical field evaluations and establish correlations for

fresh to moderately weathered BIF.

The main statements to be evaluated in the study are summarised below:

• Evaluate and compare from different types of BIF the dynamic elastic parameters and

geological and geomechanical features.

• Evaluate the influence of the anisotropy and heterogeneity, defined by the

metamorphic banding, in the dynamic elastic parameters.

• Establish a correlation between petrophysical, geological (macro and micro properties)

and geomechanical characteristics with elastic dynamic parameters.

To address this work, this PhD research project was undertaken by the lead author at the

Australian Centre for Geomechanics, School of Civil, Environmental and Mining Engineering, The

University of Western Australia, sponsored by Vale S.A. The main aim of the thesis was to

investigate the complete weathering profile characteristics, from fresh and hard to weak

residual soil of BIF from several Brazilian iron ore mine sites.

This chapter is focused on dynamic intact rock properties, trying to understand the effect of

petrophysical heterogeneity and anisotropy, and attempts to provide practical estimates and

realistic correlations that could replace the lack of intact rock strength parameters. In order to

fully represent the geological influence, these correlations consider dynamic intact rock

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properties such as mineralogical composition, fabric, bulk density, iron content and other

features.

The heterogeneity and anisotropy defined by the BIF compositional metamorphic banding have

been studied and discussed to address the limitations and drawbacks imposed by compositional

metamorphic banding in the BIF. Because of the research emphasis on intact rock properties,

anisotropy caused by discontinuities (e.g. fractures and joints) was not evaluated.

The emphasis of this research is focussed on the following unweathered BIF lithotypes:

hematitite (HHE), fresh dolomitic (FDI), amphibolitic (FAI) and quartzitic (FQI) itabirites, also

includes moderately weathered materials defined as partially weathered goethitic (PWGI) and

quartzitic (PWQI) itabirites. These rock types were characterised based on four key laboratory

tests: bulk density (ρb), petrographic thin sections, total porosity (poro-permeameter test), and

VP and VS wave velocity determination. The tests results have been summarised and reviewed

to assess the associations that are further identified and explored between these parameters

and elastic dynamic parameters.



The studied mines are located at the western limb of the Iron Quadrangle in the Moeda Syncline

and Curral Homocline ranges located in the centre of the state of Minas Gerais, Brazil, and part

of Vale’s South Ferrous Division. The seven evaluated mines are Águas Claras (MAC), Capão

Xavier (CPX), Tamanduá (TAM), Capitão do Mato (CMT), Galinheiro (GAL), Pico (PIC), and

Jangada (JGD), with locations shown in Figure 5.1.

The Iron Quadrangle forms a mega structure, which is delineated by a roughly quadrangular

arrangement, with ridges of Paleoproterozoic BIF from the Minas Supergroup, as proposed by

Dorr (1969). These iron deposits are classified as Superior Type, according to Gross (1980) and

are composed of 1,000 m of metamorphic iron ore rocks belonging to the Itabira Group, Cauê

Formation. The Minas Supergroup includes, from bottom to top, Caraça, Itabira, Piracicaba, and

Sabará groups, as a sequence of psammitic pelitic rocks, superimposed by the Itacolomi Group.

Below that sequence are the Archean greenstone terrains of the Rio das Velhas Supergroup and

domes of Archean and Proterozoic crystalline rocks (Machado & Carneiro 1992; Machado et al.

1989 and Noce 1995).

118

According to Chemale Jr et al. (1994), the regional structure is the result of two main

deformational superimposed events. The first produced the nucleation of regional synclines in

the uplift of the gneissic domes during the Trans–Amazonian Orogenesis (2.1–2 Gyr), and the



structures and was responsible for the east–west deformational gradient.


deformational gradient from greenschist to the lower amphibolite’s facies.

The Cauê Formation (Itabira Group) represents the main host of BIF and defines a marine

chemical sequence of 350 m thick, dated 2.4 ± 0.19 Gyr (Babinski et al. 1995).

The Iron Quadrangle BIF are Paleoproterozoic, metamorphic and heterogeneous banded rocks,

presenting a millimetre to centimetre rhythmic alternation banding of iron minerals (hematite,

martite and magnetite), and non-iron minerals (quartz, dolomite, and amphibolite). Two main

lithologies were defined by Dorr (1969) for this type of iron deposit: hematite, recently

redescribed by Selmi et al. (2009) as hematitite – the high-grade ore (Fe ≥ 62%), and the

low-grade ore, the itabirite (30% < Fe < 62%), with three compositional types or lithotypes,

according to the presence and relative abundance of gangue minerals: quartzitic, dolomitic and

amphibolitic (all defined as proto-ore). Tectonic, metamorphic, and weathering processes have

changed it in different ways, resulting in multiple sets of iron ore lithotypes.

Banding is the most typical characteristic defined by strong heterogeneity. This variation could

be controlled by the original sedimentary bedding, tectonic setting, metamorphic grade,

hydrothermal, or supergene processes. However, the superposition of these processes causes

partial or total mineralogical and textural changes.

The origin of itabirites and associated high-grade hematitite orebodies remains controversial,

and several studies have been produced on this topic, as largely discussed in Spier et al. (2003).

For the genesis of friable orebodies, some authors agree with a supergene process and residual

itabirite enrichment from the leaching of gangue mineral by surface waters. For such orebodies,

the 40Ar/39Ar dating of manganese minerals in Vale mines suggests that weathering processes

and the mineralisation period occurred between 61.5 ± 1.2 Myr to 14.2 ± 0.8 Myr, reaching the

peak process in 51 Myr (Spier 2005 and Spier et al. 2006). This dating indicates a tertiary

mineralisation, and after this period the further weathering may not have substantially affected

the weathering profile.

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The Moeda Syncline (MAC, CPX, TAM, CMT, GAL, and PIC mines) has been partially affected by

the younger Brazilian tectonic cycle and mainly in the eastern limb with the local development

of ductile–brittle to brittle shear zones that cut all lithologies or are subparallel to the bedding

planes. Several strike-slip faults cut across the structure dividing it into several segments.

Serra do Curral (MAC and JGD) represents the overturned southeastern limb of a truncated

northwest verging syncline–anticline couple highly strained and rotated by the right lateral

movement of the northeast–southwest trending oblique ramp of thrust fault as supported by

Chemale Jr et al. (1994). In this segment, the northwestward inverted limb of a syncline is

truncated near to the contact of the Minas Supergroup in the underlying Rio das Velhas

Supergroup by shear zones related to the thrust.

24 Figure 5.1 Studied mines localisation and Iron Quadrangle geological settings (after

Baars & Rosière 1997)


Compositional metamorphic banding is the most typical itabirite characteristic and is considered

to define a heterogeneity. This variation could have been controlled and disturbed by the

original sedimentary bedding, tectonic setting, metamorphic grade, hypogene or supergene

processes and weathering as the superposition of these processes causes partial or total

mineralogical and textural changes. Studies of BIF conducted by Rosière et al. (2001) for the

western side of the Iron Quadrangle indicate a very low internal deformation by plastic flow of

the minerals with the ductile deformation accommodated mostly by buckling and flexural gliding

with local development of a spaced cleavage. A continuous schistosity and an anastomosing

120

foliation occur, related to discontinuously developed shear zones. Brecciated zones are usually

related to sites of local water pressure build-up, during the percolation of hydrothermal fluids.

Not considering the controversy about the supergene (groundwater leaching) or hypogene

(hydrothermal water leaching) genesis for these large, rich, and weak Iron Quadrangle ore

deposits, this study is focused on the three main hard and fresh itabirites, two moderately

weathered itabirites, and hard hematitite, produced either tectonically or by a hydrothermal

event. This variation is conditioned by the original composition of sediments, the intensity of

deformation, the degree of metamorphism and the hydrothermal alteration imposed. Tectonic

structures produced a large variety of textural features due to the development of discontinuity

planes, and microstructures resulting from different tectonic types of itabirite, and the

weathering contributes to rock strength reduction.

Hematitite and itabirites are locally subdivided by using rock strength characteristics (hardness),

rated in technological crusher laboratory tests, used to simulate the industrial process for better

control of mining operations, which are also used for geotechnical purposes as a rock strength

index. This test consists of crushing a sample to less them 31.5 mm and passing it through the

6.35 mm sieve, resulting in three main different lithotypes: hard (more than 50% above 6.35

mm); medium (50% to 25% above 6.35 mm) and weak (less than 25% above 6.35 mm).

Vale’s crusher classification has been used as a guide to identify weathering grade and rock

strength index (hardness), where typology is defined as hard, representing fresh to slightly

weathered material; medium, representing moderately to highly weathered material; and weak,

representing completely weathered to residual soil. It could also be associated with the field

intact rock strength as hard, representing extremely hard to medium-hard material, medium, as

medium-hard to medium-soft, and weak, as medium-soft to extremely soft material, according

to ISRM (1981) tables.

Typically, itabirites present small mineralogical variety: hematite, martite, magnetite,

specularite, goethite, and ochreous goethite are respectively the most important iron minerals.

Quartz, iron dolomite, gibbsite, and kaolinite (weathering minerals), are the main gangue

minerals; talc, chlorite, and pyrolusite are the main accessory minerals. Several studies

described the mineralogical and fabric correlation with geological association for Iron

Quadrangle mines, including Rosière et al. (1993, 1996 and 2001), Lagoeiro (1998), and Pires

(1995).

The main geological and geotechnical macro characteristics of the studied lithotypes are

described as follows.

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Hard hematitite

The hard hematitite (HHE) presents higher intact rock strength and iron content; however, its

genesis is a subject of controversy. It is postulated that hard-massive ore bodies resulted from

hipogenetic iron remobilisation (Pires 1995) or from initial concentration on rich sedimentary

bedding (less expressive bodies). Ore bodies are correlated to shear zones, planes interceptions,

and metamorphic contact with intrusive dikes presenting strong anisotropy due to millimetre

tectonic foliation. Each domain presents a typical texture defined by Varajão et al. (2002) as the

following typologies:

• Massive – present no specularite banding or foliation.

• Banded – present centimetric bands with casual specularite.

• Foliated – present thin foliation with wide presence of specularite.

HHE is a dark grey metallic homogeneous rock with the highest-grade ore compounded by

granular martite and microplates of hematite as the principal iron mineral followed by

magnetite, quartz, and goethite. Typical dark metallic colour is observed for massive types, and

more opaques bands are observed for banded types.

Weathering is responsible for minor mineralogical changes, concentrated on the oxidation of

open discontinuities or through porous layers changing, in accordance with ISRM (1981), varying

from W1 to W2 at surface or in very fractured deep zones. Average ρb is 4.4 t/m3, and natural

moisture content is an average of 1% (Santos 2007).

As suggested by Varajão et al. (2002), at Capitão do Mato Mine, total porosity can reach 11% for

slightly weathered HHE, whereas for fresh HHE, this could be less than 2.5%. Crystal size varies

from 10 μm to 30 μm for hematite and martite granular crystals and is equal to 1 μm for

microplates of hematite. It is also suggested that primary micro pores vary from Å to 1 μm, with

the most important secondary porosity being associated with martite crystals varying from Å to

5 μm.

Figure 5.2A shows microphotography of granular martite and tabular hematite (light and dark

grey) and large pores (black) and Figure 5.2B, massive HHE on the outcrop (banded and

fractured).

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(A) (B)

25 Figure 5.2 A (left), HHE at microscope view with granular crystal of hematite (light grey)

and smaller crystal of hematite micro plates (light grey) (Horta & Costa 2016).

B (right), outcrop of fractured HHE at Capitão do Mato Mine


Fresh quartzitic itabirite (FQI) is defined by the typical heterogeneity given by the interbedding

of quartz and hematite mineral bands and interpreted as an anisotropic rock that is generally

deformed (folded or faulted). Non-iron chert band was totally metamorphosed to quartz, and

iron bands are composed of hematite, martite, and martitised magnetite.

Quartz layer fabric is granoblastic to lepidogranoblastic. Crystal shapes are euhedral due to their

metamorphic level and present crystal sizes ranging from 10 µm to 120 µm. Hematite layers

present tabular and granular shapes and crystal sizes between 6 µm to 80 µm. Total porosity is

very low (less than 5%), moisture content reaches 10% and the ρb is 3.1 ± 0.29 t/m3, as presented

by Santos (2007).

These rocks present high intact strength, especially for fresh materials (W1). However, when

some initial weathering is present (fractures or banding), it is defined as W2 (ISRM 1981a).

Figure 5.3A shows microphotography of FQI presenting typical granular quartz banding (large

coloured crystals), granular to tabular hematites (light grey), and pores (black). Figure 5.3B

shows a typical fractured mine slope.

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(A) (B)

26 Figure 5.3 A (left), microphotography of FQI presenting typical quartz bands (large

coloured crystal), granular to tabular hematite band (light grey) and pores

(black) (Horta & Costa 2016). B (right), typical outcrop presenting fractures in

Tamanduá mine


Moderately or partially weathered quartzitic itabirite (PWQI) presents a mineralogical

composition similar to FQI, except for an extensive presence of goethite that suggests an

increase in weathering. Quartz layers present higher total porosity. In some instances, the quartz

bands can be disaggregated by partial leaching (some layers can reach total porosity >40%)

(Figure 5.4B). Leaching intensification determines an increase in total porosity, iron content, and

percentage of goethite and ochreous goethite. However, intact rock strength and crystal

cohesion decrease, especially in quartz bands.

The weathering degree generally is W3 (moderately) and W4 (highly) for a rock strength of R2

and R3 (ISRM 1981a).

Figure 5.4A shows a typical microphotograph of PWQI with a fracture filled by goethite and

Figure 5.4B a mine outcrop.

Hematite band

Quartz band

1,000 μm

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(A) (B)

27 Figure 5.4 A (left), PWQI microphotography highlighting presence of goethite cementing

a fracture (Horta & Costa 2016). B (right), an overview of typical PWGI slope

at Tamanduá mine


Fresh amphibolite itabirite (FAI) presents a heterogeneity defined by layers of hematite, martite,

and goethite alternated with quartz, goethite, and rare amphibole (grunerite, tremolite,

actinolite, and others) bands (Figure 5.5A). Band textures are lepidogranoblastic to granoblastic,

and crystal size is 30 µm on average. The original mineralogy, with amphibole, is preserved only

for fresh type (W1) at very high depths, where a typical brownish-yellow colour is present. It

presents a very low total porosity (less than 5%), moisture content of 10% and ρb of

2.8 ± 0.46 t/m3, as presented by Santos (2007).

Due to weathering, amphibole minerals quickly change to fibrous goethite, and due to the depth

of the weathering profile, it is difficult to obtain fresh FAI (with amphibole) at low depths, which

has not been influenced by some mineral degradation. For this reason, in this chapter, the

so-called ‘fresh FAI’ will also consider the slightly weathered FAI (W2), even in the absence of

amphibole minerals, now replaced by fibrous goethite.

Considering the intact rock strength, this type is strong and even considering the difficulty in

obtaining samples of W1, this lithotype is still strong (W2), where some weathering and

discolouration (dark yellow or brown) could be observed in fractures and banding layers, as

shown in Figure 5.5B.

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(A) (B)

28 Figure 5.5 A (left), FAI microphotography highlighting fibrous goethite and amphibolite

acicular old crystal (dark fibre minerals) immersed in quartz bands (Horta

& Costa 2016). B (right) typical mine slope folded and fractured FAI at Jangada

mine


The moderately or partially weathered goethitic itabirite (PWGI) presents a mineralogical

composition close to FAI excepts for the absence of amphibolite represents the W3 (moderately)

to W4 (highly) weathered FAI, R2 to R3 rock strength degree (ISRM 1981a), presenting a

significant intact rock strength reduction and there is a significant increase in total porosity and

percentage of goethite, ochreous goethite, and quartz, covered by a thin layer of goethite

(Figure 5.6A). The colour of this type becomes yellow to orange (Figure 5.6B). Mean

ρb = 2.7 ± 0.45 t/m3 (Santos 2007).

(A) (B)

29 Figure 5.6 A (left), PWGI at microscope view showing in red orange the large amount of

goethite (Horta & Costa 2016). B (right), typical PWGI from Jangada mine slope

126


As presented by Rosière (2005), the heterogeneity is defined by millimetric to centimetric pink

or white iron dolomite (siderite and ankerite) and a lower percentage of iron carbonates and

quartz bands, interbedded with dark grey iron bands, constituted by tabular hematite, martite,

and martitised magnetite. Accessory minerals are sericite, talc, and chlorite. Non-iron minerals

vary in size from 2 µm to 15 µm and iron minerals present crystal size varying from 5 µm to

20 µm as shown in Figure 5.7A.

The fresh dolomitic itabirite (FDI) presents a very low total porosity (less than 5%), moisture

content around 5%, and ρb = 3.2 (±0.36) t/m3 (Santos 2007). FDI is also strong rocks and very

fresh material (Figure 5.7B), typically W1.

(A) (B)

30 Figure 5.7 A (left), FDI microphotography highlighting typical banding of dolomite and

quartz (light colour) interlayered by bands of hematite and quartz (dark

colours) (Horta & Costa 2016). B (right), typical folded FDI hand sample

5.3.3 P and S wave velocities, dynamic elastic and petrophysical properties

correlations

P and S wave velocities

Defined by Timoshenko & Goodier (1951) as cited in Moraes (2018), strain can be defined as

changes in geometry, orientation, volume, and position induced by a load or unload of stress

over a certain period of time. For each rock type (homogeneous or heterogeneous) and load

(constant or variable) applied, it is possible to write the constitutive stress–strain laws resulting

in various deformability parameters. Such parameters are derived directly (in situ or in

laboratory testing) and indirectly (rock mass classification using correlations). Laboratory tests

are categorised into static, in which samples are submitted to loading or unloading that induces

different equilibrium stresses; and dynamic, in which elastic parameters are obtained as a

500μm

127

response from a cyclic, rapid energy source such as acoustic wave emission that propagates the

rock material cyclically.

Using a laboratory test, as discussed at Kearey, et al. (2009), elastic modulus can be indirectly

obtained by calculating acoustic emission velocity (seismic waves) within the rock specimen, as

a result of particle vibration during energy release when triggered by an energy source. Such an

energy pulse is small, elastic, and independent of wave frequency. However, it differs from the

elastic modulus and material density and can be divided into two types: compressive waves or

P wave, that moves through the propagation direction (uniaxial strain), and shearing waves or

S wave, that moves perpendicular to the propagation path. The shear wave can be separated

into two polarised waves (horizontal and vertical), in relation to the transverse isotropy plane,

where the low velocity wave propagates perpendicular to the anisotropy plane (horizontal

polarisation) and the high velocity wave propagates parallel to the axis of anisotropy (vertical

polarisation).

Kearey et al. (2009) explain that wave velocity is also a function of the mineralogical

composition, texture, porosity, and presence of fluids. These factors severely affect the P wave

velocity (Vp), whereas the S wave velocity (Vs) did not propagate through discontinuities or

empty spaces. Several characteristics of Vp and Vs for common lithotypes are presented below:

• Felsic igneous rocks have lower velocities relative to mafic rocks.

• The velocity reduces as the rate of discontinuities (fractures) increases.

• Porous sedimentary rocks present velocity dispersion associated with the porosity and

density correlation, with higher velocities for high density and low porosity and lower

velocities for low density and high porosity. It also depends on the cementation grade,

consolidation, and other factors.

• Low velocities are registered for non-cohesive soils due to low crystal contact and high

porosity.

• In contrast with those with smaller crystals, lithotypes with the same mineral

composition but with larger crystals present higher velocities.

• The presence of fluids (water, oil, and air) filling pores (partially or completely)

representing a water pore pressure mainly influence the Vp. For high water pore

pressure, the velocities are higher when compared with low water pore pressure. For

low saturation degrees, the wave velocities are higher, while high saturation is lower.

128

In this context, the Vp is thus higher at rock matrix and lower at fluids and the air. The

Vs in the fluids generates minor changes as they do not generate shearing resistance.

• Higher wave velocity occurs parallel to the anisotropy direction and lower velocity is

observed in the direction perpendicular to anisotropy in metamorphic rocks. These

changes are associated with mineral and porous alignment, and the set of

discontinuities. Lower velocities are controlled by the weakness planes along banding

and schistosity. Otherwise, higher velocities are associated with the strongest crystal

contacts along the anisotropy direction.

• For the same reason, the Vp anisotropy decreases with stress and increases with

temperature. This effect may be due to the widening of pores and microcracks during

the expansion of the sample due to thermal stress.

Acoustic wave propagation velocity has been used to determine intact rock strength for several

typologies by researchers such as Inoue & Ohomi (1981) for weak rocks, Gaviglio (1989) for rock

density, Kaharaman (2001) and Yasar & Endogan (2004) for carbonate rocks. Based on their close

relation to the intact rock properties, structure, and texture, these methods were used by the

many mining companies in the world. Later studies and subsequent static tests (standard rock

strength tests) confirm static and dynamic comparisons (expressed with several formulas) with

the non-destructive laboratory test aspect of sonic wave measures. Furthermore, the low costs,

repeatability, and speed of acoustic emission tests are attractive characteristics.

Dynamic elastic waves that propagate through saturated or dry rock samples can be measured

in the laboratory by registering the transit time of each wave through the sample’s axial length

(Bourbié et al. 1987).

Classical rock mechanics provides various techniques for determining the elastic parameters,

the most widely employed being stress–strain curves that are obtained from destructive

compressive and tensile rock tests. Most recently, non-destructive tests based on acoustic wave

velocity propagation measures have been applied to obtain such parameters. The following are

benefits of using dynamic approaches: non-destructive, higher flexibility on sample geometry,

low cost, faster than destructive laboratory testing, and easier to perform the tests. Some

drawbacks can also be described, such as: obtained results are implicitly correlated with elastic

parameters and are directly related to the reliability of the database and the correlation curves

and equations used.

Additionally, acoustic emission can be coupled with laboratory static tests, facilitating constant

monitoring in different directions during the stress level increase.

129

As stated in Bloch et al. (1994), it is possible to calculate the dynamic elastic constants such as

Young’s modulus (Edyn), Poisson’s ratio (νdyn) based on compressional wave velocity (Vp), shear

wave velocity (Vs) and the sample’s bulk density (𝜌𝜌b) by the application of the elasticity theory.

Equations 5.1, 5.2, 5.3 and 5.4 show the relationship between dynamic elastic constants, bulk

density, and elastic wave velocities:

Dynamic Poisson’s ratio equation:

𝑣𝑣𝑑𝑑𝑑𝑑𝑚𝑚 =(𝑉𝑉𝑝𝑝

𝑉𝑉𝑠𝑠� )2−2

2��𝑉𝑉𝑝𝑝

𝑉𝑉𝑠𝑠� �2−1�

(5.1)

Dynamic Young’s modulus equation:

𝐸𝐸𝑑𝑑𝑑𝑑𝑚𝑚 = 𝜌𝜌𝑉𝑉𝑠𝑠2(3𝑉𝑉𝑝𝑝2−4𝑉𝑉𝑠𝑠2)(𝑉𝑉𝑝𝑝2−𝑉𝑉𝑠𝑠2)

(5.2)

Compressive wave velocity (Vp) equation:

Vp2 = 𝐸𝐸𝑑𝑑𝑑𝑑𝑑𝑑(1−υ)ρb(1+υ)(1−2υ)

(5.3)

Shear wave velocity (Vs) equation:

Vs2 = 𝐸𝐸𝑑𝑑𝑑𝑑𝑑𝑑2ρb(1+𝑣𝑣𝑑𝑑𝑑𝑑𝑑𝑑)

(5.4)

As presented in Deere & Miller (1966), it must be assumed that elastic constants determined

using acoustic wave emission (which defines the elastic behaviour) are dependent on rock type

variation as, mineralogical composition, rock fabric and structure, crystal size and shape,

density, porosity, degree of anisotropy, porewater, confining pressure, moisture content, stress

amplitude, rate and duration of loading, temperature, weathering and alteration zones, and

joint properties, and defining the most important variable could be a difficult task.

As described in Moradian & Behnia (2009), the Vp/Vs ratio has been used for oil reservoir

engineering to determine porosity, bulk density and identifying different lithologies.

Nonetheless, this ratio is very sensitive to saturating pore fluids and tends to be 10–20% lower

when the samples are dry. Carbonate rocks seem to be highly influenced by heterogeneity,

particularly by texture and porosity.

Additionally, one of the first works carried out with the objective of determine the lithology of

rocks is based on the ratio between Vp/Vs as presented by Pickett (1963). Carrying out a study

on consolidated rocks of different porosities, Pickett concluded that clean sandstones have a

Vp/Vs ratio between 1.6 and 1.7, while limestones have a value of 1.9, and dolomites 1.8. This

130

ratio is independent of the density of the rock, whereas analysing only the speed of the Vp wave,

the lithology indicator could be considered ambiguous, since Vp is a function of other physical

rock properties as total porosity, mineralogy, and rock fabric.

Recent works from Motra & Stutz (2018) suggest that, for metamorphic anisotropic rocks (quartz

mica schist and amphibolite), the velocity anisotropy can be linked to the texture and anisotropy.

To determine the anisotropy effects, studies by Kern (1993); Punturo et al. (2005); and Motra &

Wuttke (2016) using S wave velocity propagation with different polarisation (parallel and

perpendicular), enables the determination of the amplitude of shear wave splitting, which is a

function of anisotropy, to measure the spatial dependency of wave velocities (anisotropy),

particularly for metamorphic rocks.

The work of Panakkal et al. (1990) showed a linear relationship between elastic moduli and

Poisson’s ratio with ultrasonic velocities, while testing sintered iron ore with porosity up to

21.6%, indicating that ultrasonic velocities (Vp and Vs) can be used for monitoring the elastic

moduli of porous materials without measuring density.

Petrophysical parameters

It is recognised, that other physical characteristics such as total porosity, bulk density,

mineralogical composition, crystal size, and fabric are directly responsible for variations in the

strength of intact rock. However, few studies of the interrelationship between intact rock

strength and petrophysical parameters have been established worldwide for BIF. Conclusions

from Thomson (1963) for Australian iron ores have suggested a theoretical hematite–quartz

curve used to determine the bulk density and appropriate iron content approximation.

Correlations presented by Aylmer et al. (1978) have associated bulk density, iron grade and

porosity for Mount Tom Price iron ore. Studies of Box & Reid (1976) for the iron ore formation

at Cockatoo Island, attempted to correlate true specific gravity with iron content and the

influence of porosity. Finally, studies by Nel (2007), developed for Sishen iron deposits in South

Africa, developed a direct correlation between porosity and dry bulk density, providing a reliable

calculation index.

Studies correlating dynamic elastic proprieties with petrophysical and static strength parameters

on BIF have been developed by Wassermann et al. (2009) for an oolithic iron mine in Lorraine,

France. The study evaluated damage processes on UCS tests using Vp and found that mechanical

behaviour deduced from strain measurements is dilatant for some samples and non-dilatant for

the others, even when elastic properties indicate damage processes for all samples.

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Silva (2014), studying rocks Archean schists from Cuiabá mine, Brazil had measured permeability

and total porosity and elastodynamic properties that show the rigidity of these rocks and can be

used to define lithostructural constraints.

For iron oxide mineralisation in the Blötberget mine, Sweden, Maries et al. (2017), obtained

several physical properties from geophysical logging and laboratory measurements to assess the

ore-bearing rocks by increasing density and dynamic elastic modules measured from the full

waveform sonic.

For BIF in the Iron Quadrangle, the studies of Pereira (2017) using acoustic emission and other

geophysical techniques on well logging, defined rock strength parameters and discontinuity

characteristics to establish rock quality designation (RQD) and elastic in situ modules. Moraes

(2018), using part of this research database, evaluated, and correlated dynamic and static

proprieties in different anisotropy directions using statistical approaches. The author has shown

a well-adjusted regressive model for fresh itabirites. In general, the levels of the statistical

parameters are improved for initial anisotropy directions with a poorly adjusted regression

curve after the elimination of spurious data.

Bulk density is the most direct and easy-to-measure indicator of changes in compactness and level

of alteration of a sample (Van den Akker & Soares 2005). It is an essential physical property, which

has good correlation with the majority of index properties of rock, mainly with P wave porosity.

For Brazilian BIF, studies by Ribeiro et al. (2014), Santos et al. (2005), and Santos (2007) focused

on evaluating the association between bulk density and iron content for Vale’s iron ore mines

and concluded that there is a linear positive correlation between the total iron ore content and

the bulk density.

Anisotropy and heterogeneity

BIF heterogeneity is an important geological characteristic easily observed and measured on

several scales varying from a few millimetres to several centimetres and is defined by typical

metamorphic banding. This heterogeneity can result in an anisotropy described as transverse

isotropy, in which layers have approximately the same rock properties along the plane of the

band and different rock properties across the band. As suggested by Hudson & Harrison (2000),

geomechanically, the important rock properties that can be affected by anisotropy are

deformability modulus, strength, brittleness, permeability, and discontinuity frequency.

Singh (1989) and Ramamurthy (1993) pointed out that metamorphic rocks are mostly

anisotropic due to the effect of schistosity, cleavage and microcracking. For BIF, anisotropy is

132

defined by its mineralogical composition (metamorphic banding), mineralogical orientation

(alignment of minerals at the same orientation), the porosity induced by the original

composition or weathering, and different bulk densities in each layer. Additionally, secondary

anisotropy directions, defined by schistosity and foliation, are locally found mainly in hematitites

and in itabirites with some discreet shear zones, and fold axes.

The compositional metamorphic banding and heterogeneity are defined by the alternation of iron

and non-iron layers (typical itabiritic compositional metamorphic banding) and is defined in the

research as ’heterogeneity’. It is the most important itabirite feature that dictates the formation

of the intact rock strength, and is controlled by iron or non-iron layer thickness, porosity, and

mineral composition. A simple example is observed in zones where the banding contains thicker

layers of iron minerals, the itabirite will be denser, with lower porosity, relatively higher strength

values and richer in iron content.

It is expected for BIF to behave like an anisotropic material, where physical properties present

symmetric characteristics about an axis normal to compositional metamorphic banding, which

is the plane of isotropy. However, this compositional metamorphic banding also defines itabirite

heterogeneity (bands of iron minerals intercalated with non-iron mineral layers).

Ultimately, BIF heterogeneity is responsible for determining the different lithotypes, ranging

from country rocks such as dolomite, quartzite, and amphibolite (very few or non-iron bands) to

high-grade ore (very few or no waste bands) as rich itabirite or hematitite. This evaluation is

consistent with results presented by Dalstra et al. (2003) that compare the Hamersley Province

iron ore with many global deposits and correlate the proto-ore assemblages with intermediate

Fe-grades between ore and the host BIF.

For Saroglou & Tsiambaos (2007), the engineering behaviour of rock masses is strongly

dependent on the anisotropy present at different scales. At the microscale (intact rock) it

depends on the alignment of the rock crystals (i.e., inherent anisotropy). At the macroscale (rock

masses) with anisotropic structure, it is characterised by distinct bedding or schistosity planes

(structural anisotropy). The anisotropy of intact rock, which is the focus of this chapter, can be

determined by the variation of the velocity anisotropy index (IVp) as proposed by Saragou &

Tsiambaros (2007) due to the existence of bedding, foliation and schistosity planes in both intact

rock observed at the macroscale and microscale.

133

5.4 METHODOLOGY

To evaluate fresh and moderately weathered BIF, its petrophysical and geotechnical

characteristics, and achieve the proposed goals, the methodology was divided into three phases,

as described below.

The first phase included field investigations, in which several samples were collected from

surface outcrops and core drills, covering all BIF lithotypes. Geological and geotechnical

information, based on ISRM (1981) suggestions, were collected, and outcrops and samples were

photographed for additional visual information and physical parameter recognition

(e.g. anisotropy, banding, type of BIF, discontinuities, and mineralogy).

The second phase focused on petrographic thin section analyses, used to evaluate rock

mineralogy, fabric, and porosity, to provide information from a microscopy point of view and

compare it with field macro characteristics.

The third phase included undertaking and evaluating all laboratory tests used to determine the

dynamic intact rock properties, and correlations between the petrophysical characteristics and

geotechnical parameters of each BIF type. Correlations between anisotropy, ρb, Vp and Vs

velocity, Poisson's dynamic ratio, and Young's dynamic modulus were chosen as these rock

properties are considered to be the ones that play a role in the slope stability design and

evaluation for these types of rocks.

The sampling validation approach presented by Appendix I was used to reduce parameter

variability due to geological effects and defects. High variance is also motivated by intrinsic

sample variation or negligence during sample selection and grouping. Samples were grouped by

geological characteristics such as banding type, type of BIF, anisotropy direction on samples,

bulk density, and the presence of geological features, to reduce the variance and provide a

better lithotype identification.

The methodology proposed to cross-check pre-test samples was:

• Samples were grouped by level of weathering, W1 and W2 for fresh and W3 and W4

for moderately weathered, based on ISRM (1981).

• Vp measurements were used to discard inadequate samples (containing inclusions or

microfractures).

• As presented in Ribeiro et al. (2014) and Appendix I the BIF present a typical bulk

density range, and this range was used to separate fresh itabirites, ranging from

134

2.7 t/m3 to 3.8 t/m3 from non-iron or waste rock with ρb below 2.7 t/m3 and hematitite

with ρb above 3.8 t/m3.

• All samples were photographed before and after tests to find the main geological

characteristics, failure surface and failure mode.

• As presented in Appendix I, some geological features such as intense folding, the

presence of specularite levels, filling material with a different weathering degree,

quartz, and calcite veins are the main features that do not represent the typical

itabirites heterogeneity and for this reason were not evaluated.

• Itabirites samples that did not show typical banding were not evaluated.

Extreme outlier results were removed according to the box plot statistical methodology

(Whitaker et al. 2013). This technique identifies the mild outlier’s values from the quartiles (Qt)

determination, based on Equation 5.5.


QtLower = 1Qt - 1.5(3Qt - 1Qt) (5.5)

and values above the upper inner fence (Qtupper) as Equation5.6:

Qtupper = 3Qt + 1.5(3Qt - 1Qt) (5.6)




3Qt = × + 1.5.IQR (5.7)

The first or inferior quartile (1Qt) evaluates database dispersion around a central data leaving

25% of data below the sum and is defined by Equation 5.8.

1Qt = × - 1.5.IQR (5.8)

Interquartile ranges (IQR) measure how spread out from a central data the values are and these


IQR = 3Qt - 1Qt (5.9)

For BIF which present metamorphic heterogeneity in a determinate scale of observation, it is

necessary to determine the ratio of anisotropy to evaluate the degree of anisotropy. Estimating

the variation of each parameter due to the effect of anisotropy allows the differentiation of

135

spurious test results induced by rock intrinsic characteristics from the anisotropy effects which

can lead to misleading results and increasing variance.


property, different results in different directions, and the degree or ratio of anisotropy is used

to quantify how far the rock is from being isotropic. The term ‘heterogeneity’ is used for rocks

composed of layers or bands (scale related) that are different from one another that could, or

could not, present for the same physical properties, different results in different directions.

To evaluate the influence of the compositional metamorphic banding (heterogeneity) and define

the anisotropy on intact rock strength for all tests, the anisotropy angle, β (beta), as described

by Jaeger (1960), was used to test angles varying from 0° to 90° between the banding and the

loading direction.

Due to the reduced number of valid results for certain typologies and anisotropy direction, the

results were grouped on three main βangles (ranges): for loading parallel to banding, results of

0° < β ≤ 30° were grouped; for loading oblique to banding, results of 30° < β ≤ 60°were grouped;

and for loading perpendicular to banding, results of 60° < β ≤ 90° were grouped.

The anisotropy degree determination based on P wave velocity measurements was also

determinate using the method proposed by Saroglou & Tsiambaos (2007), using an equation to

determine the velocity anisotropy index (Ivp) given by the ratio presented in Equation 5.10.

𝐼𝐼𝐼𝐼𝐼𝐼 = 𝑉𝑉𝑉𝑉 0°𝑉𝑉𝑉𝑉 90°

(5.10)

Where Vp0° is the maximum velocity of P waves (when wave propagation is parallel to the plane

of anisotropy) and Vp90° (when wave propagation is perpendicular to the anisotropy plane).



31 Figure 5.8 P wave velocity anisotropic index modified from Saroglou & Tsiambaos (2007),

and βangle defined after McLamore & Gray (1962)

136

A pragmatic solution has been suggested by Chang et al. (2006) where a number of empirical

relations have been proposed for linking dynamic rock strength with physical and geological

parameters. In many situations, using such relationships is often the only way to estimate static

strength due to the absence of laboratory testing cores. The basis for these interactions is that

many of the same factors that affect rock strength also affect other physical properties, including

wave velocity, elastic moduli, bulk density, Poisson’s ratio, and porosity. This chapter shows how

good this correlation is for two sets of data that are strongly linked together. Further, an

adequate coefficient of correlation that describes the strength and the direction of the

correlations of the variables must be shown in order to measure and determine the relationship

between two variables.

According to Butel et al. (2014), the quality of the correlation is determined by the values of the

coefficient of determination (R2) value, the size of the dataset, and the visual fit of the regression

curve. During correlation evaluation analysis, tested several adjusted fitting curves (e.g. linear,

exponential, and potential) were tested. The exponential adjustment curve proved to be the

most suitable curve for the available dataset, even when some correlation proves to be better

for linear or potential regressions (e.g. the correlation ratios obtained were not significantly

different from the exponential values). The correlation coefficient has been calculated by using

the ‘best fitting’ approach using automatic procedures through several attempts on the dataset.

The exponential adjusted curve is defined as the curve best fitted to evaluate the variables,

correlations, and dispersions, and define the adjusted equation which gives the proportion of

the variance in the independent variables. In other words, the R2 is a measure of how well a

regression curve matches a set of data for an evaluated dataset. It could also be interpreted as

the adjusted model of observed effects between two dependent variables, with the strength of

the correlation is defined as 0 to 0.29 (little if any correlation), 0.3 to 0.49 (low), 0.5 to 0.69

(moderate), 0.7 to 0.89 (high) and 0.9 to 1.0 (very high correlation) as presented in Asuero et al.

(2006).

Due to the material characteristics and the lack of bibliographic references for studied rock

types, the adequate correlation ratio was considered to be equal or superior to 0.50 (moderate).


For hard strength, fresh to moderately weathered lithotypes, elastic parameters (dynamic

Young’s modulus (Edyn) and Poisson’s ratio (νdyn)), were defined based on Vp and Vs, used not only

to determine the parameters but also to evaluate the integrity of the samples. Bulk density (ρb)

137

and thin section evaluations were used to correlate and define geological and physical

properties, such as total porosity, anisotropy, mineralogy, and fabric.

The samples were prepared and analysed under a research grant, supervised by the lead author

at the Petrophysics Laboratory of the Federal University of Campina Grande (UFCG), Campina

Grande, Paraiba, Brazil, and a report was produced by Lima & Costa (2016).

A total of 21 blocks were collected, resulting in 82 samples grouped accordingly to the banded

iron formation lithotype, level of weathering, and anisotropy direction. Samples were cylindrical

with a diameter varying from 38 to 50 mm long, with a final treatment to achieve perfect parallel

endings applied to allow an ideal coupling when performing wave velocity and total porosity

measurements inside the propagation chamber ASTM D4543-01 (ASTM 2001). These

preparations were necessary to avoid losses in wave amplitude caused by irregular coupling

during the experiment. The same attention was taken for the precise measurements of pore and

total volume required to obtain the porosity and density.

After the cited preparations were completed for the porosity evaluation, the plugs were dried

under a temperature of 80°C for 24 hours and weighed using a semi-analytical electronic

balance. All dimensions were measured using a digital calliper rule.

For the laboratory tests, selected rock samples taken from the surface and drill cores were

subsampled and prepared according to each of the required standards. All test results were

separated by rock type, mine, anisotropy (β), and laboratory. The data are available in

Appendix IV.

Total porosity

The measures of total porosity (Ø) in laboratory of each sample was a function of total pressure

using an automated poro-permeameter Ultraporoperm AutoLab-500® (Figures 9A, B and C). The

apparatus measures porosity in the range of 0.1% to 40% using Boyle’s law. The measurements

are accurate to within ±0.1% (Han et al. 2011a and 2011b).

For each sample, the pressure cycle was repeated to assess the reversibility of the changes

induced by the first pressure cycle on the sample’s properties. For each sample, total porosity

was measured six times, and the mean value and standard deviation were reported. The

poro-permeameter injects nitrogen gas into the sample, and by application of Boyle’s law, it

measures the grain volume. The difference between the total volume and grain volume

corresponds to pore volume. Total porosity is determined by the ratio between pore volume

138

and total volume. Once the sample is dried, it is possible to assume that the solid sample mass

corresponds to the total mass previously measured.

A visual total porosity (Øb) was also determined using petrographical thin sections qualitative

visual determination. The comparation between total porosity (poro-permeameter measured)

and visual total porosity (thin sections evaluations) must be compared with restriction due to

the nature of measures.

(A) (B) (C)

32 Figure 5.9 A (left), poro-permeameter pressure gauges. B (centre), pressure

transductors. C (right), compression chamber (Lima & Costa 2016)

Bulk density

In this chapter, bulk density was determined according to the ratio between total dry mass and

total volume using standards outlined in ASTM 1289.6.4.1 (AS 2016) for the Australian

laboratory and ABNT NBT 6508 (ABNT 1989) for the Brazilian laboratory. The bulk density of

each specimen is calculated according to Equation 5.8:

ρb = M/V (5.8)

Where ρb is the bulk density (kg/m3), M is the mass of the specimen measured prior to testing

(kg), and V is the volume (m3) of the specimen, calculated from dimensions measured during

sample preparation.

Additionally, using the Ultraporoperm 500 ® it was also possible to obtain the grain density given

by the ratio between total mass of the sample and grain volume. Once porosity and grain density

were obtained, bulk density was estimated, neglecting density of air, which fills samples’ voids.

139

Vp and Vs wave velocity

Wave velocity experiments were undertaken according to procedures established by ASTM

D2845-08 (ASTM 2008) with an AutoLab 500® (Figure 5.10), which calculates the velocity

propagation of compressional waves (Vp) and shear waves (Vs) by measuring its time of transit

through the axial length of the sample. This equipment allows experiments to be undertaken

under controlled conditions of confining pressure, pore pressure, temperature, and fluid

saturation. Tested samples were dried in an oven and checked by comparing weight changes

versus time in the oven. After preparation, it was considered that the existing pores were free

of liquids, and temperature and confining pressure were kept under atmospheric levels.

33 Figure 5.10 The mechanical and electronic apparatus used to measure P and S wave

velocities (Lima & Costa 2016)

Once the length of each sample is known, to obtain the wave velocity the sample is placed

accurately in a confining chamber using an electronic calliper, where a pair of piezoelectric

transducers are responsible to read the compressional wave and two shear waves

(perpendicular to each other), which travels through the rock sample. The mechanical wave is

then converted into an electrical signal transmitted to an amplifier and to an oscilloscope, and

finally is translated to a computer interface. Once transit time and length of the elastic waves

are known, velocities are obtained from the ratio of these parameters.

The elastic dynamic modulus, i.e. dynamic elastic modulus (Edyn), and dynamic Poisson’s ratio

(νdyn), can be determined from the wave velocities and total density of the plugs as outlined by

140

Bourbié et al. (1987), Sheriff (1991), and Soares (1992) using Equations 5.1, 5.2, 5.3 and 5.4

presented in the previous section.

It must be noted that the best sonic couplings achieved to produce reliable P and S wave results

were obtained mostly for fresh typologies. Good coupling was not achieved for moderately

weathered materials. For this reason, these lithotypes were clustered in some analyses with

PWQI and PWGI because of the low amount of reliable results and the geological and

geotechnical similarities.


Several thin sections were prepared perpendicular to the bedding plane and the main

characteristics observed were visual total porosity (Øb), grain and pore size, shape distribution,

mineralogy, and fabric. The data were used to validate in microscale some features observed on

the macroscale, including heterogeneity, anisotropy, total porosity, and mineral distribution.

A total of 33 thin sections were prepared and evaluated. The sections were produced by The

University of Western Australia (UWA) petrography laboratory, and the description and

percentage measures were undertaken under an Australian Centre for Geomechanics (ACG)

research grant, supervised by the author’s supervisor and presented in Horta & Costa (2016).

5.5 RESULTS

5.5.1 Mineralogical and fabric overview

Mineralogy is based on BIF origin and fabric is associated with tectonical settings (regional and

local) and metamorphism. It is highly affected by the weathering process and redefined by the

weathering grade.

The mineralogical ensemble, textural characteristic (rock fabric) and total porosity are important

characteristics used to define the anisotropic behaviour and geomechanical properties for the

BIF. The analyses for FQI, FDI, FAI, PWGI and PWQI were supported using field investigations

and were used in conjunction with microscopic evaluations from Horta & Costa (2016). For HHE,

as there is a vast literature review available related to its fabric and mineralogical composition,

studies by Rosière et al. (1996 and 2001), Varajão et al. (2002) and Spier et al. (2003) were used.

As suggested by Baars & Rosiére (1997) and Rosière et al. (2001), the BIF fabric at the western

side of the Iron Quadrangle can be separated into three major domains: a subhedral to euhedral

crystals domain, with an overall granoblastic fabric, defined as the most common type and

considered to represent post-tectonic partial recrystallisation; a euhedral domain of locally

141

tabular-shaped or very elongated specularite platelets, preferred orientation, defining a

secondary schistosity induced by shearing and a high stress domain; and a locally brecciaed

fabric domain, that can be induced by tectonical or physical collapse zones.

Such domains reflect distinct microcharacteristics that are not always noticed on the

macroscale. These domains were associated with the three geological settings identification on

this research that influence the geomechanical and anisotropic behaviour:

• Post-tectonical domain (weathering), characterised by the complete mineral leaching

or mineral boundary contact leaching.

• Tectonical high stress zones, describing orientated mineral assemblage.

• Brecciated zone (weathering or tectonical), with porous level either orientated or not.



Also presented are the visual total porosity percentage, and maximum and minimum size

(between brackets) of pores for three types of measures. Intragranular pores correspond to the

percentage of pores inside crystals generally not interconnected. Intergranular pore

corresponds to porosity between crystals and are generally interconnected. Secondary pore

refers to fracture pore percentage. The visual total porosity (Øb) is the sum of the three pores’

percentage and was considered the maximum total percentage. For Øb in brackets, the minimum

value does not consider the secondary pores (intact rock) and for maximum value, the secondary

pore is included.

8Table 5.1 A sum

mary of the results of the m

icroscopic examination from

Horta &

Costa (2016)

Rock

Type

Granular

Hematite (%

)

Size (min-

max) m

m

Tabular

Hematite (%

)

Size (min-

max) m

m

Specular

Hematite (%

)

Size (min-

max) m

m

Goethite (%

)

Size (min-

max) m

m

Ochreous

Goethite (%

)

Size (min-

max) m

m

Quartz (%

)

Size (min-

max) m

m

Gibbsite/Kaolinite

(%)

Size (min-m

ax)

mm

Amphibolite

Pseudomorph

(%)

Size (min-m

ax)

mm

Carbonate

(%)

Size (min-

max) m

m

Intragranular

Pores (%)

Intergranular

Pores (%)

Secondary

Pores (%)

Visual total

porosity (%)

Size (min-

max) m

m

Num

ber of thin

sections

FAI 34

(0.002–1.16) –

– 4

(0.005–0.6) –

43

(0.02–0.8) –

14 –

42

– 5

(0.005–0.08) 5

FDI 25

(0.020–1.5)

7

(0.009–0.08)

1

(0.09–0.2)

2

(0.005–0.08) 10

17

(0.015–0.4) –

– 32

(0.025–0.5) 3

3 –

6

(0.005–0.18) 5

FQI

26

(0.002–1.9)

7

(0.002–0.8) –

1

(0.002–0.15)

2

(0.005–1.5)

36

(0.01–1.7)

5

(0.005–0.03) –

-7

4 2

11–13

(0.001–0.11) 13

HHE (0.005–0.015)

(0.001–0.081) (0.05–1.5)

– –

– –

– –

3 5 to 10

0 3 to 10

Rosière et al. (1996

and 2001), Spier et

al. (2003) and

Varajão et al. (2002)

PWGI

28

(0.01–1.8)

12

(0.005–0.8) –

34

(0.005–0.9)

8

(0.07–1.)

24

(0.01–0.4)

2

(0.06–0.2) –

– 6

3 7

9–16

(0.002–0.12) 5

PWQ

I 36

(0.01–1.2)

20

(0.002–0.10) –

8

(0.002–0.45)

4

(0.01–1.5)

18

(0.01–0.9)

2

(0.03–0.1) –

– 5

6 4

11–15

(0.002–0.12) 5

142

143

In general, the analyses of these thin sections indicated three main characteristics of pervasive

fabric responsible for inducing penetrative planes that could represent heterogeneity and result

in rock anisotropy:

• Band boundary contacts between layers with different mineral compositions, defining

the compositional metamorphic banding; mainly quartz for FQI, PWQI, FAI, PWGI, and

carbonates for FDI. This characteristic is not observed for HHE as it is a monomineralic

rock.

• Planar or granular minerals orientated in one direction (mineral orientation), defined

by tabular hematite and recrystalised quartz and carbonate crystals.

• Orientated porous layers (pore orientation), defined by orientated intergranular pores

aligned along the metamorphic banding (band boundary and mineral orientation) or

along discontinuities. This characteristic defines the metamorphic banding for HHE.

It is noted in Table 5.1 that fresh or moderately weathered BIF presented no significant changes

in crystal or pores size; the main changes are the increase in goethite, tabular hematite,

ochreous goethite and presence of kaolinite and gibbsite not seen in fresh types and total

porosity percentage increase from fresh to moderately weathered types. The anisotropy

observed in these rocks could be associated with these characteristics and not with the rock

fabric (mineral orientation) that generally are responsible for anisotropic effects.

It is noted that Øb increases from fresh (5% to 13%) to moderately weathered (9% to 16%) on

average. This increase is attributed to mineral leaching and alteration of iron and non-iron bands

highly affected by the degree of weathering. Additionally, FQI presented higher Øb from 11% to

13% comparing with other fresh BIF, this difference can be attributed to the presence of slightly

weathered thin sections that could influence the results.

A description of each lithotype is presented below.

Fresh to slightly weathered quartzitic itabirite

Fresh quartzitic itabirite (FQI) mineralogy is marked by the presence of quartz, granular

hematite, tabular hematite, goethite, and ochreous goethite. Kaolinite and gibbsite occur as

accessory minerals.

The presence of a slightly weathered portion is observed by the higher percentage of goethite

and ochreous goethite as small crystals in quartz rich bands, and anhedral shapes can also occur

filling pores. Bands of quartz have higher Øb = 20%, mainly resulting from intergranular pores. In

144

these bands where microplates of hematite occur in larger quantities the porosity is slightly

smaller.

Bands of hematite present lower Øb = 7% but with the presence of lobed crystals, tabular

hematite crystals larger than 0.80 mm and granular hematites larger than 1.3 mm in some zones

Øb can reach 30%. Hematite crystals lower than 1.3 mm were considered part of the banding

itself.

In general, FQI presented higher Øb from 11% to 13% compared to another fresh BIF. This

difference can be attributed to the presence of slightly weathered thin sections that could

influence the results.


Fresh amphibolitic itabirite (FAI) mineralogy is mainly defined by the presence of quartz,

hematite, goethite as a pseudomorph of amphibolite, goethite, and ochreous goethite. The

pseudomorph of amphibolite is generally altered to fibrous goethite and only occurs in some of

the thin sections. Goethite usually occurs with an anhedral shape in between quartz crystals (as

a cement) but also as crystals.

Quartz bands are well preserved and large hematite crystals (1.1 mm) were interpreted as being

part of the preserved hematite band. Hematite bands present lower total porosity and the limits

between quartz and hematite bands are not very clear due to the pore distribution and lower

concentration in the boundaries as observed in FQI.

Ochreous goethite, gibbsite and kaolinite occur as accessory minerals. All the samples present

low Øb (5%).


Fresh dolomitic itabirite (FDI) mineralogy is defined by the presence of hematite, iron dolomite,

quartz, goethite, and ochreous goethite. Granular hematite is predominant and has its size

decreased when iron dolomite (ankerite and siderite) occurs in higher quantities. Iron dolomite

occurs as granuloblastic crystals interlayered with hematite bands, as well as filling spaces

between crystals (calcite). Ochreous goethite occurs as cement, filling spaces between crystals of

hematite and crystals of calcite. Its occurrence is better observed in the boundaries of hematite

bands where carbonate and crystals of hematite in smaller size dominate.

Quartz is present in bands where crystals and sub-crystals are orientated. Talc occurs as an

accessory mineral.

145

FDI presents a low Øb of 6%. Bands of hematite also present some percentage of iron dolomites

and are characterised by relatively lower Øb. In this band, hematite occurs as granular and/or

tabular crystals and iron dolomites as a matrix with very fine crystals. For iron dolomite bands,

the percentage and intergranular pores are considerably larger.

Hard hematitite

From Rosière et al. (1996 and 2001) and Varajão et al. (2002) and Spier et al. (2003), the hard

hematitite (HHE) is constituted mainly of iron oxides and hydroxides (e.g. hematite and martite)

with the percentage of pores between 3% and 10% (Table 5.1). The heterogeneous

metamorphic banding is represented by layers with higher porosity interlayered with more

massive ones, conditioned by mineral and pore orientation. In addition, in shallower depths for

slightly weathered types these porous layers became a preferential patch for weathering

(oxidation of iron minerals) increasing anisotropy effects and weakness of this band. Locally, the

presence of specularite can induce a higher anisotropy degree.


The moderately or partially weathered quartzitic itabirite (PWQI) mineralogy is characterised by

hematite, quartz, goethite, and kaolinite, ochreous goethite, and gibbsite in a smaller percentage.

Hematite is characterised by crystals with low intergranular porosity and some pores can be

partially to completely filled with goethite or ochreous goethite as cement. In general, kaolinite

and gibbsite appear filling intragranular pores. Quartz bands and hematite bands were hardly

identified, and bands of hematite present lower relative total porosity and quartz bands have

higher relative total porosity. However, this is reduced when goethite is filling the space (cement)

between quartz and hematite. For this reason, the Øb varies from 11% to 15%.


Moderately or partially weathered goethitic itabirite (PWGI) is characterised by hematite,

goethite, quartz, ocherous goethite, and gibbsite and kaolinite occur in smaller percentage.

Anhedral goethite or ochreous goethite are common in spaces between crystals of quartz and

hematite (cement). However, they are also observed as a crystal in smaller concentrations.

Hematite is present as lobed crystals and microplates.

Bands of hematite present lower relative total porosity. Quartz bands have a higher relative total

porosity. However, this is reduced when goethite is filling the space (cement) between quartz

and hematite. For this reason, the Øb varies from 9% to 16%.

146

The main difference between PWQI and PWGI is the increase of goethite and ochreous goethite,

and the consequent hematite reduction for PWGI. The PWGI total porosity seems to be relatively

lower due to the goethitic cementation. However, evaluated thin sections did not confirm this

field observation.

5.5.2 Heterogeneity and anisotropy evaluations

It is expected for heterogeneity promoted by typical compositional metamorphic banding of the

BIF to be responsible for a transversal isotropy. Analysis conducted on this research showed at

least three different types of fabric characteristics that could induce heterogeneity, which could

induce anisotropy.

Analyses conducted in Section 5.5.1, found three main characteristics of pervasive fabric that

could be responsible for inducing penetrative planes representing heterogeneity and resulting

in rock anisotropy: boundary contacts between layers with different mineral composition,

defining the compositional metamorphic banding; planar or granular minerals orientated

according to one direction (mineral orientation); and orientated porous levels (pore

orientation).

The heterogeneity observed in itabirites is different from that observed in hematitites, however

both satisfy the heterogeneity characteristic defined by Amann et al. (2013) which states that

they visually appear heterogeneous, with evidence of non-uniform crystal size distribution at

the specimen scale, variable mineral grain composition and total porosity.

Bands boundary contact

For fresh itabirites, millimetric to centimetric bands with different mineralogy (e.g. hematite,

quartz, or iron dolomite) define an itabirite heterogenous compositional metamorphic banding.

While evaluating itabirite fabric under a microscope, one can observe relative higher differences

in crystal sizes (0.002 mm to 1.9 mm) for iron bands and smaller differences (0.01 mm to 1.7 mm)

for non-iron bands representing a high roughness due to the size difference, as shown in Figure

5.11A.

The band boundary is defined for hard hematitite by millimetric to centimetric banding

alternation of different iron minerals (e.g. granular hematite, martite or microplates of

hematite) each mineral can present different crystal sizes. Its supported by Varajão et al. (2002)

defined for this type, crystal sizes varying from 10 μm to 30 μm for hematite and martite granular

crystals, and 1 μm for microplates of hematite. The contact between these minerals (boundary

147

surface) presents a high degree of roughness, due to the difference of the crystal sizes as

presented in Figure 5.11B.

(A) (B)

34 Figure 5.11 A (left), FQI microphotography showing bands of larger granular quartz (light

brown crystals) in contact with microplates of hematite (small grey and light

grey crystals). B (right), HHE thin section showing contact between larger

granular hematite crystals (dark band) with smaller tabular hematite crystals

(light band)

The band boundary for fresh itabirite is defined by alternation of iron and non-iron minerals

bands, and the, FDI presents higher mineral assemblage variation (iron dolomite, quartz, and

hematite). In contrast, HHE with an almost monomineralic composition, presented no iron

mineral band strength contact heterogeneity behaving as an isotropic material.

Mineral orientation

Minerals can have a pervasive fabric with orientation that are defined by the tabular hematite,

specularite and granular minerals (quartz and or iron dolomite).

According to Varajão et al. (2002), mineral orientation for HHE is characterised by microplates

of hematite (more common), orientated granular hematite and larger crystals of specularite in

high-stress zones as seen in Figure 5.12A, representing continuous homogeneous orientated

bands parallel to the metamorphic banding.

For fresh itabirites, mineral orientation is defined by orientated granoblastic quartz and iron

dolomite crystals bands, interlayered with tabular hematite or granular hematite, orientated

according to the metamorphic banding, as shown in Figure 5.12B.

500μm

500μm

148

(A) (B)

35 Figure 5.12 A (left), HHE thin section presenting an orientated specularite large platelets

(light grey elongated minerals) orientated according to the banding. B (right),

FDI thin section showing bands of granular dolomite (light bands) and bands

of tabular hematite (brown) and pores in contact (black) Horta & Costa (2016)

Orientated elongated minerals are defined by orientated planes of iron and non-iron minerals

(planar or tabular). This orientation spreads over all bands and could represent levels of

anisotropy (penetrative and persistent) or single discontinuities, depending on the thickness.

Some good examples are represented by layers of specularite, which define a strong strength

anisotropy orientated by schistosity or banding, quartz, and orientated microplates of hematite

(banding). These characteristics are associated with the local and regional tectonical setting and

are influenced by the weathering grade.

It is observed at greatest depth, without the influence of iron enrichment or weathering, the

variety of deformational and metamorphic fabrics placed on BIF, with a simple mineralogical

composition, led to a mineral size and shape orientation that developed a systematic

heterogeneity, but not necessarily high weakness surfaces outlining strong anisotropy.

Nonetheless, these surfaces can represent single weaker planes, responsible for a non-pervasive

anisotropy due to the weaker mineral contact relative to the elongated mineral values. Bands

that are composed of large and elongated minerals (e.g. specularite and tabular hematite)

produce a relatively weaker contact with reduced crystal surface contact on the microscope. In

these cases, the band thickness of lepidoblastic or granuloblastic minerals can cause a continuity

interruption for the intact rock, forming important discontinuities that could represent their

non-pervasive anisotropy surfaces.

500μm 100μm

500μm 1,00μm

149

Pore orientation

Fresh BIF presents, in general, total porosity lower than 10% with pore sizes ranging from 1 μm

to 0.2 mm. There pores layers can define weaker planes easily observed in slightly to moderately

weathered typologies, resulting from leaching of prone minerals (boundary corrosion or mineral

dissolution), with representation (iron oxides or hydroxides), preferentially using initial banding

contact or mineral orientation. These pore concentrations and orientations can originate

continuous or discontinuous bands of pores defining anisotropic layers that contribute to the

heterogeneity mainly for hematitite but also for itabirites.

It´s suggested by Rosière et al. (2001), for HHE a banding defined by bands of iron minerals with

larger crystal size variation and higher total porosity (up to 10%), and bands of non-iron minerals

with smaller crystal size variation and lower total porosity (3%), defining what is called

hematitite heterogeneous metamorphic banding as present in Figure 5.13A.

For fresh itabirites, pore orientation is concentrated at the boundary of iron and non-iron bands,

and also in orientated mineral bands, as seen in Figures 5.13A and B. For moderately weathered

types (PWQI and PWGI), pores can be cemented by goethite (mainly PWGI), as shown in Figure

5.13C.

For PWQI and PWGI the weathering progressively reduces the strength of the contacts (leaching

and oxidation), increasing the original visual total porosity from 3% to over 16%, resulting in

effective weakness planes.

Some of these porous can be filled with iron oxide and hydroxide (PWGI), which improves

strength and decreases the anisotropy effects possibly produced by the pores increase.

However, in some of these the pores can be filled with iron hydroxides or weaker minerals, such

as calcite or manganese. In this case, the anisotropy effect due to the pores increase remains as

seen in Figure 5.13C.

When the increase of total porosity (weathering) embraces all bands, weathering can drastically

change the intact rock to a completely weathered state.

150

(A) (B)

(C)

36 Figure 5.13 A (left), pore bands in massive HHE. B (centre), FWQI microphotography showing

porosity (delimited in red lines) between quartz (lightly coloured) and hematite

(light grey) contact. C (right) PWGI presenting hematite (white) and quartz (grey)

bands with different crystal sizes at contact (dark); Horta & Costa (2016)

Total porosity seems to be the most important characteristic that defines the anisotropy of the

HHE and very important for others BIF. The overall porosity distribution defines the intact rock

fabric as a response to the banding boundary and orientated elongated mineral fabrics, which

are the aspects that define the different BIF internal textural web ’skeleton ‘, providing weakness

surfaces or bands. These voids are weathering preferential zones that will induce a continued

intact rock strength reduction as weathering increases, inducing effective mineral contact

reduction by leaching and chemical alteration, and ultimately increasing the total porosity.

In some cases when concentrated, this process causes real discontinuities, generating intact rock

interruption (continuum break).

1000 μm 0.5 mm

0.5 mm

151

Furthermore, this research did not evaluate quantitatively the boundary banding, elongated

orientated minerals and total porosity effects. For this reason, it is not possible to define the

most important characteristic that defines the anisotropy index for fresh, slightly, and

moderately weathered BIF.

5.5.3 Anisotropy evaluations of dynamic elastic parameters and petrophysical

properties

This section presents the dynamic elastic parameters, obtained from Vp and Vs wave velocity

propagation tests conducted in different anisotropy angles (βangle) evaluation of fresh and

moderately weathered BIF. These results were used to determine anisotropy effects and elastic

parameter based on empirical correlations and determine the relationship between wave

velocity propagation, dynamic elastic parameters (Young’s modulus and Poisson’s ratio) and

petrophysical properties (bulk density, and total porosity).

Table 5.2 summarises test results for FQI, FDI, FAI, and PWQI/PWGI for each βangle (0°, 45°, and

90°). Basic statistical evaluation is presented in this table for each type: mean, standard deviation

(SD), maximum values (Max), minimum values (Min), and number of tested samples (n).

Considering the inadequate number of total test results for HHE (n = 4) has not been evaluated

in this section. Some results can present bias, due to the reduced number of tests in some

direction, mainly FAI0° (n = 2). Due to the reduced number of available tests, the PWQI and PWGI

were considered as a single group, supported by the similar geological and geomechanical

features.

152

9 Table 5.2 Basic statistical summary test results and parameters evaluated for all fresh

and moderately weathered BIF, considering anisotropic results

Lithotype Anisotropy (β) Parameter Total porosity (%) Bulk density (t/m3) VP (m/s) VS (m/s) E dyn (GPa) v dyn Vp/Vs

FAI

90°

Mean 3.4 3.44 5,245 3,005 77 0.211 1.9 SD 2.2 0.07 340 968 35 0.157 0.8 Max 5.3 3.54 5,713 3,714 107 0.442 3.1 Min 1.2 3.38 4,898 1,579 25 0.098 1.3 n 4.0 4 4 4 4 4 4

45°

Mean 5.5 3.33 4,960 2,599 62 0.294 2.1 SD 3.5 0.12 771 915 37 0.105 0.7 Max 9.5 3.49 5,998 3,563 104 0.440 3.0 Min 1.9 3.24 4,255 1,482 21 0.209 1.6 n 4.0 4 4 4 4 4 4

0°

Mean 3.7 3.25 5,565 2,927 73 0.305 1.9 SD 0.2 0.01 40 287 11 0.056 0.2 Max 3.8 3.25 5,593 3,130 80 0.345 2.1 Min 3.5 3.24 5,537 2,724 65 0.265 1.8 n 2.0 2 2 2 2 2 2

FDI

90°

Mean 2.5 3.14 5,195 2,582 55 0.318 2.1 SD 1.2 0.49 756 480 17 0.089 0.4 Max 3.9 3.68 6,944 3,425 88 0.431 2.9 Min 0.1 2.41 4,444 1,825 32 0.120 1.5 n 9 9 9 9 9 9 9

45°

Mean 2.1 3.12 5,537 2,917 68 0.265 1.9 SD 1.2 0.52 719 471 23 0.141 0.4 Max 3.8 3.82 6,494 3,611 103 0.420 2.7 Min 0.7 2.30 4,457 2,215 36 0.049 1.5 n 8 8 8 8 8 8 8

0°

Mean 2.2 3.16 5,402 2,653 59 0.336 2.1 SD 1.4 0.38 660 519 17 0.052 0.3 Max 4.7 3.71 6,364 3,621 85 0.423 2.7 Min 0.5 2.56 4,375 1,856 33 0.261 1.8 n 9 9 9 9 9 9 9

FQI

90°

Mean 3.3 3.14 4,377 2,241 40 0.297 2.0 SD 2.6 0.56 597 504 9 0.103 0.5 Max 7.8 3.58 5,273 3,140 55 0.439 3.0 Min 0.6 2.08 3,619 1,737 31 0.121 1.5 n 6 6 6 6 6 6 6

45°

Mean 3.9 3.32 3,908 2,286 45 0.204 2.0 SD 2.1 0.15 1,130 598 26 0.111 0.4 Max 7.0 3.51 5,179 3,301 88 0.366 2.5 Min 1.2 3.12 2,471 1,695 19 0.056 1.6 n 8 8 8 8 8 8 8

0°

Mean 4.5 3.30 5,002 2,774 65 0.263 1.9 SD 3.1 0.07 550 653 24 0.111 0.3 Max 9.4 3.41 5,745 3,373 89 0.401 2.5 Min 1.4 3.18 4,262 1,733 28 0.066 1.5 n 7 7 7 7 7 7 7

PWQI/ PWGI

90°

Mean 26.6 2.77 2,096 1,265 11 0.195 1.6 SD 2.7 0.17 381 192 3 0.087 0.2 Max 30.0 2.94 2,484 1,449 14 0.293 1.7 Min 22.6 2.54 1,373 893 5 0.053 1.3 n 6 6 6 6 6 6 6

45°

Mean 26.6 2.68 2,308 1,378 12 0.214 1.7 SD 4.0 0.31 286 242 3 0.087 0.2 Max 32.1 3.04 2,543 1,667 15 0.320 1.9 Min 22.1 2.38 1,818 1,074 8 0.078 1.5 n 5 5 5 5 5 5 5

0°

Mean 25.6 2.80 3,056 1,635 20 0.287 1.9 SD 2.2 0.19 395 430 9 0.096 0.4 Max 27.6 3.07 3,609 2,072 29 0.418 2.7 Min 22.0 2.47 2,377 1,056 9 0.147 1.6 n 10 10 10 10 10 10 10

Mean: mean value; SD: standard deviation; n: number of tested samples; Max: highest value obtained; Min: lowest values obtained

153

To assess the effects of the anisotropy imposed by the compositional metamorphic banding of

BIF, for different dynamic parameters was applied the modified Saroglou & Tsiambaos (2007)

methodology, defined the anisotropy index using P wave velocity anisotropic index (IVp).

10Table 5.3 Summary table of dynamic propriety anisotropy index for all lithotypes

Lithotype Ivp Class IVp FDI 1.0 Isotropic FAI 1.1 Fairly FQI 1.1 Fairly PWQI/PWGI 1.5 Moderately

It can be noted from Table 5.3 that:

• FDI presents an IVp = 1 defined as isotropic material.

• FAI and FQI present an IVp = 1.1 defined as fairly anisotropic material.

• PWQI/PWGI presents an IVp = 1.5 defined as moderately anisotropic. For all fresh

material there, is no significant difference between Vp and Vs either parallel or

perpendicular to the heterogeneity (banding). They were considered isotropic to fairly

anisotropic. Since the IVP was defined using P wave velocity (Table 5.3), it is possible to

claim that the slightly higher anisotropy effect observed at moderately weathered BIF

is due to the ρb decrease and total porosity increase changing the original unweathered

boundary contacts and pore ratio.

Anisotropic correlations from wave velocity propagation and elastic dynamic parameters

Vp and Vs velocities were measured for the three βangle, used to calculate Edyn and νdyn values and

evaluate the effects of BIF anisotropy stablishing correlations able to determine the anisotropy

effect for all the lithotypes studied.

The figures presented in this section show, for each type, graphs that use the mean in each βangle

to define the anisotropy curve, correlating measured (Vp and Vs) and calculated parameters (Edyn

and νdyn) for each anisotropy direction (0°, 45°, and 90°).


Figures 5.14A, B, C and D show for FDI respectively, Vp, Vs, Edyn and νdyn results from each βangle

equal to 0°, 45°, and 90°.

154

(A) (B)

(C) (D)

37 Figure 5.14 A (top left), graph shows Vp values for each βangle. B (top right) shows Vs for

each βangle. C (lower left), graph shows Edyn values for each βangle. D (lower right)

shows νdyn for each βangle. In all figures, the dashed line is the anisotropic curve,

coloured triangles are the FDI value results and the black triangles represent

β0°, red β45° and blue β90°

In Figures 5.14A, B, C, and D the total variation for FDI show Vp varying from 5,537 m/s to

5,195 m/s, Vs from 2,917 m/s to 2,582 m/s, Edyn varying from 68 GPa to 55 GPa, and νdyn varying

from 0.336 to 0.265. Vp, Vs, and Edyn mean values obtained at β45° are slightly higher than those

obtained at β0°. This unexpected behaviour (higher values were expected for β0°) is attributed to

the isotropic to fairly IVP obtained for FDI. For νdyn, the curve presents a different shape (upward

5195

5537 5402

2582

2917

2653

59 55

68

0.32

0.34

0.26

155

concavity) attributed to the low SD. From Table 5.3 and Figures 5.14A, B, C, and D one can note

that FDI is isotropic material with a slightly higher value for β45°.

• Fresh amphibolitic Itabirite

Figures 5.15A, B, C and D show for FAI respectively, Vp, Vs, Edyn and νdyn results from each βangle

equal to 0°, 45°, and 90°.

(A) (B)

(C) (D)




coloured top-down triangles are the FAI value results and the black triangles

represent β0°, red β45° and blue β90°

5245

4960

5566

3005

2599

2927

77

62

73 0.294

0.211

156

In Figures 5.15A, B, C, and D the total variation for FAI shows Vp varying from 5,245 m/s to


from 0.305 to 0.211.

For Vp, the highest mean value was obtained at β0° and for Vs and Edyn at β90° was the highest value.

It can be attributed to the minor difference obtained for different βangles still inside the SD range.

For νdyn, the curve also presents a different shape (upward concavity) attributed to the low SD.

From Table 5.3 and Figures 5.15. A, B, C, and D, one can note that FAI is, in general, fairly

anisotropic material.


Figures 5.16A, B, C and D show for FQI respectively, Vp, Vs, Edyn and νdyn results from each βangle

equal to 0°, 45°, and 90°.

157

(A) (B)

(C) (D)




coloured squares are the FQI value results and black squares represent β0°, red

β45°and blue β90°

In Figures 5.16A, B, C, and D the total variation for FQI shows Vp varying from 5,002 m/s to


from 0.2045 to 0.297. The mean values of Vp, Vs, and Edyn obtained at β0° are slightly higher and

νdyn presented slightly higher value for β90°.

4377

3908

5002

2241

2286

2774

45

65

40

0.297

0.26

0.204

158

From Table 5.3 and Figures 5.16 A, B, C, and D, one can note that FQI is, in general, fairly

anisotropic material.

Moderately weathered goethitic and quartzitic itabirite

Figures 5.17A, B, C and D show for PWQI and PWGI respectively, Vp, Vs, Edyn and νdyn results from

each βangle equal to 0°, 45°, and 90°.

(A) (B)

(C) (D)




coloured crosses are the PWQI/PWGI value results and the black crosses

represent β0°, red β45° and blue β90°

2096 2308

3056

1378

1265

1635

12 11

0.2 0.21

0.29

159

In Figures 5.17A, B, C, and D the total variation for PWQI/PWGI show Vp varying from 3,056 m/s

to 2,069 m/s, Vs from 1,635 m/s to 1,265 m/s, Edyn varying from 20 GPa to 11 GPa, and νdyn

varying from 0.287 to 0.195.

Vp, Vs, and Edyn, and νdyn mean values obtained at β0° are higher and is considered as an expected

behaviour for moderately anisotropic materials even presenting IVP from fairly anisotropic for

Poisson’s ratio.

From Table 5.3 and Figures 5.17A, B, C, and D is possible to note that PWQI/PWGI is moderately

anisotropic material. Showing a clear weathering effect in the ancient fresh rocks behaviour.

5.5.4 Isotropic evaluations of dynamic elastic parameters and petrophysical

properties

Due to the isotropic to fairly IVP for all BIF described in Section 5.5.3, in this section, the

anisotropy was not considered even though it can influence the results and interpretation.

This section presents the dynamic elastic parameters obtained from Vp and Vs velocity

propagation tests not considering the different anisotropy angles (βangle) and evaluating all types

of fresh to moderately weathered BIF as a single group. These results were used to identify

empirical correlations and determine the relationship between wave velocity propagation,

dynamic elastic parameters (Edyn and νdyn) and petrophysical properties (ρb) considering the BIF

as an isotropic material.

Table 5.4 summarises all test results for fresh types and moderately weathered types presenting

the basic statistical evaluation for each type, the mean, standard deviation (SD), maximum

values (Max), minimum values (Min), upper and lower inner fence, covariance (CV), margin of

error, values of the first and third quartiles (1 Qt and 3 Qt), and number of tested samples (n).

Considering the low number of test results, PWQI and PWGI were grouped, also considering the

similarity presented in previous sections. For the same reason, HHE has no statistical support

and some parameters were not presented.

Some results can present bias due to the low number of tests. This affects mainly the HHE tests

with a total of four tests. Even with low test numbers for HHE the anisotropic index obtained

was IVp = 1.1 (fairly anisotropic).

160

11Table 5.4 Basic statistical summary test results and parameters evaluated for all fresh

and moderately weathered BIF, considered as isotropic results

Lithotype Anisotropy (β) Parameter Ø (%) Bulk density

(t/m3) VP (m/s) VS (m/s) Edyn (GPa) vdyn Vp/Vs

FAI Total

Mean 4.3 3.36 5195 2827 70 0.263 2.0

SD 2.6 0.11 541 800 31 0.119 0.6

Max 9.5 3.54 5998 3714 107 0.442 3.1

Min 1.2 3.24 4255 1482 21 0.098 1.3

1 Qt 2.3 3.25 4941 2386 49 0.180 1.6

3 Qt 5.3 3.42 5579 3408 88 0.333 2.0

Lower inner fence 0.00 3.00 3983 852 0 0.000 1.0

Upper inner fence 9.80 3.68 6536 4941 146 0.564 2.6

CV 0.599 0.033 0.104 0.283 0.439 0.453 0.317 Margin of error 42% 2% 8% 20% 32% 32% 23%

N 10 10 10 10 10 10 10

FDI Total

Mean 2.2 3.14 5372 2710 60 0.308 2.0

SD 1.2 0.45 698 492 19 0.099 0.4

Max 4.7 3.82 6944 3621 103 0.431 2.9

Min 0.1 2.30 4375 1825 32 0.049 1.5

1 Qt 1.1 2.81 4921 2402 46 0.276 1.8

3 Qt 3.0 3.52 5896 3004 73 0.365 2.2



CV 0.538 0.142 0.130 0.182 0.313 0.322 0.178

Margin of error 22% 6% 6% 7% 15% 13% 7%

N 26 26 26 26 26 26 26

FQI Total

Mean 3.9 3.26 4407 2436 50 0.250 1.9

SD 2.5 0.31 922 614 24 0.111 0.4

Max 9.4 3.58 5745 3373 89 0.439 3.0

Min 0.6 2.08 2471 1695 19 0.056 1.5

1 Qt 2.5 3.18 3955 1929 31 0.188 1.7

3 Qt 4.1 3.44 5152 3021 69 0.315 2.2

Lower inner fence 0.09 2.79 2160 291 0 –0.003 1.1


CV 0.642 0.094 0.209 0.252 0.469 0.442 0.201

Margin of error 29% 4% 10% 12% 22% 20% 9%

N 21 21 21 21 21 21 21

HHE Total

Mean 1.6 5.10 6865 3218 143 0.354 2.1

SD 1.4 0.07 309 213 17 0.041 0.2

Max 3.3 5.16 7164 3404 158 0.392 2.4

Min 0.2 5.01 6538 3019 129 0.315 1.9

N 4 4 4 4 4 4 4

PWQI/ PWGI Total

Mean 26.1 2.76 2604 1468 15 0.244 1.8

SD 2.7 0.21 569 364 7 0.097 0.3

Max 32.1 3.07 3609 2072 29 0.418 2.7

Min 22.0 2.38 1373 893 5 0.053 1.3

1 Qt 23.8 2.57 2270 1238 11 0.200 1.6

3 Qt 27.5 2.93 2978 1667 16 0.320 1.9



CV 0.104 0.077 0.219 0.248 0.485 0.399 0.175

Margin of error 5% 4% 10% 11% 22% 18% 8%

n 21 21 21 21 21 21 21

SD: standard deviation; COV: covariance; Max: highest value obtained; Min: lowest value obtained; 1 QT: first quartile; 3 QT: third

quartile, n: number of tests

161

From Table 5.4 results is possible to note that:

• Total porosity

High SD for fresh lithotypes.

Lower relative values obtained for HHE = 1.6% (±1.4) and FDI = 2.2% (±1.2); moderate

for FQI = 3.9% (±2.5) and FAI = 4.3% (±2.6); and higher values for PWQI/PWGI = 26.1%

(±2.7).

Comparing visual total porosity (Øb), from thin sections with total porosity (Ø), from

poropermiameter tests, is possible to note that for FAI (Øb = 5% and Ø = 4.3 ± 2.6), for

FDI (Øb = 6% and Ø = 2.2 ± 1.2), for FQI (Øb = 12% and Ø = 3.4 ± 2.5), for HHE (Øb = 6%

and Ø = 1.6 ± 1.4) and for PWQI/PWGI (Øb = 13% and Ø = 26 ± 2.7). Reaching a proper

correlating for FAI but in general, Øb overestimate the total porosity for FDI, FQI and

HHE, and underestimate for PWQI/PWGI.

• Bulk density

Low SD for all tested lithotypes.

Lower relative values obtained for PWQI/PWGI= 2.76 (±0.21) t/m3; moderate for FAI=

3.36 (±0.11) t/m3, FQI= 3.26 (±0.31) t/m3 and FDI= 3.14 (±0.45) t/m3; and higher values

for HHE = 5.10 (±0.07) t/m3.

• P wave velocity


Low relative values for PWQI/PWGI = 2,604 (±569) m/s; moderate for

FQI = 4,407 (±922) m/s; high for FAI = 5,195 (±541) m/s and FDI = 5,372 (±698) m/s;

and very high for HHE = 6,865 (±309) m/s.

• S wave velocity


Lower relative values for PWQI/PWGI = 1,468 (±364) m/s; moderate for FQI =

2,436 (±614) m/s; high for FAI = 2,927 (±800) m/s and FDI = 2,710 (±492) m/s; and very

high for HHE = 3,218 (±213) m/s.

• Dynamic elastic modulus

High SD for all tested lithotypes.

162

Lower relative values for PWQI/PWGI = 15 (±7) GPa; moderate for FQI = 50 (±24) GPa,

FDI = 60 (±19) GPa and FAI = 70 (±31) GPa; and higher values for HHE = 143 (±17) GPa.

• Dynamic Poisson’s ratio

High SD for all tested lithotypes;

Lower relative values for PWQI/PWGI= 0.24 (±0.10), FQI= 0.25 (±0.11) and

FAI= 0.26 (±0.12); and high values for FDI= 0.31 (±0.10) and HHE= 0.35 (±0.04);

• Vp/Vs ratio


Lower correlation values for PWQI/PWGI = 1.8 (±0.2), FQI = 1.9 (±0.2), FAI = 2 (±0.3), and FDI =

2 (±0.2) and higher correlation for HHE = 2.1.

Table 5.5 summarises the range obtained for dynamic parameters, and petrophysical properties

for fresh to moderately weathered BIF types. To provide a simple evaluation, the results were

separated into three value ranges, each one being associated with a geological and

geomechanical characteristic, with range and correlations likely to be important when

describing the behaviour and characteristics of these rocks.

12 Table 5.5 Evaluated parameters trend summary table for all fresh BIF

Parameters Moderately weathered itabirites (low)

Fresh itabirites (typical)

Fresh hematitite (high)

Total Porosity Ø (%) Ø > 22 22 ≥ Ø ≥ 10 Ø < 10

Bulk Density ρb (t/m3) ρb < 3.2 3.2 ≤ ρb ≤ 3.8 ρb > 3.8

P wave velocity Vp (m/s) Vp < 3,800 3,800 ≤ Vp ≤ 6,200 Vp > 6,200

S wave velocity Vs (m/s) Vp < 2,1000 2,100 ≤ Vp ≤ 3,000 Vp > 3,000

Dynamic Young's modulus Edyn (GPa) Edyn < 35 35 ≤ Edyn ≤ 125 Edyn > 125

Ratio VP/Vs VP/Vs < 1.9 1.9 ≤ VP/Vs ≤ 2.0 VP/Vs > 2.0

For a better correlation understanding, the same correlations presented in Section 5.5.3 for

anisotropic parameters were established for isotropic condition as presented in the following

section. For this evaluation, all test results were used and evaluated as a single group.

163

In contrast to previous sections where PWQI and PWGI were presented as a single group, for

this evaluation it was considered separated as it presented a sufficient number of tests.

Isotropic correlations using dispersion graphs

The visual separations (grouping) presented in this section must agree with the statistical values

(mean, SD, maximum, and minimum values) presented in Table 5.4 and the coefficients of

determination presented in Table 5.7.

• Vp and Vs dispersion graphs

Figures 5.18A and B show Vp and Vs dispersion graphs with each lithotype represented by distinct

colours and marks. These graphs provide a visual clustering for each lithotype separating the

maximum and minimum VS and Vp variation.

(A) (B)

41 Figure 5.18 A (left), shows the dispersion graph for Vp. B (right), shows the Vs dispersion

graph. For these graphs, each colour and symbol represent a single lithotype

as presented in the legend. Dashed light blue lines are the group limits and

dashed dark blue circles are highlighted samples grouping

For Vp values from Figure 5.18A the range of PWQI and PWGI is grouped at low Vp < 3,800 m/s,

for fresh itabirites from 3,800 m/s ≤ Vp ≤ 6,200 m/s, and for HHE high Vp > 6,200 m/s. For the Vp

graph, only a few samples of FDI are higher than this range overlapping the HHE group

(highlighted in dashed black circle), and a few samples of FQI are overlapping the PWQI and

PWGI group (highlighted in dashed blue circle). A group of PWGI with higher relative velocity in

the border limit is noted.

3800

6200

2100

E E

164

For the Vs graph (Figure 5.18B), the PWQI and PWGI are grouped at low Vs < 2,100 m/s, the main

group of fresh itabirites at moderate 2,100 m/s ≤ Vs ≤ 3,000 m/s and HHE and a small number of

fresh itabirites samples at high Vs > 3,000 m/s. Also noted (dashed circles in Figure 5.18B), is a

group of PWGI samples presenting higher Vs values compared with the rest of this group.

• Vp/Vs ratio dispersion graph

Figure 5.19 shows the wave velocity ratio between Vp and Vs. Adjusted linear curve were plotted

covering all BIF (black dashed line) and the respective correlation equation and R2 was determined.

42 Figure 5.19 The correlation between Vs/Vp (wave velocity ratio) for the set of samples

analysed. The dashed line is the linear adjusted curve with its equation. Point

’a’ shows the limits between moderately weathered and fresh itabirites and

point ‘b’ shows the limits between fresh itabirites and HHE. Each colour and

symbol represent a single lithotype

Figure 5.19 shows a positive linear curve (dashed line) with high R2 = 0.7 and the correlation

equation, Vp = 1.54 Vs + 839.

Point ‘a’ defines the lower limit between moderately weathered to fresh lithotypes

(Vp = 3,800 m/s and Vs = 2,100 m/s) with Vp/Vs = 1.8. Samples above point ‘a’ are considered

fresh, and below this point, moderately weathered. Point ’b’, indicates the position where the

ratio Vp/Vs = 2 sets the limit between fresh itabirites and HHE (Vp = 6,200 m/s and Vs = 3,000 m/s).

Some samples are plotted out of the limits; FQI at low ratio due to the presence of W2 samples

and FDI as high ratio due to the high Vp and Vs noted for iron dolomite bands.

a

b

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6200

2100

165

• Vp and Vs correlation with Edyn and νdyn dispersion graphs

Figures 5.20A and C (upper and lower left side) present, respectively, the Vp correlation with Edyn

and νdyn, and Figures 5.20B and D (upper and lower right side) present, respectively, Vs

correlation with Edyn and νdyn for all lithotypes. Adjusted linear curves were plotted separately to

fit the fresh BIF (red dashed line) and moderately weathered itabirites (black dashed line) and

the correlation equations were also presented.

(A) (B)

(C) (D)

43 Figure 5.20 A (top left) and C (lower left) show, respectively, the graphs Vp with Edyn and Vp with νdyn

for all evaluated lithotypes. B (top right) and D (lower right) show, respectively, graphs

for Vs with Edyn and Vs with νdyn for all of the lithotypes evaluated. The dashed lines are

the correlation lines for fresh itabirite (red) and moderately weathered itabirite (black)

materials, and the legend shows the coefficient of determination. Each colour and

symbol represent a single lithotype

a

6200

3800

35 125

35 125

2100

166

In Figure 5.20A, a positive linear adjusted fit curve (red dashed line) was determined for fresh

BIF, the obtained equation is Vp = 21.81Edyn + 1,591 with a R2 = 0.54 (moderate). The adjusted

curve for moderately weathered itabirites, the second curve (black dashed line) presents the

equation Vp = 65.7Edyn + 1,591 presents a strong R2 = 0.76.

In this figure point ’a’ defines the lower limit between moderately weathered to fresh lithotypes

(Vp = 3,800 m/s and Edyn = 35 GPa). Samples above point ’a’ are considered to be fresh, and the

ones below are moderately weathered. Point ‘b’ sets the limit between fresh lithotypes and HHE

(Vp = 6,200 m/s and Edyn = 125 GPa).

Figure 5.20.B presents a positive linear curve (red dashed line) for fresh BIF and the equation

obtained is Vs = 1.27Edyn + 744 with a strong R2 = 0.70. For moderately weathered itabirites (black

dashed line), the equation obtained is Vs = 47.01Edyn + 743 with R2 = 0.93 (very strong). The use

of a linear adjusted curve best configures and returns higher coefficients of correlation for fresh

and moderately weathered types separately.

In this figure point ’a’ defines the lower limit between moderately weathered to fresh lithotypes

(Vs = 2,100 m/s and Edyn = 35 GPa). Samples above point ’a’ are considered fresh, and below are

considered moderately weathered. Point ’b’ sets the limit from fresh itabirites to HHE (Vs =

3,000 m/s and Edyn = 125 GPa).

In Figures 5.20C and D, due to the scatter obtained for νdyn it was not possible to establish valid

correlations.

• Bulk density dispersion graph

Figure 5.21 shows the bulk density dispersion graph presenting the distribution of the results for

each lithotype represented by distinct colours and marks. In this graph, it is possible to separate

maximum and minimum variation velocity, and it provides a visual clustering for each lithotype.

167

44 Figure 5.21 The distribution of ρb for the set of samples analysed. Each colour and symbol

represent a single lithotype and the dashed light blue lines represent the

grouping limits

PWGI, PWQI and part of FDI presented lower ρb values of 2 t/m3 > ρb ≥ 3.2 t/m3. Fresh itabirites

have moderate values between 3.2 t/m3 ≥ ρb ≥ 3.8 t/m3. The highest values were obtained for

HHE, with ρb > 3.8 t/m3.

There is a lack of ρb results from fresh itabirites cluster (below 3.8 t3/m) to HHE cluster (above

5.0 t3/m) that can be associated with the low number of HHE tests and to the extreme high

difference from the iron content between these lithotypes.

• Vp and Vs correlations with bulk density dispersion graph

Figure 5.22A shows a graph correlating between ρb and Vp, while Figure 5.22B presents the

correlation between ρb and Vs.

E

3.2

3.8

168

(A) (B)

45 Figure 5.22 A (left), shows the correlation between ρb and Vp. B (right), shows the

correlation between ρb with Vs for all tested samples. For these graphs, each

colour and symbol represent a single lithotype as presented in the legend. The

dashed light blue lines represent the group limits and circles and squares are

highlighted sample groupings

The obtained correlation curve shows R2 < 0.5, not presented for both graphs. In Figure 5.22A,

point ’a’ defines the lower limit between moderately weathered to fresh lithotypes

(Vp = 3,800 m/s and ρb = 3.2 t/m3). All samples above point ’a’ are considered to be fresh and

below this point, moderately weathered. Point ’b’ sets the limit between fresh itabirites and

HHE (Vp = 6,200 m/s and ρb = 3.8 t/m3).

It is noted that from the itabirites group FDI presented the highest Vp values and highest ρb

dispersion. The HHE group presented highest Vp and Vs correlations with ρb (blue dotted circle

and square) from all BIF. PWQI/PWGI showed lowest Vp and Vs correlations with ρb having a low

dispersion (green dotted circle and square) from all BIF.

In Figure 5.22B, point ‘a’ is the lower limit between moderately weathered to fresh lithotypes,

with Vs = 2,100 m/s and ρb = 3.2 t/m3. All samples above point ‘a’ are considered to be fresh and

those below as moderately weathered. Point ’b’ sets the limit between fresh itabirites and HHE,

with Vs = 3,000 m/s and ρb = 3.8 t/m3.

• Edyn and νdyn correlation with ρb dispersion graphs

b

a

b

a

3.8 3.8

3.2 3.2

3800 6200 2100

169

Figures 5.23A and 5.23B show the correlation between ρb with Edyn and νdyn, respectively,

presenting the distribution of the results for each lithotype outlined with distinct colours and

marks. The black dashed line is the linear correlation curve and the related equation is

presented.

(A) (B)

46 Figure 5.23 A (left), shows the graph for ρb and Edyn for all tested samples. Different marks

and colours represent different lithotypes. B (right), shows the correlation

between ρb and νdyn for all the samples tested. For these graphs, each colour

and symbol represent a single lithotype as presented in the legend. The

dashed light blue lines are the group limits

In Figure 5.23A, point ’a’ (ρb = 3.2 t/m3 and Edyn= 35 GPa) is the lower limit between moderately

weathered to fresh lithotypes. All samples above point ’a’ are considered fresh, while those

below are moderately weathered. Point ’b’ sets the limit between fresh itabirites with HHE, with

ρb = 3.8 t/m3 and Edyn= 125 GPa.

Additionally, Figure 5.23A shows a positive linear curve (black dashed line) that was determined

for all of the lithotypes evaluated. The equation correlated is ρb = 0.011Edyn+2.61 with a

moderate R2 = 0.50.

HHE represents higher ρb and Edyn values, in contrast to the moderately weathered material, which

has lower values. The FDI presented the higher ρb dispersion.

In Figure 5.23B, due to the scatter obtained for νdyn, it was not possible to establish valid

correlations.

a

b 3.8

3.2

35 125

170

• Total porosity dispersion graph

Figure 5.24 is the total porosity (Ø) dispersion graph, presenting the results for each lithotype as

represented by distinct colours and marks. This graph shows sample results dispersion and

provides a visual clustering for each type.

47 Figure 5.24 Dispersion graph of total porosity. For this graph, each colour and symbol

represent a single lithotype as presented in the legend. The dashed light blue

lines are the grouping limits (weathering grades)

Figure 5.24 shows the spatial distribution of the Ø test results for all lithotypes. PWQI and PWGI,

present higher values of Ø ≥ 22%, representing moderately weathered itabirites (blue and black

crosses). FQI (red squares) and FAI (upside down black triangles) presented a moderate total

porosity (5% ≤ Ø ≤ 10%), representing slightly weathered lithotypes. HHE (black diamonds), FDI

(blue triangles) and a large amount of the FAI and FQI groups show lower total porosity with

Ø < 5% representing fresh lithotypes.

FAI and FQI present higher Ø dispersion for the fresh itabirite group and are associated with the

presence of W2 samples. Additionally, there is a rise of Ø values from fresh and slightly

weathered itabirites and fresh HHE to moderately weathered itabirites. This can be attributed

to the weathering escalation changing fresh rock (W1 and W2) to moderately weathered rock

(W3 and W4).

E

Fresh

Slightly weathered

Moderately weathered

5

22

Tota

l

171

The FDI dataset presented no samples in the slightly weathered range. This lack of result is

attributed to the high solubility of iron dolomite minerals where the weathering is more

effective ranging from fresh (W1) directly to completely weathered type (W5 and W6).

• Bulk density correlation with Ø dispersion graph

Figure 5.25 shows the relationship between ρb and Ø, with the results for each lithotype

represented by distinct colours and marks.

48 Figure 5.25 Dispersion graph showing the relationship between bulk density and total

porosity for all tested samples. For this graph, each colour and symbol represent a

single lithotype as presented in the legend. The dashed light blue lines are the group

limits and the dashed coloured circles are highlighted sample groupings

In Figure 5.25, the point ‘a’ defines the lower limit between moderate and slightly weathered

BIF (ρb = 3.2 t/m3 and Ø = 22%); above this point samples are considered moderately weathered.

Point ‘b’ sets the limit between slightly weathered and fresh itabirites with (ρb = 3.2 t/m3 and

Ø = 5%); above this point are considered fresh itabirites and bellow are considered fresh poor

itabirites. Black dotted circle defines fresh HHE group, with high bulk density and point ‘c’

(ρb = 3.8 t/m3 and Ø = 5%) sets the limits between fresh itabirites to hematitite.

It is possible to see a clear separation of fresh and hard BIF (FAI, FDI, FAI, and HHE) from

moderately weathered lithologies (PWQI and PWGI). Fresh samples show low Ø and a

5 22

3.2

3.8

a b

c

Total

172

substantially higher ρb when compared to weathered rock samples; the exception being FDI due

to the presence of iron dolomite minerals as non-iron bands.

The FDI ρb presents a dispersion (blue dotted circle) and exhibits a very low total porosity

correlation, while for FQI (red dotted circle) and PWQI/PWGI (black dotted circle) there is a

negative correlation between Ø and ρb.

• Vp and Vs correlation with Ø dispersion graphs

Figures 5.26A and 5.26B show, respectively, the relationship between Vp and Vs correlation with

Ø presenting the results for each lithotype represented by distinct colours and marks. The black

dashed lines represent the linear correlation fitting curves and related equation.

(A) (B)

49 Figure 5.26 A (left), shows the relationship between total porosity and Vp. B (right), shows

the relationship between Ø and Vs for all the samples. For these graphs, each

colour and symbol represent a single lithotype as presented in the legend the

dashed light blue lines are the group limits and the dashed black line is the

adjusted curve

In Figure 5.26A, point ‘a’ defines the lower limit between moderately to slightly weathered

samples (Ø = 22% and Vp= 3,800 m/s). Samples above point ‘a’ are considered moderately

weathered to fresh and below this point are considered slightly weathered. Point ’b’ sets the

limit from slightly weathered to fresh samples (Ø = 5% and Vp= 3,800 m/s) and Point ‘c’ set the

limits from fresh itabirite to HHE (Ø = 5% and Vp= 6,400 m/s. A negative linear adjusted fit curve

(black dashed line) was determined for all BIF. The obtained equation is Vp = −1,095Ø + 5,460

and has a strong R2 = 0.65.

b b

a a

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5 22 5 22

c c

173

In Figure 5.26B, point ’a’ defines the lower limit between moderately weathered to slightly

weathered samples (Ø = 22% and Vs= 2,100 m/s). All samples below point ‘a’ are considered

slightly weathered to fresh and above are considered moderately weathered. Point ’b’ sets the

limit from slightly weathered to fresh samples (Ø = 5% and Vs= 2,100 m/s) and point ‘c’ sets the

limit from fresh itabirite to HEE (Ø = 5% and Vp= 3,000 m/s. A negative linear adjusted fit curve

(black dashed line) was determined for all BIF, and the obtained equation is Vs = -51.09Ø + 2,822

with a moderate R2 = 0.5.

Figures 5.26A and B show similar trends and sample distributions for total porosity correlations

with Vp and Vs with low velocities, and high porosity for PWQI and PWGI. Additionally, high

velocities and low Ø are observed for fresh and slightly weathered samples.

Edyn and νdyn correlations with total porosity dispersion graphs

Figure 5.27A shows the relationship between Edyn and the Ø, and Figure 5.27B shows the

correlation between νdyn and Ø, presenting the results for each lithotype with distinct colours

and marks. The black dashed line represents the linear correlation curve and related equation.

(A) (B)

50 Figure 5.27 A (left), shows the relationship between dynamic Young’s modulus

and total porosity for all of the samples tested. B (right), shows the

relationship between dynamic Poisson’s ratio and Ø for all of the samples

tested. For these graphs, each colour and symbol represent a single lithotype

as presented in the legend. The dashed light blue lines are the group limits and

the dashed line is the adjusted curve

b 35

c

a

174

In Figure 5.27A, a negative linear adjusted fit curve (black dashed line) was defined, considering

all samples, with a low R2 = 0.41. For this reason, the correlation equation was not presented.

The correlation graph presents at point ‘a’ the limits between moderately to slightly weathered

itabirite (Ø = 20% and Edyn = 35 GPa), above this point samples are moderately weathered. Pont

‘b’ set the limit between slightly weathered and fresh itabirite (Ø = 5% and Edyn = 35 GPa) above

this point samples are fresh itabirites. Point ‘c’ set the limits between fresh itabirite with HHE

(Ø = 5% and Edyn = 120 GPa).

From the dispersion graph presented in Figure 5.27A, it is possible to separate by the point ‘a’ that

sets the limits between moderately weathered from slightly weathered degree (Ø = 20% and Edyn

= 35 GPa), above this point samples are considered moderately weathered and below are

considered slightly weathered. Point ‘b’ set the limits between slightly weathered from fresh rocks

(Ø = 5% and Edyn = 35 GPa), above this point samples are slightly weathered and above are fresh.

Point ‘c’ sets the limits between fresh itabirites with hematitite, (Ø = 5% and Edyn = 120 GPa).

From Figure 5.27A, due to the scattering obtained for νdyn it is not possible to stablish a valid

correlation.

Figure 5.6 summarises moderately to very strong coefficients of determination and curve

equations obtained for Vp and Vs test results with dynamic elastic Modulus and petrographic

properties correlations. These equations represent the best -adjusted curve for each BIF

grouping defined all BIF (isotropic), Fresh BIF (W1 and W2), and moderately weathered BIF (W3

and W4) according with ISRM (1989a) tables.

13Table 5.6 Summary table for elastic propriety correlations presenting coefficient of

determination above moderate (R2 ≥ 0.5) and the best-fitted curve equations

Proprieties correlations BIF groups Correlation equations Coefficient of determination – R2 (%) Class

Vp/Vs Total Vp = 1.54 Vs + 839 0.7 High

Vp/Edyn

Fresh Vp = 21.81Edyn + 1,591 0.6 Moderate

Moderately weathered Vp = 65.7Edyn + 1,591 0.8 High

Vp/ɸb Total Vp = (-)109.5ɸ + 5,460 0.7 High

Vs/Edyn Fresh Vs = 1.27Edyn + 744 0.7 High Moderately weathered Vs = 47.01Edyn + 743 0.9 Very high

Vs/ɸb

Total

Vs = (-)51.09ɸ + 2,822

0.5

Moderate

ρb/Edyn Total ρb = 0.011Edyn + 2.61 0.5 Moderate

Table 5.7 presents for each type the R2 obtained from Excel toll, showing just the high coefficient

of determination R2 ≥ 0.5. HEE was not evaluated due to the reduced number of test results.

175

Table 5.7 summarises moderate to very strong coefficients of correlation and curve equations

obtained for Vp and Vs test results with dynamic elastic modulus and petrophysical property

correlations. These equations represent the best-adjusted curve for each BIF grouping defined

as all BIF (total), fresh BIF (W1 and W2), and moderately weathered BIF (W3).

14Table 5.7 Correlation coefficient (R2) for each type correlating wave velocities, elastic

dynamic parameters and petrophysical proprieties

Lithology Ø v ρb Ø v Vp Vp v Vs Vp v Edyn Vsv x νdyn Vs v Edyn νdyn v Edyn

R2 FQI -0.53

(moderate) –

0.74 (high)

0.80 (high)

– 0.94

(very high) –

R2 FAI -0.6

(moderate) -0.51

0.74 (high)

0.79 (high)

-0.92 (very high)

0.99 (very high)

0.88 (high)

R2 FDI – – – 0.52

(moderate) -0.6

(moderate) 0.88

(very high) –

R2 PWQI/PWGI -0.59

(moderate) –

0.81 (high)

0.86 – 0.97

(very high) –

5.6 DISCUSSION

5.6.1 BIF compositional metamorphic banding heterogeneity and the anisotropy

behaviour

It is suggested that BIF are transversally isotropic materials, due to the heterogeneity defined by

the compositional metamorphic banding. However, as presented in Appendix I for fresh BIF, the

results of the UCS tests undertaken in different anisotropy directions were not significantly

different, remaining within SD.

Analyses conducted in this chapter found that three main types of petrophysical characteristics

of pervasive rock fabric are responsible for inducing penetrative planes that can result in

anisotropic behaviour in fresh to moderately weathered BIF:

• Heterogeneity defined by the contrast at the boundary contact between bands of iron

and non-iron minerals (band boundary).

• Planar or granular minerals orientated along a single or multiple direction (mineral

orientation).

• Oriented pore levels or bands (pore orientation).

176

In addition to these fabric characteristics, the rock strength and anisotropy of each BIF type is also

influenced by chemical mineral affiliation (similar mineral hardens index and bulk density), similar

crystal sizes and the crystal contact surface, which ultimately is responsible for the roughness.

These fabric characteristics will exist in persistent and repetitive layers or bands of the fresh to

moderately weathered BIF that may define an anisotropy wherein the anisotropy index,

depending on P wave velocity propagation through the BIF samples, varies from isotropic (IVp =

1) to fairly anisotropic (IVp = 1.5).

Otherwise, discontinuities are not necessarily associated with the heterogeneity induced by

compositional metamorphic banding. They can be associated with slightly weathered levels with

higher porosity associated with structural features, such as fractures, and since these

discontinuities did not follow a pattern, they cannot be characterised as pervasive and persistent

anisotropy plans. However, when weathered they can form patterns of anisotropic-like surfaces

that cut thought the intact rock, especially from moderately weathered to highly weathered BIF.

This heterogeneity is characterised in different way for each of the BIF lithologies:

• HHE is a monomineralic (iron minerals) rock, with metamorphic heterogeneity banding

defined by interlayered bands of low or high Ø and normally associated with the

presence of tabular hematites and granoblastic hematite minerals associated with

different crystal sizes in each band. The IVp = 1.1 (fairly anisotropic) and Vp/Vs= 2.1.

• FAI, FDI and FQI are heterogeneous rocks with compositional metamorphic typical

itabirite banding defined by the repetition of iron and non-iron mineral bands that can

differ in size and crystal orientation. From the geomechanical perspective, both can

represent transverse anisotropy as defined by the three fabric characteristics (band

boundary, pore orientation, and mineral orientation), that in fact did not represent an

important anisotropy behaviour. The anisotropic index for FAI and FQI is IVp = 1.1 (fairly

anisotropic) and FDI IVp = 1.0 (isotropic) and Vp/Vs = 2 for all itabirites.

• For PWQI and PWGI, even with the higher total porosity observed (reaching 30%), the

anisotropic index (IVp = 1.5) is positioned at the limit between fair and moderately

anisotropic index. This higher relative anisotropic behaviour is also confirmed by the

lower relative wave velocity ratio (Vp/Vs = 1.8) from all BIF. However, it was found that

a significant increase in this index from fresh to moderately weathered BIF attributed

to the Ø increases and ρb decreases as the main changes occurring during the first

stages of the weathering. For BIF, the weathering processes, as already mentioned

177

uses the three pervasive planes as preferential paths for water percolation, inducing

leaching and mineralogical alteration (oxidation) reducing the parental intact rock

strength.

In summary, the HHE FAI and FQI showed fairly anisotropy index and FDI showed an isotropic

behaviour attributed to the non-band iron mineral composition (iron dolomites) that presents

higher P and S wave velocity when compared with quartz bands (closer to hematite). For

moderately weathered BIF, as the total porosity increases from 5% in average (fresh BIF) to

closer to 30%, this characteristic must control the anisotropic behaviour (and intact rock

strength). It is important to note that water content was not considered in this evaluation due

to the nature of samples and test preparation.

In evaluating Vp and Vs velocity propagation through BIF samples, the higher velocities were

obtained in general for β0° (along the banding heterogeneity) where the wave propagation is

especially faster, using preferentially the mineralogical continuity promoted by mineral

orientation and composition avoiding pore layers. Lower velocity was observed as expected at

β90° and β45° as the waves must cross porous and different minerals bands (non-iron and iron

bands). Although it was not possible to observe a typical U-shape (Jaeger 1960) curve due to the

lack of results in more than three anisotropy directions, it is possible to note that FQI, FAI, and

PWQI/PWGI presented similar asymmetric shape curves with upward concavity and maximum

at β0°. FDI, on the contrary, presented an almost symmetric shape curve with downward

concavity showing the largest velocities at β45°. This behaviour could be attributed to the low

difference between the P and S wave velocity along β45° compared with β0° of just 3%.

Band composition and thickness are important characteristics that define the BIF behaviour, the

intact rock strength and the elastic parameters. Where banding presents thicker layers of

non-iron minerals (e.g. carbonates or quartz bands), the result is a relatively less dense,

proportionally more porous rock. Also, the intact rock strength will be lower than other

specimens with more iron bands (thicker iron layers), which increases the iron content, reduces

porosity, and increases the ρb. This behaviour is remarkable for FDI where the increase in

non-iron bands results in a strength reduction even without a total porosity increase and Vp

decrease related to the petrophysical characteristics of the iron dolomite minerals already

discussed.

This evaluation is in accordance with Dalstra et al. (2003), who compare the Hamersley province

with other deposits worldwide and makes correlations of proto-ore assemblages with Fe grades

intermediate between the ore and the host BIF.

178

5.6.2 Correlations between wave velocity propagation, dynamic elastic, and

petrophysical proprieties

Considering all BIF types as an almost isotropic single group, the high coefficient of

determination between ρb, P and S wave velocity, total porosity and dynamic Young’s modulus

obtained for some lithotypes provide a reliable basis for defining parameters and property

correlations. Also, for describe the geological characteristics and geomechanical behaviour of

each lithotype. Even the lower coefficient of determination obtained for some lithotypes

represents useful data for describing rock type geomechanical behaviour.

The linear regression curves obtained for the petrophysical and geometrical correlations are

summarised and outlined in the following sections; first in general, considering the correlation

sets and then for BIF type.

Dynamic young’s modulus and P and S- wave velocity

Figures 5.20A and 5.20B, presented the expected linear positive correlations between Edyn and VP

and VS. The fresh and hard samples group presented VP R2 = 0.6 (moderate) and Vs R2 = 0.7 (high).

The moderately weathered samples group (W3 and W4) presented VP R2 = 0.8 (high) and

Vs R2 = 0.7 (high). The lower correlation for the fresh group is attributed to the low number of tests

from HHE.

Dynamic young’s modulus and bulk density

Figure 5.23A shows a linear positive regression curve adjusted to best fit expected for HHE

samples (higher ρb). For this reason, the correlation is moderate R2 = 0.5.

Rock samples with low ρb present lower VP and Vs compared with denser materials, that present

higher VP and Vs. This behaviour is expected and can be explained by the nature of the

compressional wave, which travels faster through more compact materials and low porosity

such as HHE. Although it is an expected result, experiments show weak to moderate correlation

for the entire group.

P and S wave velocity and total porosity

Figures 5.26A and 5.26C present, as expected, a linear negative correlation between total

porosity and P and S wave velocity. A single feet curve was obtained for the total porosity

correlation with VP R2 = 0.7 (high) and Vs R2 = 0.5 (moderate). The dependence of Vs on total

porosity is complex and multivariable. What can be suggested is the total porosity of fresh types

179

is quite low (Ø < 5%) and the bulk density varies from different compositional type influencing

the Vp and Vs correlations with the total porosity.

P and S wave velocity correlation

Figure 5.19 shows the expected linear correlation curve, with R2 = 0.7 (high).

As predicted in theory, using the Vp/Vs ratio it is possible to discriminate the rock type as

presented in Figure 5.19. Even though this study was carried out on a few samples, it was

possible to find Vp/Vs ratio values very close to those predicted by Pickett (1963), on average 2.1

to HHE, 2 to FAI and FDI, 1.9 to FQI, and 1.8 to PWQI and PWGI.

Due to the scatter obtained for νdyn it was not possible to establish valid correlations.

The correlation showed a similarity between different types of BIF that were grouped in Tables

5.5, 5.6 and 5.7 into three groups. The main characteristics of each group are described as follows.

• Hard hematitite group

The HHE high intact rock strength is mainly defined by the higher ρb (above 3.8 t/m3), low total

porosity (Ø < 5%) and small mineral size (0.001 mm to 0.08 mm). The higher Vp mean obtained

is 6,865 m/s and on average is 30% higher than the other BIF, and the Edyn > 125 GPa. The

heterogeneous metamorphic banding is represented by layers with higher total porosity

interlayered with more massive ones, conditioned by mineral and pore orientation inducing fair

anisotropy.

Since the anisotropy effects are not important for HHE, the high standard deviation noted could

be attributed to the reduced number of tests.

• Fresh to slightly weathered itabirites group

Fresh itabirite is the most common BIF lithotype, presenting compositional metamorphic

banding conditioned by concentrated quartz and/or iron dolomite bands interplayed with iron

bands representing the typical itabirite heterogeneity. The high intact rock strength is mainly

defined by the typical bulk density (3.2 t/ m3 ≤ ρb ≤ 3.8 t/m3) and typical total porosity (W2 - 10%

≥ ɸ ≥ 3% - W1). Vp varies from 3,800 m/s to 6,200 m/s and Edyn from 35 GPa to 125 GPa. The

higher percentage of porous layers are concentrated on non-iron bands or at the contact

between bands inducing some fair anisotropy parallel to the typical compositional metamorphic

banding identified in great depths.

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The higher heterogeneity in crystal size observed in this type seems not to influence by the

anisotropic effects measured from Vp and Vs values as values the IVP are similar from HHE (more

homogeneous crystal size type,).

The anisotropy degree is low. Observed is a different anisotropy behaviour for FDI (Rc=2),

partially attributed to the presence of iron dolomite minerals with lower mineral hardness as

compared with quartz and similar Vp and Vs velocities, compared with hematite. Nevertheless, it

must be considered that the iron dolomite bands contributed to reduce the strength and the

elastic parameters even keeping high Vp and Vs and Ø values of this type.

• Moderately weathered itabirites group

PWQI and PWGI present the same geological characteristics compared with fresh itabirites. It is

considered the first weathering product from fresh itabirites (weathering grade changed from

W1/W2 to W3/W4). IN shallower depths, this weathering change is easily identified in field and

laboratory samples. The lower intact rock strength is mainly defined by the lower bulk density

(ρb < 3.2 t/m3) higher total porosity (Ø > 22%), lower P wave velocity with values below

3,800 m/s, and Edyn lower than 35 GPa. These higher percentages of porous layers are

concentrated in both non-iron and iron bands, and mainly at the contact between bands

inducing some fair to moderately anisotropy index parallel to the compositional metamorphic

banding (higher from all BIF). They are caused by weathering (leaching of quartz and

recrystallising different iron ore oxides and hydroxides). Increasing percentages of the layers

potentially generate preferential weathering paths that can increase the anisotropy and change

to moderately weathered lithotypes.

Lower intact rock strength is mainly defined by the high pore content. The higher heterogeneity

in crystal size observed in this type seems to influence the strength and anisotropy mainly due

to the lack of cementation for PWQI. The intact rock strength observed for PWGI due to the

goethite cementation normally observed in this type are higher when compared with the PWQI.

The leaching effectiveness on iron dolomite bands for FDI results in the absence of moderately

weathered (W3 and W4) rocks in the weathering profile. Contrarily, from quartz based itabirites

(FAI and FQI), as the leaching is not efficient it is present in the weathering profile a full range of

the weathering grade from W1 to W6 according to ISRM (1989a) tables.

Even with several attempts to reduce data dispersion, some parameters showed high SD. This may

represent the natural dispersion promoted by intrinsic geological features, or by the observed

higher anisotropic effects especially for FDI, PWQI and PWGI.

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5.6.3 Comparing dynamic with static parameters

Results presented in Table 4.5 (Chapter 4) and Table 5.4, are summarized it Table 5.8 that allows

the correlation between these two different approaches to obtain elastic parameters. Between

brackets is present de standard deviation obtained for each parameter

15Table 5.8 Correlation table between elastic dynamic and elastic parameters

Lithology ρb

(t/m3) Ø

(%) UCS

(MPa) Estat

(GPa) νstat

Edyn

(GPa) νdyn

Edyn/Estat

FAI 3.36

(0.11) 4.3

(2.6) 172 (89)

93 (29)

0.200 (0.050)

70 (31)

0.263 (0.119)

0.75

FDI 3.40

(0.45) 2.2

(1.7) 135 (64)

95 (36)

0.220 (0.070)

60 (19)

0.308 (0.099)

0.63

FQI 3.26

(0.31) 3.9

(2.5) 173 (96)

86 (39)

0.190 (0.050)

50 (24)

0.250 (0.111)

0.58

HHE 5.1

(0.07) 1.6

(1.4) 159

(134) 70

(61) 0.290

(0.040) 43

(17) 0.354

(0.041) 0.61

From Table 5.8 it is noted that obtained results from dynamic and static elastic parameters are

very close, belonging to the standard deviation, even for HHE that presents the higher bulk

density values. The ratio between dynamic (lower values) and static modulus proved a

correlation (Edyn/Estat < 1), generally attributed to the wave velocity attenuation due the presence

of discontinuities and pores layers. However, the lower total porosity obtained, and the

approach used to reduce the presence of micro cracks at tested samples, can reduce this effect

and also attributed this low correlation with mineral composition and crystal size.

5.7 CONCLUSION

In this chapter, the anisotropy behaviour of BIF (fresh, slightly, and moderately weathered)

dynamic elastic parameters, petrophysical properties, and their physical and empirical

correlations were evaluated and outlined in detail. This approach emphasised the role of

identifying the geological and geomechanical field characteristics and features that were crucial

in defining a proper sample grouping to separate and better represent the BIF rock types and

understand its geomechanical behaviour.

It is possible to identify a natural variation obtained experimentally in petrophysical and dynamic

elastic parameters (considering the absence of microcracks), largely due to randomness of

geological and geomechanical nature and experimental errors, which should manifest themselves

even in samples of the same lithology tested in a common direction.

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As noted, geological features induce some dispersions of the results, since they may have

different microstructural variations in fabric, affected by rock heterogeneity and early influence

of weathering, inducing in some rock types little, but not irrelevant anisotropy effects, and

variations in the petrophysical properties of the samples (e.g. ρb and Øb). Even so, using several

methodologies (samples separation and statistical approach) it was possible to group similar

types verifying reliable trends of a correlation based on P and S wave velocity propagation

(dynamic parameters).

For HHE, FAI and FQI, the anisotropy index is fair, and FDI behave as a moderately anisotropic

material due to the high P and S wave velocities obtained for iron dolomitic minerals bands.

A moderately IVP was also identified for PWQI and PWGI, mainly caused by the total porosity

increase and ρb decrease (more weathered type) noted in these types.

Considering the typical heterogeneity defined by the compositional metamorphic banding for

itabirite, evaluations of macroscopic and microscopic characteristics revealed that there is very

little effective anisotropic behaviour, as the anisotropic index varies from isotropic to little

anisotropy for fresh and slightly weathered BIF. However, moderately weathered BIF presented

moderately anisotropy index, and for these material, transverse anisotropy must be considered.

Three main petrophysical characteristics were found to be responsible for inducing penetrative

planes that could result in this little anisotropy observed are:

• Contact between bands of iron and non-iron minerals, which define the itabirite

compositional metamorphic banding (heterogeneity).

• Planar or granular minerals orientated along a single direction or multiple directions

(mineral orientation).

• Porous layers (total porosity percentage and orientation) which define the hematite

heterogeneity.

In general, the low strain and low metamorphic grade (green schist) observed along the western

side of the Iron Quadrangle resulted on the relatively simple mineralogical assemblages of the

BIF and creating a poorly orientated fabric with small mineral size, that imposed fair anisotropy

on the intact rock generally observed in great depths where the enrichment and weathering

process did not reach. This behaviour can be attributed to the presence of minerals with similar

mineralogical strength (mineral hardness), bulk density, and strong contacts between the

individual crystal defined by BIF fabric. The relationships evaluated and the adjusted regression

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curves provided equations that can be used to estimate and characterise each lithotype and the

difference from fresh to moderately weathered BIF types.

The study also indicates that the effects of the level of weathering directly influence the dynamic

elastic and rock proprieties for different kinds of BIF. Anisotropy is responsible for little

variations, mainly for FDI, HHE, FAI and FQI and moderately for PWQI and PWGI. This behaviour

is defined by using dynamic and non-destructive methods using P and S wave velocity

propagation, supported by total porosity, and bulk density measurements.

In general, ρb and Ø are the main petrophysical properties that control the dynamic elastic

behaviour, with a strong contrast between fresh from moderately weathered rocks, easily noted

from Vp and Vs results. The high correlation determination and empirical correlation obtained for

each lithotype proved to be a useful approach to describe the total porosity and Edyn with ρb. This

behaviour is noted when comparing dynamic with static elastic modules where were obtained a

correlation Edyn/Estat < 1 showing wave velocity attenuation mainly induced by the samples

inherent microcracking (noted for HHE), pores layers, mineral composition and fabric.

The techniques used to determine the total porosity, using a qualitative micro thin section

evaluation (Øb) correlated with laboratory tests (Ø), even with inconsistence related to the

measurements techniques, showed to be reliable to define the ranges of each fresh to moderate

lithotype. Total porosity is the petrophysical property that presents, as expected, inverse linear

correlation with S and P wave velocity, and this correlation can be used to estimate the

weathering grade and, indirectly, the iron content as proposed in the Appendix I. Bulk density is

also appropriate to estimate Edyn correlations. Attention must be taken to FDI correlations, due

to the presence of iron dolomites minerals that induce changes in some correlations.

The evaluated assemblage of itabirites and hematitites could be regrouped into three groups of

similar behaviour and relative parameters definition:

• PWQI and PWGI (moderately weathered itabirites group), with lower wave velocities,

Edyn values and ρb, and higher total porosity.

• FQI, FDI and FAI (fresh to slightly weathered itabirites group), with typical high range

for wave velocities and Edyn values and moderate ρb, and low total porosity.

• HHE (hard hematitite group), with higher wave velocity, Edyn values, and ρb, and very

low total porosity.

These groups are directly associated with the intact rock strength, and the weathering process

increase is mainly associated with the total porosity increase for FAI and FQI grading from fresh

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(W1) to slightly (W2) and finally to moderately weathered (W3/W4) types. In the same context,

this grading is not observed for FDI (lack of moderately weathered types), resulting in

weathering profile with just two different weathering horizons – fresh (W1/W2) and completely

weathered (W5/W6). This behaviour is attributed to the efficiency of the leaching process

(which decreases with depth), with is more prone for FDI than FQI and FAI. This behaviour is in

accordance with the distinct weathering profiles observed in Iron Quadrangle iron mines as

presented in Appendix I and II.

The use of empirical correlations graphs and equations to predict dynamic elastic and intact rock

strength parameters and petrophysical properties has proved to be a reliable methodology to

estimate the geomechanical characteristics that are directly associated with lithotype.

Additionally, the weathering grade and heterogeneity were defined as important features that

control anisotropy, bulk density, elastic parameters and, ultimately, the intact rock strength of the

BIF.

The field geological and geomechanical investigations and laboratory tests undertaken during

the research programme enabled to assess the effect of physical heterogeneity and anisotropy

on the geomechanical properties of the BIF deposits in the western side of Brazil’s Iron

Quadrangle and develop procedures that will optimise pit slope design, promote a better

understanding of potential slope failure mechanisms and reduce the risk of slope failure – gains

that will help Vale and other iron ore mines improve the operational productivity and safety of

their iron ore mines.

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CHAPTER 6. WEAK ROCK BEHAVIOUR OF HIGHLY TO COMPLETELY

WEATHERED BANDED IRON FORMATIONS FROM IRON QUADRANGLE –

BRAZIL

This chapter presents the third unpublished manuscript.

ABSTRACT

Weathering is defined as the most important factor responsible for reducing the original high

intact rock strength and converting the hard itabirite rocks into weak rocks, which display a

variety of soil-like properties, while in some cases retaining the relic structures comprising mainly

the heterogeneity derived from the original compositional metamorphic banding. As a

consequence, strength and petrophysical parameters, and other physical characteristics are

significantly affected.

In the deep weathering profiles, normally observed in Brazilian iron ore mines, there are

predominantly saprolite and residual soil in the first four hundred metres from surface.

Additionally, a climate with high pluviometry during long periods of the year and a cycle of

discharge and recharge by the rainy season impose alternating periods of saturated and

unsaturated conditions in near surface and subsurface within the banded iron formations (BIF).

For these weathered materials, saturated soil mechanics concepts dominate slope failure

analyses and slope design evaluations not only for residual soil materials, but also for weak and

completely weathered BIF for iron ore mines in the Iron Quadrangle, Brazil. These studies were

supported by laboratory tests and previous experience and has proven to be adequate to back-

analyse several observed failure mechanisms. However, the approach was not sufficient in itself

to avoid global failures and has therefore led to the adoption of very conservative slope designs,

including leaving large buttresses of rich ore at the toe of slopes.

The purpose of this chapter is to evaluate field geological petrophysical and geotechnical

characteristics and to compare with laboratory tests that determine the saturated and

unsaturated shear strengths as a means of describing the geomechanical behaviour of four

soil-like highly to completely weathered BIF types. These completely weathering typologies are

locally termed weathered argillaceous itabirite, weathered quartzitic itabirite, weathered

goethitic itabirite and weak hematitite. These materials represent more than 70% of the mined

ore in Vale mines.

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The studies undertaken included geological and geotechnical field investigation, testing to assess

in situ and laboratory (permeability, natural moisture content) particle size distribution,

Atterberg limits, drained and undrained triaxial and direct shear tests, soil–water characteristic

curves and bulk density, as well as petrographic thin section analyses for each type. From these

investigations, petrophysical characteristics, soil classification and shear strengths (saturated

and unsaturated) parameters were obtained, and shear strengths with respect to matric suction

were evaluated, considering the anisotropic effects induced by compositional metamorphic

banding observed in these material types.

Results show that mineralogical composition, bulk density, and total porosity are strongly

associated, leading to distinct shear strength properties as indicated by the Unified Soil

Classification System (USCS) soil classification and plasticity chart. Anisotropy ratio is low for

weathered argillaceous, quartzitic and goethitic itabirites, and the weak hematitite is isotropic.

Saturated strength parameters increase from the lower-strength for weathered argillaceous

itabirites to weathered quartzitic and goethitic itabirites, with the highest strength observed in

the weak hematitite. The mineral fabric, mineralogy and higher total porosity layers can induce

ductile failures at low stress level, and at higher stress levels could induce brittle failures as a

result of the post-peak loss of strength (strain softening).

Unsaturated direct shear tests show that shear strengths involving matric suction exhibit

unsaturated friction angles higher than the saturated direct shear test as a result of the suction

effect playing a role in the apparent cohesion. This phenomenon is associated with the dilatation

at low stress levels induced by slightly higher clay content of the weathered argillaceous itabirite,

presence of kaolinite and gibbsite. For this reason, unsaturated behaviour must be considered

for less conservative slope stability analyses, essentially for temporary slopes during the dry

season and in the near surface transient unsaturated zones.

It is possible to classify the weak, highly to completely weathered BIF as a saprolite to residual

soil material of thoroughly weathered rock that has not been transported. As such, the BIF can

be defined as immature residual soils, i.e. weak rocks, mainly composed of very dense sand and

silt materials. Variations in secondary clay and clast contents also define different behaviour,

highly depending on the stress level. In general, at low stress levels (below 400 kPa) BIF behave

like loose sandy material, while for high stress levels (above 400 kPa) they behave like dense

sandy material. However, variations in total porosity, bulk density and mineralogy composition

can change these behaviours.

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6.1 INTRODUCTION

In the Iron Quadrangle mines of Brazil, banded iron formations (BIF) are divided into ‘itabirite’,

the low-grade ore, and ‘hematite’, the high-grade ore. The genesis of these lithologies is

controversial, but it is assumed that hypogene and supergene processes were responsible for

the iron concentration that changes itabirites into hematitites. In addition, it is understood that

tectonic events, as well as weathering, were responsible for reducing hard itabirites (considered

original proto-ore) to weak weathered materials (saprolite) and are also responsible for

generating the deep weathered profiles commonly observed in iron ore mines.

In this chapter, the influence of the weathering on the shear strength behaviour of highly to

completely weathered BIF observed at surface and in deeper weathering profiles is evaluated.

The four weak material types studied represent more than 70% of mined ore and are classified

based on the main non-iron mineral band into weathered argillaceous itabirite (WAI), weathered

quartzitic itabirite (WQI), weathered goethitic itabirite (WGI) and weak hematitite (WHE).

As presented in Lao (2013), for current slope design, the effect of unsaturated strength and flow

on the evaluation of slope stability in residual soils has not usually been considered. This is

mainly because the mechanics of unsaturated soils are poorly understood and neglecting the

strength increase due to partial saturation is a conservative approach. Also, conventional

laboratory testing is usually carried out under saturated conditions. Consequently, this approach

probably leads to a difference between real conditions and the analytical results. Especially for

the residual soils in tropical or subtropical regions where the groundwater table is usually deep,

evapotranspiration often potentially exceeds infiltration and groundwater tables are often low

because of the climatic conditions and dense vegetation, resulting in an unsaturated top layer

with continuous air in the voids above the watertable.

In the Iron Quadrangle iron ore mines, these materials are commonly observed at depths of up

to 400 m, and this zone is normally called the superficial weathered profile. The slope stability

approach and failure mechanism analyses in this profile are based on saturated soil mechanics

principles. While this approach has been largely satisfactory, there are several other key

geotechnical issues that are still not well understood, and that appear to have caused large slope

failures that have resulted in significant disruption to mining. Otherwise, this approach has been

shown to be very conservative, mainly for short-term slopes above the water level leading to

the maintenance of larger safety berms in slope toes blocking part of the ore reserve.

It is also recognised that failure mechanisms in weak rock slopes are controlled mainly by intact

rock shear strength and locally due to anisotropic effects conditioned by the compositional

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metamorphic banding, as well as secondary factors such as structural control and groundwater.


tectonic settings of these weak and weathered rocks, which are very often located at the toe of

high slopes, where the stress is concentrated, and few studies have been conducted on these

effects.

The current approach used to support slope stability analyses in these types of weak rocks (with

soil behaviour) is based on limit equilibrium analyses using the Mohr–Coulomb failure criterion

and effective shear strength parameters of the saturated materials. The shear strength

parameters are normally supported by a limited number of laboratory tests, mainly saturated

tests (drained triaxial and direct shear tests), supported by adapted rock mass classifications.

While this approach has been mostly adequate to minimise slope stability problems, it would

appear from experience that they are conservative. Despite that, there have been examples of

large slope failures not captured by these design approaches that have had significant negative

impacts for mine production. In this matter, this charter evaluated approaches that consider the

permeability and porewater pressure effects in unsaturated slope stability analyses in order to

consider the real unsaturated soil slope behaviour.

In addition, some classical soil mechanics parameters such as total porosity, bulk density,

particle size distribution, Atterberg limits, mineral shape and mineralogy have not been properly

evaluated and considered in relation to the observed slope stability analyses and failure

mechanism behaviour and evaluations. Due to the difficulties associated with sample

preparation, representativeness and testing, a correct evaluation of these characteristics and

parameter correlations remain a key challenge for a complete behaviour understanding.

In summary, this section presents a brief review of the geological setting weak BIF and the key

geological and geotechnical characteristics of studied material types are presented. The

petrophysical characterisation and material parameter definitions are based on Vale’s

geotechnical laboratory test database, as well as WHE petrographic thin sections from Costa

(2009) and laboratory tests specially undertaken to determine bulk density, natural moisture

content, particle size distributions, Atterberg limits, soil–water characteristic curves, direct

shear and triaxial tests (drained and undrained) and permeability (field and laboratory). These

test results were summarised and evaluated to determine correlations between petrophysical

characteristics and geomechanical parameters. The chapter also discusses and arrives at

conclusions regarding the relationship between saturated and unsaturated shear strength

parameters and presents a bilinear envelope with respect to a matric suction for each typology.

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Finally, the implications of these test results on pit slope stability are discussed and presented

in Appendix II.


The aim of this section is to evaluate the geomechanical behaviour, shear strength parameters

and petrophysical characteristics based on laboratory tests and geological/geotechnical field

evaluation for highly to completely weathered BIF types. The research aims to address some key

statements summarised as follows:

• Identify and compare the differences in the geological and geomechanical behaviour,

and shear strength characteristics from different highly to completely weathered BIF

types.

• Evaluate the heterogeneity, provided by the compositional metamorphic banding, and

the importance of the anisotropy in the geomechanical behaviour and shear strength.

• Determine the parameters and evaluate the saturated and unsaturated behaviour for

completely weathered BIF.

Secondly, the behaviour characterisation is evaluated with regard to the importance of the

porewater pressure and suction effects in these soil-like materials.

The chapter is focused on the shear strength properties, as well as assessing the effect of

heterogeneity and anisotropy to establish a practical correlation as a function of the

mineralogical composition, fabric, bulk density, permeability and other petrophysical

characteristics. It does not include discontinuity characterisation.

This section is part of a PhD research project undertaken by the lead author at the Australian

Centre for Geomechanics, School of Civil, Environmental and Mining Engineering, The University

of Western Australia, sponsored by Vale S.A. The thesis research investigates the complete

weathering profile characteristics, from fresh and hard to weak and completely weathered BIF

from several Vale iron ore mine sites.

The studied mines are part of Vale’s south ferrous division and comprise of five mines (Figure

6.1) located in the centre of Minas Gerais state, Brazil, named Capão Xavier (CPX), Tamanduá

(TAM), Capitão do Mato (CMT), Sapecado (SAP) and Jangada (JGD).

190

51 Figure 6.1 Mine locations and Iron Quadrangle geological settings (modified from

Morgan et al 2013)


6.3.1 Regional geological setting

The studied area is located at the western part of the Iron Quadrangle, in the southern border

of São Francisco Craton, and comprises five mines along the Moeda Syncline and Curral

Homocline as shown in Figure 6.1.

The Iron Quadrangle macrostructure is delineated by a rough quadrangular arrangement of

Paleoproterozoic BIF of the Minas Supergroup, as proposed by Dorr (1969). This supergroup is

composed of hundreds of metres of iron-rich metamorphic rocks belonging to the Itabira

Group/Cauê Formation. The Minas Supergroup comprises, from bottom to top, Caraça, Itabira,

Piracicaba and Sabará groups, overlain by the Itacolomi Group. Below that sequence are the

Archean greenstone terrains of the Rio das Velhas Supergroup and domes of Archean and

Proterozoic crystalline rocks as studied by Machado et al. (1989), Machado & Carneiro (1992),

and Noce (1995).

The regional structure is the result of two main deformational super-positional events, as

described in Chemale Jr. et al. (1994). The first produced the nucleation of regional synclines in

the uplift of the gneissic domes during the Trans Amazonian Orogenesis (2.1–2 Gyr), and the


(0.8–0.6 Gyr) described by Marshak & Alkmim (1989). This event was mainly responsible for the

191

deformational gradient in the western side with low strain and green schist metamorphic grade,

as described by Hertz (1978). The geological settings are also shown in Figure 6.1.

The Iron Quadrangle BIF are heterogeneous banded rocks presenting a millimetre to centimetre

rhythmic alternation of iron and non-iron minerals which have been recrystallised to quartz from

original chert or jasper bands. Compositional metamorphic banding is the most typical

characteristic and defines a strong heterogeneity. This variation could be controlled by the

original sedimentary bedding, tectonic setting, metamorphic grade, hydrothermal or supergene

processes. However, the superimposition of these processes causes partial or total mineralogical

and textural changes, making it difficult to identify the heterogeneity control.

Dorr (1969) defined, for this type of iron deposit, two main lithologies: hematite or hematitite,

the high-grade ore (Fe≥ 64%), and itabirite, the low-grade ore (Fe<64%), which is divided into

three compositional lithotypes denominated by the main non-iron minerals: quartzitic,

dolomitic and amphibolitic itabirites. From these proto-ores, tectonic, metamorphic, and

weathering processes changed them in different ways and magnitudes, resulting in a multiple

setting of iron ore typology which are subdivided by the weathering profile level in horizons and

zones, rock strength and structural setting.

The origin of itabirites and associated high-grade hematite orebodies remains controversial, and

several works have been produced on this topic (Spier et.al 2003). For the genesis of the friable

orebodies some authors agree on a supergene process and residual itabirite enrichment, with

leaching of the gangue minerals by surface waters. For these orebodies, the 40Ar/39Ar dating

of manganese minerals in Vale mines, presented by Spier (2005) and Spier et al. (2006), suggest

that weathering and the mineralisation period occurred between 61.5 ± 1.2 Myr to 14.2 ±

0.8 Myr, reaching the peak process in 51 Myr. This suggests a tertiary mineralisation (66 Myr),

and after this period the weathering may not have substantially affected the weathering profile.

6.3.2 Banded iron formation geological settings

In Vale mines, hematitite and the three compositional itabirites are subdivided according to rock

hardness by a technical crusher laboratory test used to simulate the industrial process. This test

consists of crushing a known mass of sample to less than 31.5 mm, then sieving it through a 6.35

mm sieve, resulting in three main types: hard or compact (more than 50% above 6.35 mm);

medium hard or semi-compact (50% to 25% above 6.35 mm); and weak or friable (less than 25%

above 6.35 mm). Using this approach, Vale’s crusher test has been used as a guide to identify

the weathering level and the strength index (hardness), with classification into hard (R5 or R6)

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are associated to fresh (W1) to slightly weathered material (W2); medium hard (R4 and R3),

representing moderately (W3) to highly weathered material (W4); and weak (R2 to R0),

representing completely weathered (W5) to residual soil (W6) as presented in ISRM (1981)

tables.

As suggested by the new strength table present in Martin & Stacey (2018), the strength index of

the studied materials is classified as lower-strength varying from R2 to R0, and the weathering

grade varies from highly weathered (W4) to residual soil (W6). Average uniaxial compressive

strength (UCS) values of these materials are below 1 MPa, with the upper boundary, for

moderately weathered materials, reaching 10 MPa and defined as R2+. The lower boundary

overlaps soil classifications corresponding to a very stiff soil (S5) to hard soil (S6) which places

these materials in a boundary zone between rock and soil behaviour.

For the hydrogeological characteristics, the WHE, the WQI and the WGI behave as granular

aquifers with low primary but high secondary porosities and storage coefficients, which reduces

in less fractured zones. Due to the relatively high percentage of clay minerals, the WAI behaves

as an aquitard and can locally modify the aquifer flow or generate confined aquifers.

Additionally, due to annual rainfalls of between 1,500 mm to 1,700 mm with approximately 90%

of annual precipitation concentrated between October to March (summer), superficial aquifers

(transient aquifers) can occur, closely associated to the climate seasonality fluctuation (order of

tens of metres) (Reboita et al. 2015). Conversely, the deep aquifer is minimally affected by the

climate seasonality (metric order). However, it is strongly affected by mine dewatering.

The main geological and geotechnical characteristics obtained from previous reports and the

database of these types (lithotypes) can be described as follows.

Weak hematitite

Weak hematitite (WHE) is the main supergene ore type and the most extensive and richest in

iron ore mines. The original term ‘weak hematite’, defined for this type, was changed to

hematitite as suggested by several authors in order to define the rock type differently to mineral

type. It consists of thin (millimetric to centimetric) opaque/dark grey colour layers of hematite

and martite, with low apparent cohesion (friable) and high visual total porosity (Øb) alternating

with more cohesive layers of dark metallic bands of hematite and martite with higher relative

strength and lower Øb.

The Øb varies significantly from 25% to 30%, according to Costa et al. (2009), or even higher,

from 29% to 37% according to Ribeiro (2003). This lithotype presents a heterogeneity well

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marked by varying the porous levels. The anisotropic effect was defined by Costa et al. (2009)

and represents the lowest strength hematitic ore, below R2 (ISRM 1981a). In most cases it

exhibits a tectonic foliation but can also preserve the original banding in the form of high porous

layers. Figure 6.2A shows a field sample and 6.2B shows microphotographs of WHE.

(A) (B)

52 Figure 6.2 A (left), microphotograph of WHE at slope scale showing typical banding. B

(right), is a microphotograph showing micro-banding in WHE, granular and larger

hematite crystals (larger, light grey) and micro-plates of tabular hematite (smaller,

light grey) (Costa 2009)

WHE presents a bulk density (ρb) average of 3.5 t/m3 and petrographic thin sections analyses

show a monomineralic composition, essentially hematite crystals in a variable shape and size,

and secondarily martite, specularite, magnetite, quartz and argillaceous minerals (kaolinite and

gibbsite) as gangue.

Typically, these materials have a weathering grade varying from W5 to W6 and field intact rock

strength grade varying from R0 to R2 (ISRM 1981a). The associated UCS values are lower than 5

MPa, varying from 1 MPa to 2 MPa and RMR class of IV or V, according to Bieniawski (1989).

These lithotypes are associated with synclines, as a halo around hard hematite in the fold limbs

and in the lower surfaces above the itabirites. They can also occur at shear and brittle failure

zones involving specularite as described by Costa (2009). In addition, authigenic breccia is very

common and can represent a load-deformation structure associated with collapse imposed by

the leaching process and respective reduction in the itabirite volumes (Ribeiro 2003).

Crystal size and the shape of the minerals vary with tectonic settings. Larger hematite crystals

are granoblastic and smaller are micro-plates, and the specularite exhibits a lepidoblastic

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texture. In size, these iron minerals vary from 0.005 mm to 0.5 mm, with a maximum of 1 mm

(Rosière 2005) (Figure 6.2B).

Heterogeneity is characterised by the orientation of crystals, porosity variations and void

alignments. In some cases, it is possible to see two orientations (foliation and banding). Using

these micro characteristics, Costa (2009) defines three distinct sub-types of WHE:

• Subtype banded: Composed of by centimetric banding layers with less apparent

cohesion interfingered with more cohesive layers. They occur in the fold limbs and

above the itabirite, with low deformation and constituting the main type in open pits

in terms of extent. Crystal orientation is determined by metamorphic recrystallisation,

and heterogeneity is defined by the interfingered layers, with porous and tabular

hematite determining an anisotropy direction, and a medium void ratio commonly

occurring in areas with low deformation. The micro-bedding typically involves less

porous granular crystals interfingered with micro-plates of more porous hematite, as

illustrated in Figure 6.3A.

• Subtype foliated: Millimetric foliation and specularite are responsible for apparent

strong anisotropy, low lateral extension, low apparent cohesion and low porosity, are

associated with shear zones. Specularite minerals are concentrated in high

deformation areas (length increased by deformation and oriented), predominated as

a granuloblastic to lepidoblastic texture, defining an oriented foliation especially at

tabular and specular hematite crystals. All of this is shown in Figure 6.3B.

• Subtype brecciated: Typical bimodal composition with ‘clast’ layers with very low

apparent cohesion and isotropic, predominately formed by highly porous granular

crystals of hematite, and ’matrix’ layers dominated by macro-plates of low-porosity

hematite. The breccias are associated with collapsed structures (not tectonic

autogenic breccias) as defined by Ribeiro (2003) and fault zones; Figure 6.3C features

this description.

(A) (B) (C)

53 Figure 6.3 A (left), microphotography of hematite crystal, with pores and granuloblastic

texture defining the banded subtype (Costa 2009). B (centre), oriented

micro-plates of hematite, defining the foliated subtype (Costa 2009). C (right),

clast texture defining the brecciated subtype (Costa 2009)

Other forms of WHE material exist that may not originate exclusively from weathering as

previously described. When supergene, hypogene and tectonic processes are considered other

types can be identified as proposed by Costa (2009).

The mineral texture and related physical properties are important characteristics of WHE due to

the potential for brittle failure in past failures mechanisms; for example, the Patrimônio wall

failure at Águas Claras mine, as discussed by Franca (1997) and Costa (2009). As argued by

Martin & Stacey (2018), when the cohesion is destroyed this can lead to a rapid failure

mechanism, mainly caused by the overload transferred to granular particle contacts, thus

mobilising the frictional strength, which could not support the total load, resulting in rapid

failures.

An uncommon variety, not evaluated in this research, is the W6 residual soil from the complete

alteration (oxidation and hydration) from iron and non-iron bands resulting in ochreous goethite

representing the lower-strength weathered type. The strength reduction is associated with the

weathering due to the very high goethite and ochreous goethite content and the very high visual

total porosity (higher than 30%), with very low apparent cohesion as presented by Costa (2009).

This apparent cohesion is supported only by the skeleton framework as proposed by Martin &

Stacey (2018).


WAI is characterised by a dark brown colour, as shown in Figure 6.4A, resulting from the high

goethite content and clay minerals such as gibbsite and kaolinite. It has a fine laminated

structure formed by layers of hematite micro-plates, granular hematite and goethite alternated

with layers of micro-plates of hematite, goethite, quartz, gibbsite, kaolinite, and ochreous 195

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goethite. Some manganese minerals such as pyrolusite and cryptomelanite are also found

cementing the rock fragments as illustrated in Figure 6.4B.

With a restricted spatial distribution this type has a low iron content ranging from 30% to 52%;

when the iron content is greater than 52% it is considered rich and is termed rich WAI or weak

argillaceous hematite. The average ρb = 2.7 t/m3 and the natural moisture content has an average

of 10%. The hydraulic conductivity is considered low with an average value equal to

4.6×10-6m/s; as such it generally represents an aquitard or, depending on the clay band

thickness, an aquiclude.

Typically, this material has a weathering grade of W5/W6, a field intact rock strength grade

varying from R0 to R1 according to ISRM (1981), associated UCS values lower than 1 MPa and is

classified as RMR class V according to Bieniawski (1989).

According to studies by Zapparoli et al. (2007) the visual total porosity is mainly associated with

the type, defined the most common micro-texture (mineral shape and structure), as brecciated

texture, as shown in Figure 6.4B, followed by mylonitised or foliated textures; the first resulting

from collapses mainly associated with the supergene process (mimetic and laterisation) and the

second due to discrete shear zones. Brecciated laminated hematites or altered authigenic

breccias are constituted by clasts of laminated hematite surrounded by granular hematite that

could be oriented or not, and that are cemented by ochreous goethite, goethite, gibbsite, and

kaolinite. The porosity is around 15% due to the cementation.

Spier (2005) argues that the WAI are formed by the effective leaching process over the dolomitic

itabirites. However, studies by Suckau et al. (2005) and Zapparoli et al. (2007) suggested two

different geneses for the WAI: a basal unit, associated with the Batatal Formation, representing

a gradational pelitic to chemical deposition, and an intraformational unit associated with

exhalative volcanic deposits.

For WAI, the mineral texture and composition are crucial to the geotechnical behaviour due to

the association with the plasticity index and negative porewater pressure effects, which can

drastically change the shear strength, and therefore affect failure mechanisms and overall slope

stability behaviour.

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(A) (B)

54 Figure 6.4 A (left), WAI at slope scale showing layers of clay minerals. B (right),

microphotography showing brecciated texture of WAI (Horta & Costa 2016)

Weathered quartzitic itabirite

Weathered quartzitic itabirite (WQI) is the highly to completely weathered variant of quartzitic

itabirite and the second most common type in terms of spatial distribution. The iron content

ranges from 30% to 52 %. When the iron content is greater than 52% it is considered rich and is

termed rich weathered quartzitic itabirite. The average ρb = 2.2 t/m3 and the average natural

moisture content is 2.3%. The visual total porosity is mainly associated with the non-iron bands.

The weathering grade varies from W4 to W5 and field intact rock strength from R3 to R0

according to ISRM (1981). UCS values can reach 5 MPa for less weathered R2 samples, but in

general it has a UCS lower than 2 MPa and is classified as RMR Class IV and V by Bieniawski

(1989).

WQI differs from the original hard type by an increase in visual total porosity due to silica

leaching and iron band oxidation. Macroscopically, these rocks are friable, with dark metallic

grey colour bands of hematite and martite, and white to yellow friable quartz bands with a minor

amount of goethite as shown in Figure 6.5A.

The original mineralogy remains with addition of remobilised micro-plates of hematite, goethite

and gibbsite resulting from the weathering. However, there is an important increase in iron

content driven by silica leaching. The quartz bands are highly leaching layers (friable), where the

visual total porosity easily reaches 40%.

As described by Rosière (2005), WQI occurs with the normal banded fabric composed mainly of

granular hematite and lesser anhedral crystals of martite or showing a banded fabric rich in

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oriented specularite plates that define the fabric. The iron oxide band fabric is very similar to

WHE and quartz bands have a typical granoblastic polygonal fabric as illustrated in Figure 6.5B.

(A) (B)

55 Figure 6.5 A (left), photograph of WQI at slope scale showing subvertical banding

(author’s personal archive). B (right), microphotography showing banding of

tabular hematite (light grey) and granuloblastic crystals of quartz (dark grey)

of WQI (Horta & Costa 2016)


WGI differs mineralogically from other types through a higher content of iron oxides and

hydroxides (goethite and ochreous goethite). Average ρb = 2.4 t/m3 and the average natural

moisture content is 7%. The total porosity is mainly associated with non-iron bands.

Weathering grades vary from W5 to W6 and field intact rock strength from R3 to R0 according

to ISRM (1981). UCS can reach 5 MPa for less weathered R3 samples, but in general the UCS is

lower than 2 MPa. The RMR is classified as Class V by Bieniawski (1989).

This unit is characterised by an orange-brown and yellow colour as shown in Figure 6.6A, caused

by high goethite and ochreous goethite contents which acts mainly as cement, partially replacing

quartz crystals, interlayered with friable quartz bands, as can be seen in Figure 6.6B. WGI is the

result of the intense weathering mainly on itabirites but also on hematitite. It occurs mainly at

surface or along superficial fractures as a typical BIF residual soil, above WQI and WHE, but it

can also be found at depth as layers in open faults or shear zones.

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Continuous weathering increase can produce a residual soil, which is defined as ochreous

goethitic itabirite, composed basically of ochreous goethite and quartz. Additionally, there can

be an abundance of gibbsite, kaolinite, and other deleterious minerals, producing a high total

porosity and very low shear strength in this material. This type has a small areal distribution; it

was not considered in this study.

The geological and geotechnical characteristics of WGI are the same as WQI. The main

characteristic is the higher degree of weathering that increases the ochreous goethite as a

cement and can, is some cases, increase the shear strength when compared to WQI.

(A) (B)

56 Figure 6.6 A (left), photography of WGI at slope scale showing banding of ochreous

goethite, hematite, and quartz. B (right), microphotograph showing layers of

goethite, quartz and gibbsite and layers of hematite and goethite of WGI

(Horta & Costa, 2016)

6.3.3 Banded iron formations weathering profile

As described by Ribeiro & Carvalho (2002), Ribeiro (2003), and Spier (2005), the supergene iron

enrichment and the weathering are directly responsible for the reduction of the original BIF

proto-ore strengths. Other geological processes such as tectonic events (e.g. shear zones and

brecciation) as suggested by Pires (1979 and 1995), hypogene and metamorphism as exposed

by Spier et al. (2008), could also change the original rock strength.

In this chapter, the supergene concentration and weathering sequences are considered to be

the main phenomena responsible for the chemical and physical changes that reduced the

original hard and fresh rocks to weak residual soils. For several authors (Dorr 1969 and 1973;

Varajão et al. 1997; Ribeiro 2003; Spier et al. 2006; Ramanaidou 2009 and Ramanaidou & Morris

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2010) the original high-strength rocks (proto rock) were affected by weathering resulting in a

partial or complete mineral leaching, and chemical alteration.

Evaluating the canga origin, Spier et al. (2019) proposes a genetic and evolution model of the

weathering profile for itabirites of the Iron Quadrangle that includes a sequence of chemical

weathering and pedogenic processes. This model allows an independence of the pedolith

(ferruginous duricrusts) and saprolite (supergene iron ore) formation processes and the

evolution of the weathering profiles developed in itabirite. It also indicates less effective

weathering in the quartz itabirite than in the dolomitic itabirite, resulting in smaller ore bodies,

but with the same geochemical trends.

As presented by Ramanaidou and Morris (2010), the supergene iron enrichment and subsequent

strength reduction can be divided into two main processes:

• The supergene mimetic mechanism, which occurs below the watertable Morris (1980,

2002, 2002a and 2003) and the association with presence of structures, topography

and climate. It is also responsible for producing deep and large iron-rich deposits.

• The supergene lateritic weathering, which is the result of ganga mineral dissolution

and iron reconcentration above the watertable. This process produces several

weathering profiles characterised by different relative iron enrichment and ganga

mineral composition.

Studies by Ramanaidou (1989), Ribeiro and Carvalho (2002), Ribeiro (2003) and Ramanaidou

(2009) suggest that during the first leaching processes there were no volumetric changes in the

quartz to goethite substitution. Progressively, when gangue minerals start to be leached

volumetric changes take place (more effective on dolomites, but also in quartz bands) and the

replacement of quartz to goethite increases porosity. In cases where this porosity was

completely cemented by goethite or iron hydroxide at superficial levels, a hard crust called canga

was created.

As presented by Spier et al. (2003), superficial water is the main physical and chemical agent

responsible for the effective dissolution and leaching of carbonates and siliceous minerals, and

for the oxidation and hydration of iron-based minerals. The BIF form an important granular

aquifer (especially in the weathered zones), with high secondary porosity and storability that

reduces with depth, and the aquifer morphology is highly influenced by the structural domain

and fracture distribution.

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As proposed by Dorr (1969), due to the efficiency of the weathering in tropical climates induced

by high pluviometric index (rainfall), the high primary and secondary permeability of BIF rocks,

favourable topography, seasonal climate temperature changes, and physical/chemical reactions

lead to a weathering profile that can often extend to 200 m and even reach over 400 m in depth.

In addition, synclinal and anticlinal geological features, together with the presence of favourable

subvertical structural controls, as defined by high banding angles and extensional fractures,

facilitated superficial and groundwater penetration and circulation, supporting the effectiveness

of the weathering. Thus, considering the weak rocks associated with these deep weathering

profiles, found at or close to the surface, may range from highly weathered rock to residual soil.

Consequently, the strength of the weathered material might vary due to original proto-lithology

differences, mainly mineralogy, in the original three compositional types of fresh BIF, as

described by Dorr (1969), weathering intensity and geological features (such as fold or faults),

and all this geological information must be considered when evaluating strength characteristics

and behaviour.

It is also recognised that original anisotropy of some lithotypes (induced by compositional

metamorphic banding or foliation) represents an extra complexity for weathering profile

delimitation, mainly due to lateral physical and chemical variations along and through the

banding. This feature can cause strength and lateral permeability variations at the same depth

in the weathering profile, making anisotropy an important characteristic to be considered. All of

this reinforces the idea that a more pragmatic geological approach is required to define

geomechanical or lithotypes domains.

Mineral changes are not the only noted effect of the weathering. Porosity increase is also

recognised as an important change seen in those rocks. Studies by Morris (2002 and 2002a) and

Taylor et al. (2001) argue that the supergene process can reduce the thickness of the BIF by 32%

to 40% and increase total porosity from 6% to 30% (Mourão 2007). Based on that, weathering

produces an effective softening from the leaching, and the remaining iron oxide bands have a

high void ratio that could be cemented by secondary iron oxides (recrystallised hematite or

goethite). This material exhibits weak strength, and the iron enrichment promotes an increase

in bulk density due to new iron mineral concentrations and non-iron mineral leaching.

The latter finding agrees with Aylmer et al. (1978), when evaluating bulk density, iron grade, and

total porosity for Mount Tom Price iron ore, concluding that iron grade and bulk density presents

good correlation. However, accuracy is primarily affected by high values and a variability of total

porosity. According to Box & Reid (1976), for iron ore formation from Cockatoo Island, true

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specific gravity could be expressed as a function of iron content. However, due to the complexity

and multiple factors involved, the same could not be proven for porosity. Thomson (1963),

concludes that, for iron ore samples from Australia, a theoretical hematite-quartz curve can be

used for bulk density definition and can allow an approximate iron content calculation. The same

was found for South African iron ore as presented by Nel (2007), which established for the Sishen

deposits, that porosity is directly correlated to dry and bulk density, providing a reliable

calculation index.

For Brazilian iron ore mines, studies by Ribeiro et al. (2014); Santos (2007) and Santos et al.

(2005) have evaluated the association between bulk density and iron content for Vale Iron

Quadrangle BIF and concluded that there is a positive linear correlation between the total iron

content and the bulk density. In addition, those authors argue that the weathering has an

important influence on the bulk density and iron content dispersion imposed by the total

porosity.

6.4 SATURATED AND UNSATURATED APPROACHES BRIEF LITERATURE REVIEW

6.4.1 Industry overview

Brazilian iron ore mines are exposed to cyclic hydrogeological conditions that have ranged from

almost fully saturated conditions that existed before mining began to near fully depressurised

conditions during mining that have resulted in substantial reduction in groundwater levels.

Additionally, Brazil’s seasonal rainy and dry seasons can induce water level variations that can

reach a dozen metres during a single hydrological cycle considered not large give weathering

profile that can reach 400 metres depth. This cyclical watertable charge and discharge (long and

short-term) will impose alternating saturated and unsaturated conditions to the highly

weathered BIF and therefore understanding the resulting changes posed by this variation in

shear strength is the focus of this section.

The Iron Quadrangle general annual pluviometry varies between 1,500 mm to 1,700 mm with

approximately 90% of annual precipitation concentrated during October to March (summer).

During this high rainfall period it has been identified that superficial aquifers (or transient

aquifers) can form, which are intimately associated with the seasonality fluctuation. These

typically vary in depth from 1 m to 10 m depending on different variables (Reboita et al. 2015).

The deep aquifer, however, is generally only minimally affected by the climate’s seasonality but

is strongly influenced by mine dewatering.

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Concerning the BIF hydrogeological characteristics, the WHE, WQI, and WGI behave as granular

aquifers with high porosity and storage coefficients. However, due to the relatively higher

percentage of clay minerals WAI behaves as an aquitard and can locally modify the aquifer flow

or generate confined aquifers (Ventura 2009).

The development of unsaturated soil mechanics theory has principally taken place in the civil-

geotechnical engineering field (Fredlund et al. 1978). Consequently, up until recent times most

applications of unsaturated soil mechanics theory in the field of civil slope stability have been

limited to relatively shallow (in mining terms), well instrumented and monitored excavations. In

addition, as this field is relatively young in Brazil, few mining engineers have been educated in

this theory and been confident in applying it to mining engineering designs. For this reason, few

commercial laboratories offer unsaturated soil testing and main development came from

academic researches.

In situ testing in unsaturated soils could not be disregarded, however there are no proven

methods of investigations for cone penetration, pressure meters, or standard penetration test

for these types of soils similar to BIF as presented by Miller el al (2015).

For decades, slope stability analyses in weathered and weak BIF (with soil behaviour) were based

on a saturated approach using effective stress parameters. The idea of using unsaturated

concepts in the analyses is more recent and considers the pore air and water pressures existing

in the voids near surface, above the water level. In regard to weathered BIF, there has been an

industry concern that due to the typical high void content and high permeability of the materials

the amount of matric suction that could develop would be relatively low and easily lost.

However, even a minor presence of clay minerals and silt content can change these

characteristics, thereby inducing sufficient negative pore pressures that are able to influence

slope stability, as presented by Lu and Likos (2004) and Likos et al. (2010) who argue that matric

suction contributes to cohesion and increases shear strength of unsaturated soils. Also, studies

of iron formations by Grgic et al. (2005) suggested that an improvement on strength and

cohesion of unsaturated oolithic iron ore can occur due to the negative porewater pressure

effects.

In current Brazilian iron mining operations, the incomprehension of unsaturated material

behaviour and the absence of unsaturated parameters able to evaluate negative pore pressure

effects (suction) above the watertable for temporary and or permanent, has forced the

geotechnical mining engineers to adjust the unsaturated field conditions to saturated

parameters adopting effective results for this condition. With this approach, it has been possible

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to estimate possible suction effects on slope stability analyses. This incorrect approach can be

easily replaced by unsaturated laboratory tests able to properly define the material water

content (saturation) and porewater content (pore pressure) above the watertable that could

define for some completely weathered BIF negative porewater content (suction) able to add an

apparent cohesion on slope stability analyses.

The importance of evaluating the unsaturated soil characteristics has been primary noted by

researchers undertaken outside the mining industry where the shear strength of soils and

residual soils with negative porewater pressure can increase the stability of slopes, especially

those having shallow but steep surfaces, as supported by Fredlund et al. (1978). So far, matric

suction effects have rarely been considered in Brazilian open pit slope stability evaluations, so

design parameters and effective stress parameters have been obtained through conventional

saturated laboratory testing. This latter approach seems to be adequate for high rainfall areas,

where long-term slope stability is required and soil materials with high permeability and not

suitable to seasonal water changes. However, it tends to be too conservative when applied for

short-term slopes above the water level. The use of unsaturated soil mechanics is therefore an

opportunity to correct the balance between safety and economical slope design.

For Brazilian iron ore mines, regarding the application of the unsaturated theory to mining

geotechnical engineering, there has been a delay in both the transfer and application of

knowledge from the civil to the mining fields with several significant challenges presenting

themselves in larger-scale mining applications, which include:

• The increasing scale, height and variability of mining slopes.

• The difficulty and cost of instrumenting large and high slopes particularly regarding

measuring in situ matric suction.

• The exposure of the slopes to the climatic variability and inability to control matric

suctions (e.g. preventing the ingress of water).

• The fact that most soil is highly to completely weathered rocks that contain complex

relic structures.

• Unlike shallow slopes, confining stress affects the unsaturated characteristics of samples

(there is a considerably higher range of confining stresses experienced in larger slopes).

• Difficulty to find commercial laboratories able to undertake appropriate matric suction

tests.

Despite these challenges, consideration has been given to using unsaturated soil mechanics

theory for open pit iron ore mines. Grgic et al. (2005) proposed for France iron ore deposits a

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constitutive model for unsaturated behaviour of oolithic iron ore rock considering the evolution

of the strength with suction and the validity of the effective stress principle in an elasto-plastic

constitutive law. They support this model assuming that many phenomena and hypothesis were

considered by (Li 2014) and different types of iron formations by (Bourgeois et al. 2002).

6.4.2 Application of unsaturated theory to open pit mining

As postulated by Rahardjo et al. (2010), when a failure surface is situated above the water level

it could be important to evaluate the contribution of negative porewater pressure to the shear

strength. Including negative pore pressure effects in slope stability analyses may facilitate an

optimisation of pit slope design and promote a better understanding of potential failure

mechanisms. In turn, it can lead to a reduced risk of slope failure, and hence result in an

improvement of operational productivity and safety for both temporary and final slope designs.

This optimisation is based on estimating and comparing suction stress where the water content

and matric suction are first evaluated by using a soil–water characterisation curve (SWCC)

laboratory tests and the apparatus to measure the axis translation technique proposed by

Fredlund & Xing (1994) models.

From Tindal et al. (1999) the total soil suction (Ψ) is defined in terms of the free energy or the

relative water vapour pressure (moisture) of the soil. It consists of two components: matric

suction, which is attributed to capillary actions in the soil structure and is defined by air pressure

minus water suction (μa - μw), and osmotic suction, which is associated with physic-chemical

interactions between soil minerals and porewater associated with the salt content of the pore

water (π) (Equation 6.1). Both components are due to differences in relative humidity of water

soil vapour.

Ψ = (μa - μw) + π (6.1)

where:

Ψ = total soil suction.

μa = pore air pressure.

μw = porewater pressure.

π = salt content of the water.

Matric suction in general is associated with the size of the soil pores that decreases with a

decrease in soil particle size which then affects the size of the radius of curvature, consequently

increasing the matric suction pressure. Porewater pressure decreases as the degree of

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saturation decreases. For osmotic suction, the presence of dissolved salts in water decreases

the soil–water pressures, which then increases the osmotic suction.

Unsaturated soils behaviour is controlled by total suction, ψ that in turn is composed of matric

and osmotic suction. However, authors such as Fredlund et al. (2012) and Lu and Likos (2004)

have suggested that osmotic suction be ignored for low salinity engineering applications and

according to Nishimura et al. (1999) matric suction is of primary interest for engineering

problems involving unsaturated soils. This suggestion is supported for BIF due to the low salinity

normally observed at iron ore aquifers (13.2 μs/cm) as defined by Mourão, 2007 for Cauê

formation, host of iron ore mines. Thus, for Brazilian iron ore deposits negative pore pressures

should be defined by evaluating only the matric suction.

For indirect determination of the porewater pressures, Vanapalli et al. (1996) and Vanapalli &

Fredlund (2000) have proposed techniques that correlate unsaturated shear strength to

saturated shear strength, and that the SWCC describes the amount of water retained in a soil

(expressed as mass or volume water content) when it is in equilibrium at a given matric

potential. The SWCC is an important hydraulic property related to size and connectivity of pore

spaces and is therefore strongly affected by soil texture and structure. The curve is plotted on a

logarithmic scale and is highly nonlinear due to the matric potential extending over several

orders of magnitude for the range of water contents. Typical curves represent different soil

specimens (textures), demonstrating the effects of porosity (saturated water content) resulting

from variable pore size distributions (Perucho et al. 2014).

For an unsaturated soil, the matric suction and unsaturated friction angle (ɸb) are additional

parameters that increase the shear strength compared with saturated shear strength. The shear

strength of an unsaturated soil as a function of ψ can be estimated experimentally. The effective

friction angle (ɸ’) and effective cohesion (c’) as the intercept of the ‘extended’ Mohr–Coulomb

(MC) failure envelope in the shear stress axis when the net normal stress and the matric suction

at failure are equal to zero and are determined by conventional direct shear or triaxial tests on

saturated soil specimens. The slope of shearing resistance with respect to ψ is defined by the ɸb

which can assumed to be either linear, bilinear of nonlinear.

The air entry value (AEV), point ‘A’ in Figure 6.7, is defined in Fredlund & Rahardjo (1993) as the

matric suction value that must be exceeded before air is drawn into the soil pores. In terms of

strength, it is the point at which the strength of the soil deviates from its saturated effective MC

behaviour. If matric suction exceeds the AEV, the strength is assumed to follow a linear envelope

with ɸb ≈ ɸ' used for positive pore pressures. Considering changes observed above the AEV

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point, a bilinear MC envelope was proposed where ɸb < ɸ' with negative pore pressures (matric

suction). From this point ɸb is used when negative pore pressure exceeds the air entry value as

illustrated in Figure 6.7 from Nejad & Manahiloh (2017). Additionally, attributing the nonlinear

regime to the loading condition and saturation direction (wetting/ drying) changes and in some

instances, could present a linear ɸb association with Ψ.

57 Figure 6.7 Bilinear unsaturated Mohr–Coulomb envelope, modified from Nejad &

Manahiloh (2017)

With the lack of appropriate laboratory test results able to define unsaturated shear strength it

is indicated to use estimation equations based on saturated shear strength parameters and soil

classification properties as supported by Vanapalli et al. (1996), Fredlund et al. (1996), Oberg &

Sallfors (1997), Khalili and Khabbaz (1998), Bao et al. (1998) and Goh et al. (2014). Equations

that can be best fit to a dataset for determination of one or more fitting parameters are

described by Fredlund et al. (1978 and 1996).

Similarly, Rahardjo et al. (2010), using an axis translation technique from an apparatus capable

of controlling and measuring pore air and pore water pressure, were able to measure the shear

strength for the unsaturated condition, plotting and interpreting suction effects on total

material strength in an extended three-dimensional MC envelope. Procedures and apparatus

for this technique were first described by Fredlund at al. (2012) and Purwana et al. (2013). Using

this technique, according to Fredlund (2014) the failure envelope intersects the shear strength

versus matric suction plane at a total cohesion. From this, it is possible to obtain the different

matric suction values and extended MC failure envelope to define the ɸb angle. The unsaturated

direct shear strength modified apparatus used in laboratory tests and the linear extended shear

MC failure envelope are presented in Figures 6.8A and 6.8B, respectively.

A

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Measurement of the shear strength of soil under unsaturated conditions can be carried out

using modified triaxial cell or direct shear apparatuses. The modified direct shear apparatus, as

presented in Figure 6.8A, can control and measure pore air and porewater pressures in the soil

specimen independently using the axis translation technique. Details and procedures for

unsaturated tests are presented in Fredlund et al. (2012) and Purwana et al. (2013). The results

of unsaturated tests are interpreted using an extended MC envelope. Mohr circles at failure can

be plotted in a three-dimensional graph as presented in Figure 6.8B.

The results are presented as stress ratio, which is defined as the ratio of shear stress to net

normal stress, and dilatancy against horizontal displacement, under different matric suctions.

(A) (B)

58 Figure 6.8 A (left), schematic modified suction controlled direct shear test apparatus for

shear strength of unsaturated soils testing (Gan et al. 1988). B (right),

extended MC failure envelope for unsaturated soil (Gan et al. 1988)

The shear strength for unsaturated soils in which two independent stress state variables are

used was proposed by Fredlund et al. (1978) and can be viewed as an extension of theories used

for describing saturated shear strength. The MC theory can be extended to embrace a shear

strength component related to soil suction and as such it is referred to in literature as the

extended MC shear strength theory. The extended MC shear strength theory requires the

definition of two new stress state variables.

In saturated soils the effective stress (σ’) is used which is equal to (σ – uw), and in the stress state

variables referred to as the net normal stress (σ – ua) and matric suction (ua – uw), are used,

where ua is the pore air pressure.

These two new stress state variables can be used to define the extended MC envelope. The

extended MC envelope is a 3D surface rather than a 2D line reflecting the saturated condition

209

only. While the initial theory proposed a purely linear relationship (i.e. a surface made up of a

flat plane) more recently, as proposed by Vanapalli et al. (1996), this has been extended to

accommodate the nonlinear characteristics of unsaturated materials with respect to suction (i.e.

a surface made up of a curved plane). The equation of the linear extended Mohr–Coulomb

envelope in terms of net normal stress and matric suction is shown in Equations 6.2 and 6.3 as

presented in Fredlund el al. (1978).

ꞇ𝑓𝑓𝑓𝑓 = 𝑅𝑅′ + (𝑢𝑢𝑎𝑎 − 𝑢𝑢𝑤𝑤)𝑓𝑓 tanɸ𝑏𝑏 + (𝜎𝜎 − 𝑢𝑢𝑎𝑎)𝑓𝑓 tanɸ′ (6.2)

where:

ꞇff = shear stress on the failure plane at failure (kPa).

c’ = effective cohesion.

ɸ’ = effective friction angle.

ɸb = unsaturated friction angle.

(ua-uw) = matric suction at failure.

(𝜎𝜎 − 𝑢𝑢𝑤𝑤) = effective normal stress on the failure plane at failure.

𝑅𝑅 = 𝑅𝑅′ + (𝑢𝑢𝑎𝑎 − 𝑢𝑢𝑤𝑤) tanɸ𝑏𝑏 (6.3)

Where ‘c’ is the total cohesion of the soil including the effect of suction increment.

In Equation 6.3, the ɸb angle is indicating the rate of increase in shear strength with respect to

a change in matric suction (ua-uf).

The Equation 6.2, the above extended Mohr–Coulomb failure envelope, can be expressed in

terms of matric suction (ua – uf) and the effective stress (𝜎𝜎 − 𝑢𝑢𝑤𝑤) as implemented in

Equation 6.4 as follows:

ꞇ𝑓𝑓𝑓𝑓 = 𝑅𝑅′ + �𝜎𝜎𝑓𝑓 − 𝑢𝑢𝑤𝑤�𝑓𝑓 tanɸ′ + (𝑢𝑢𝑎𝑎 − 𝑢𝑢𝑤𝑤)𝑓𝑓 tanɸ′′ (6.4)

Where ɸ’’ is the friction angle associated with the matric suction stress variable (ua-uw) when

using the (𝜎𝜎 –uw) and (ua – uw) stress state variables in formulating the shear strength equation.

Tan ɸ’’ can be shown mathematically to have the following relationship expressed by

Equation 6.5:

𝑡𝑡𝑡𝑡𝑡𝑡 ɸ′′ = tanɸ𝑏𝑏 − tanɸ′ (6.5)

210

This is still a linear relationship (planar extended Mohr–Coulomb surface). The extended

Mohr–Coulomb relationship established for the Vale weathered rocks retains the linear

relationship on the effective stress plane (σ – uw) but the curve fits a nonlinear relationship for

the matric suction (ua – uw) plane.

6.4.3 Soil–water characteristic curves

As presented by Gerscovich (2001), the knowledge of the relationship between moisture and

suction is essential to address the behaviour of unsaturated soils and various field testing and

laboratory testing techniques have been proposed in the literature. Most of these methods are

hardly used in practice in view of the associated costs, lack of capable laboratory, and the

heterogeneity of the materials. Faced with this problem, several researchers proposed

mathematical (empirical) models for modelling the soil–water characteristic curve (SWCC). Most

models are based on the interdependence between the shape of the characteristic curve and

the void volume distribution or basic soil properties such as particle size distribution or porosity.

In these cases, the equations are formulated based on regression curves of experimental results.

However, the reference is rare when considering BIF soils and high slopes in iron ore mines.

According to Satyanaga et al. (2013), an SWCC is the most important soil property in unsaturated

soil mechanics. The water flow and storage characteristics of unsaturated soils are closely

related to the amount of water contained in the pores, which can be related to the negative

porewater pressure in the soil. An SWCC defines the relationship between water content and

suction (negative porewater pressure) of the soil and is used for modelling water flow through

saturated–unsaturated systems and for estimating the engineering properties of unsaturated

soil, such as permeability and shear strength. The SWCC has therefore been used for

characterising the flow of water in unsaturated embankments and the stability of slope due to

rainwater infiltration into slopes.

It is important to have a reasonably accurate characterisation of SWCC. Leong and Rahardjo

(1997) evaluated numerous SWCC fitting equations and showed that for fitting unimodal SWCC

can be derived from a single generic form. It was concluded that the Fredlund & Xing (1994)

equation gave the best fit among other. In this report, the Equation 6.6 proposed by Fredlund &

Xing (1994), was used to best fit the unimodal SWCC. The correction factor used in Equation 6.6

is shown in Equation 6.7.

211

(6.6)

where:

e = base of natural logarithm.

θs = saturated volumetric water content.

θ = calculated volumetric water content.

ψ = matric suction under consideration (kPa).

a = fitting parameter related to the air entry value of the soil (kPa).

n = fitting parameter related to the maximum slope of the curve.

m = fitting parameter related to the curvature of the slope.

ψr = fitting parameter related to the residual suction of the soil (kPa).

(6.7)

Leong & Rahardjo (1997) recommended using a correction factor C (ψ) = 1 in the Fredlund

& Xing (1994) equation for fitting the SWCC with residual matric suction less than 1,500 kPa

(Equation 6.8). This would reduce the computation effort in determining the SWCC fitting

parameter and result in the better performance of SWCC equation in fitting unimodal SWCC.

(6.8)

Zhai & Rahardjo (2012) formulated the equations for determination of SWCC variables for SWCC

data that are best fitted using Fredlund & Xing (1994) and Leong & Rahardjo (1997). The curve

fitting parameters from these equations were used to derive the variables defined in Figure 6.9.

This allows for consistent SWCC variables to be derived as compared to the conventional

graphical method where tangent lines are drawn to obtain them.

+

+

+

−=mn

s

r

r

aeC

C

ψ

θψ

θ

ln101ln

1ln1

6

factor correction101ln

1ln1

6=

+

+

−

r

r

ψ

ψψ

m

n

ae

+

=ψ

θ

ln

1

212

θs: saturated volumetric water content; θr: residual volumetric water content; ψb: AEV; ψr: residual matric suction;

(Ψi, θi ) and (a, θi): inflection point; (ψ’, θ’): point where curve starts to drop linearly; S1: slope at the infection point;

S2: slope at point where curve starts to drop linearly.

59 Figure 6.9 Definition of SWCC variables (Zhai & Rahardjo 2012)

6.5 METHODOLOGY

6.5.1 Overview

To determine the saturated and unsaturated shear strength parameters for all weak BIF and

evaluate the geological and geotechnical characteristics, a methodology was established and

divided into three phases.

The first phase included a bibliographic review and field investigations, where several samples

were collected from surface and core drills, covering all weathered types. Geological and

geotechnical information, based on ISRM (1981) suggestions, were described, and outcrops and

samples were photographed for additional visual information and physical parameter

recognition (e.g. anisotropy, bulk density, and permeability) during laboratory tests.

The second phase involved microscope work, laboratory testing and interpretation of laboratory

testing with the following key focus points:

• Petrographic thin sections analyses used to evaluate rock mineralogy, fabric and visual

total porosity to provide information from a microscopy point of view and compare it

with field macro characteristics obtained at the first phase.

Ψr Matric Suction, ψ (kPa)

Volu

met

ric W

ater

Con

tent

, θw

213

• Laboratory tests which included determination of in situ and laboratory permeability,

natural moisture content, saturated and unsaturated shear strength parameters,

porewater pressure, and soil–water characteristic curves to evaluate unsaturated

behaviour.

• UCS parameters were determined using RocData 5.0 (Rocscience 2021) adjusted fitted

curves used to determine the anisotropic ratio.

• Geotechnical characterisation using Unified Soil Classification System (USCS) soil

classification, bulk density, particle size distributions, and Atterberg limits to provide

bases for geomechanical behaviour determination and support a correlation between

geological characteristics and geotechnical parameters of each material.

The third phase consisted of interpreting the dataset derived from the previous phases

proposing correlations involving physical characteristics, geotechnical parameters and

geomechanical behaviour of each material type. It also included writing this manuscript.

Special attention was given to investigate anisotropic effects on geotechnical behaviour for

saturated and unsaturated conditions, and total and effective shear parameters. This involved

the use of single stage saturated direct shear tests with free drainage (IDST), single stage

unsaturated direct shear tests with matric suction measurements (UIDST), consolidated

undrained triaxial tests (CIU) parallel and perpendicular to banding, and soil–water

characteristic curve (SWCC) determination with wetting and drying curves.

As usual in soil tests, large variations in values of rock sample strength were obtained, and these

could often be attributed to e.g. sampling errors, the accuracy of sample preparation, testing

procedures and incorrect geological sample identification. To reduce variability due to geological

effects and defects a sampling validation approach presented in Appendix I was used. This

procedure involves a sample grouping to reduce the variance and provide a better lithotype

characterisation.

This approach checks the samples before carrying out the tests, as follows:

• Samples were grouped based on core logging or superficial mapping defined by the

level of weathering based on the ISRM (1981) table, geological characteristics (e.g.

banding features), transverse isotropy direction (β angle) and ρb.

• Geological features such as intense folding, presence of specularite, infill material with

a different weathered degree, quartz or calcite veins and others that do not represent

typical itabirite heterogeneity was discarded.

214

• Itabirite samples that did not have the typical visible compositional metamorphic

banding (heterogeneity) were not evaluated to avoid the scale effect represented by

different thickness of banding layers.

• Extreme outlier results were removed according to the box plot statistical

methodology (Whitaker et al. 2013). This technique identifies the mild outlier’s values

from the quartiles (Qt) determination, based on the Equation 6.9.


QtLower = 1Qt - 1.5(3Qt - 1Qt) (6.9)


Qtupper = 3Qt + 1.5(3Qt - 1Qt) (6.10)


The 3Qt evaluates database dispersion around a central data leaving 75% of data below

the sum and is defined by Equation 6.11:

3Qt = × + 1.5.IQR (6.11)

The first or inferior quartile (1Qt) evaluates database dispersion around a central data

leaving 25% of data below the sum and is defined by the Equation 6.12.

1Qt = × - 1.5.IQR (6.12)

Interquartile ranges (IQR) measure how spread out from a central data the values are

and these form what are called outliers and are defined by Equation 6.13:

IQR = 3Qt - 1Qt (6.13)

The first or inferior quartile (1Qt) evaluates database dispersion around a central data

leaving 25% of data below the sum and is defined by the Equation 6.14.

1Qt = × - 1.5.IQR (6.14)


and these forms what are called outliers and are defined by Equation 6.15:

IQR = 3Qt - 1Qt (6.15)

In cases of reduced number of test results (around 10), the arithmetic mean (mean) was used

to determine material property average values.

215

For BIF which presents metamorphic heterogeneous banding, it is necessary to determine the

ratio of anisotropy to evaluate the anisotropy behaviour. Estimating the variation of intact rock

strength due to the anisotropy effect allows the differentiation of spurious test results induced

by soil/rock intrinsic characteristics which can lead to misleading results and increasing variance.

In the present study, the variation of UCS due to anisotropy was considered and was determined

by the degree of strength anisotropy (RC), as first proposed by Singh et al. (1989) with defines

the degree of anisotropy as the variation in compressive strength (measured in uniaxial and

triaxial tests) depending on the angle between the direction of the load applied to the tested

samples and the direction of the anisotropy.

To evaluate the influence of compositional metamorphic banding and define an anisotropy in

intact rock strength for all tests, the anisotropy β (beta angle) as described by Jaeger (1960) was

considered by testing different angles between banding and the loading direction, varying from

0° to 90°. However, due to the reduced number of valid results for some typologies, results were

grouped into three main βangles ranges: for loading parallel to banding (β0°), all tests results from

0° < β ≤ 30° were considered; for the direction of loading oblique to banding (β45°), results from



The anisotropy ratio was defined, as presented in Singh et al. (1989), as the ratio between the



perpendicular to the planes of anisotropy and σcmin, the lowest compressive strength value

obtained. The range and classification of the degree of anisotropy established by Equation 6.16

and presented in Figure 6.10 as well as the diagram representing the βangle definition by

McLamore & Gray (1967).

𝑅𝑅𝑅𝑅 = 𝜎𝜎𝜎𝜎90°𝜎𝜎𝜎𝜎𝑚𝑚𝑚𝑚𝑚𝑚

(6.16)

where:

Rc = anisotropy ratio.

σc90° = compressive strength value for βangle perpendicular to the planes of

anisotropy.


216

60 Figure 6.10 Classification based on anisotropic ratio, Ramamurthy et al. (1993) and βangle


The anisotropic ratio applies to UCS testing results, and this test is not recommended for soil

samples with low strength as observed in weathered BIF. For this reason, triaxial laboratory

shear strength test results plotted on (σ1–σ3) graphs, and the linear regression (best fit) lines in

σ1 σ3 stress space obtained from RocData 5.0 (Rocscience 2021). Each soil type was evaluated

for the three different anisotropy directions (0°, 45° and 90°) from tests undertaken for this

research and additional data from Vale’s database. The analysis of this enhanced set of test

results provides the equivalent Mohr–Coulomb strength parameters and UCS values presented

and used to define the Rc ratio.

The quality of a correlation is determined by the coefficient of determination (R2) value, the size

of the dataset, and the visual fit of the regression curve. R2 is a measure of how well a regression

curve fits a dataset and could be interpreted from the results obtained from the adjusted model

for observed values between two dependent variables, and the strength of the correlation rank

is described by 0–0.29 (little if any correlation), 0.3 to 0.49 (low), 0.5 to 0.69 (moderate), 0.7 to

0.89 (high) and 0.9 to 1.0 (very high correlation) (Asuero et al. 2006).


For weak material types (R<2) as presented in Section 3.3.1, the laboratory tests were based on

soil mechanical methods, reflecting the low strength values. Due to this characteristic, some

samples were prepared using frozen or grouted methodology to provide proper samples for

direct shear and triaxial tests mainly.

Samples were collected from undisturbed surface blocks and drill holes by using one of several

undisturbed tube sampling techniques. The core samples were assumed to be representative of

each material type, even considering the compressive strength induced by the tube insertion,

which is in the order of 2% to 4%, according to Baligh (1985). Samples were obtained from

original 77.8 mm diamond drillhole and sub-sampling to 61.8 mm to provide a correct anisotropy

angle (β).

217

Tests were undertaken in Brazil and Australia using standard tests according to the location of

the laboratory. Where necessary, the results were normalised to be properly compared. The

undertaken tests and standards are described in the following sections.


Sample thin sections (perpendicular and parallel to banding) were prepared to evaluate crystal

and visual qualitative pore size, shape and percentage, mineralogy percentage and

micro-texture at the microscale for weathered BIF. A total of 15 thin sections were produced at

The University of Western Australia’s petrography laboratory and evaluated by Horta & Costa

(2016). Additionally, an extra 10 thin sections evaluated by Costa (2009), and another six thin

sections from a Vale internal report prepared by Zaparolli et al. (2007) were also used.

All samples were analysed using standard petrographic techniques based on a description table

created for better visualisation of the obtained information. This table contains a classification

of the soil type in the thin section, and important additional information observed during the

inspection. Microphotography was taken to show important features, textures, and

particularities.

The techniques used to determine size in thin sections are highly influenced by the angle at

which the banding is cut. For this reason, some bias is expected for these results. In order to

avoid this problem, only samples cut at 90° to the compositional metamorphic banding were

conducted for percentage proposals.

In situ and laboratory bulk density test

For the Brazilian laboratory, bulk density (𝜌𝜌b) of soil experiments in the laboratory was carried

out in accordance with ABNT NBR 16867 2020 (ABNT 2020), and for the Australian laboratory,

according to AS 1289.5.1.1-2017 (AS 2017). Additionally, results presented as Vale´s in situ 𝜌𝜌b

experiments were obtained by sand filled in accordance with NBR 7185/2016 (ABNT 2016) which

is applied to soils of any granulation that can be excavated with hand tools, and the pore spaces

need to be small enough not to be penetrated by the sand used during the experiment. Tested

material needs to be sufficiently cohesive and strong to avoid deformation during the

experiment.

Natural moisture content test

Natural moisture content of samples was based on removing moisture from samples by

oven-drying until its weight remains constant. Moisture content (%) was calculated from the

sample weight before and after drying. Experiments performed in Australia followed Standards

218

Association of Australia AS 1289.2.1.1 - 2005 (AS 2005) and NBR 6508/84 (ABNT 1984) for the

Brazilian laboratories.

In situ soil permeability test

In situ soil permeability tests were carried out to determine permeability coefficients of residual

soils and completely weathered rocks. Such experiments were carried out according to ABGE n°

04/1996 (ABGE 1996).

Particle size distribution and hydrometer

Particle size distribution (PSD) and hydrometer (sedimentation) tests were undertaken

according to AS 1289.3.6.1 -2009 (AS 2009), AS 1289.3.6.3.-2000 (AS 2000), and ASTM D422

2007 (ASTM 2007) at Australian laboratories and NBR 7217/1987 (ABNT 1987) at Brazilian

laboratories. For soil classification, USCS as defined in ASTM D-2487-98 (ASTM 1998) was used.

Atterberg limits

Atterberg limits (ATT) test were undertaken according to AS 1289.3.1.1 (AS 2009) and

AS 3.2.1 – 1995 (AS 2009) at Australian laboratories and NBR 7180 and NBR 6459 (ABNT 1984)

at Brazilian laboratories. The plasticity index was determined based on ASTM D4318-10 (ASTM

2010).

Constant and falling head (hydraulic conductivity) coefficient of permeability

The coefficients of permeability describe the rate at which a liquid moves through a soil

(hydraulic conductivity). Results are affected by fluid density and viscosity, void size, continuity,

soil particle shape, plasticity of the fines, and surface roughness. The coefficient of permeability

value is very often normalised to 20°C, marked as k20, since the viscosity of a fluid depends on

the temperature.

The constant head test was used for permeable soils (k>×10-6 m/s) and the falling head test for

less permeable soils (k<×10-9 m/s) to obtain the coefficient of permeability (k20). Tests were

undertaken according to AS 1289.6.2.1 (AS 2001) for constant head and AS 1289.6.7.2 (AS 2001)

for falling head in Australian laboratories and NBR 13292, (ABNT 1995) for constant head tests

and NBR 14545 (ABNT 2000) for variable head tests in Brazilian laboratories.

Bulk density

Bulk density was determined according to ASTM 1289.5.1.1:20017 (AS 2017) for Australian

laboratories and NBR 16867 (ABNT, 2001) for Brazilian laboratories. It was calculated according

to Equation 6.17.

219

ρb=𝑀𝑀𝑉𝑉

, (6.17)

where:

ρb = bulk density (kg/m3).

M = mass of the specimen measured prior to testing (kg).

V = volume (m3) of specimen calculated from dimensions measured during sample

preparation.

Natural moisture content

Natural moisture content testing was undertaken according to AS 1289.2.1.1 - 2005 (AS 2005)

in Australian laboratories and NBR 6508/84 (ABNT 1984) in Brazilian laboratories.

Single stage consolidated undrained triaxial compression test

For rock-like soils types, single stage, isotropic consolidated undrained tests (CIU) were



shear. This does not allow drainage at all phases of the test, and is the most common test used

for weak, porous, and granular (permeable) materials. In view of the assumed high porosity of

the large majority of highly weathered BIF (except for the WAI and some WGI), this test was

considered to be the most appropriate method for establishing results in terms of total and

effective stress (with porewater pressure measurement). Results are presented as total or

effective stress and associated porewater pressure are determined for saturated behaviour.

Australian and Brazilian laboratory triaxial tests were undertaken according to ASTM D4767-95

(ASTM 1995) and both used cylindrical specimens with a diameter of 50 mm. Confinement

stresses (σ3) of 100 kPa, 200 kPa, 400 kPa, 600 kPa and 800 kPa were applied. Test stress levels

were limited to 800 kPa maximum due to a historical soil profile depth of 250 m for shallow

mines.

Single stage intact direct shear test under consolidated drained conditions

Brazilian and Australian laboratories undertook the drained single stage intact direct shear test

(IDST) under consolidated drained conditions tests following ASTM D-3080 (ASTM 2011) and

using normal stresses of 100 kPa, 200 kPa, 400 kPa, 600 kPa and 800 kPa. For better comparison

between the different materials, the total strain (displacement rate) was set to 8%.

220

Single stage intact unsaturated direct shear test with matric suction definition

The single stage intact unsaturated direct shear tests (UIDST) with matric suction definition were

undertaken only at an Australian laboratory. These were undertaken according to AS 1289.6.2.2

– 1998 (AS 1998) with some inhouse adequations using a modified apparatus as presented in

Figure 6.8A. The main modification is related to a high air entry disk installed in the pedestal of

the pressure cell that maintains the separation of the air and water pressures that are either

applied to or measured in the soil specimen. For this test, the sample is contained in a

conventional shear box which creates a predefined horizontal failure plane between the upper

and lower components of the shear box. The shear box was 60 mm long × 60 mm wide × 56 mm

deep.

The sample is sheared at a predefined βangle by trimming and inserting the sample in the shear

box with the anisotropic fabric at the required angle to the predefined horizontal failure plane

formed by the shear box.

The consolidated drained direct shear test on an unsaturated soil specimen can be conducted

using the modified direct shear apparatus shown in the cross-sectional view in Figure 6.8A. This

machine can perform unsaturated direct shear testing and pressure plate testing to obtain the

SWCC required for air entry value analysis and suction matric analysis determined by ASTM D

6836 02 (ASTM 2002)

The test procedure is similar to that used for a conventional direct shear apparatus. However,

as the laboratory responsible for carrying out this test uses an in-house adequation, the full text

procedures were presented here in accordance with Rahardjo & Fredlund (1996). From these

authors, the direct shear test apparatus consists of a split box with a top and bottom portion.

The soil specimen is sheared by moving the lower portion of the shear box relative to the upper

portion of the box. The shear box is housed in a pressure chamber.

A motor that provides a constant horizontal shear displacement rate is connected to the shear

box base. The shear box is seated on a pair of rollers that can move along grooved tracks on the

chamber base. The top box is connected to a load cell, which measures the shear load resistance.

Vacuum grease is placed between the two halves of the shear box prior to mounting the soil

specimen in the shear box. The plumbing layout for the control board of the modified direct

shear apparatus is similar to that used for a triaxial test; however, it is difficult to measure the

volume changes of the water because of the small water volumes involved. For this reason, the

saturation of the high air entry disk and the flushing of entrapped air from the base plate and its

connecting lines should be performed prior to commencing the test. The initial air and water

221

pressures to be applied to the soil specimen can be set on the pressure regulators while the soil

specimen is being assembled.

The tests are generally performed as consolidated drained direct shear tests. The soil specimen

is usually given access to water after being placed into the direct shear box. The soil sample

needs to be fully saturated prior to being placed into the cell to ensure the test is started off on

the drying curve from the SWCC so the correct matric suctions are applied to obtain the correct

moisture content. For dry and wet curve validation tests the moisture content must be a

minimum of 0.5%, and for matric suction of 1 MPa or 1.5 MPa a maximum of 40% used to define

SWCC curves.

The chamber cap is applied following saturation of the soil. The predetermined vertical normal

load, air pressure, and water pressure are then applied to the specimen in this sequence. The

vertical normal load is applied through the loading ram at the same time that the air and water

pressures are applied.

Once the sample is placed into the cell, a check of the Skempton’s parameter ‘B’ response is

required. This should be 0.98 and above. The sample is then allowed to consolidate vertically

using an applied normal stress.

A secondary consolidation phase is required using the matric suction once the vertical

consolidation is complete. Shearing should be slow enough to ensure the equalisation of the

vapour loss and pore pressure so that matric suction remains constant.

The soil specimen is sheared at an approximate horizontal shear displacement rate after

equilibration has been reached. The horizontal shear load resistance is measured using a load

cell. Readings are also taken of vertical deformation, horizontal shear displacement, and water

volume change during shear. Shearing can be terminated either when the horizontal shear stress

resistance has reached its peak value or when the horizontal shear displacement has reached a

designated limiting value. In the case of a multistage test, the shearing process for each stage

should be stopped when the peak horizontal shear stress appears to be imminent.

Direct shear testing equipment was set with a constant horizontal displacement rate of

0.2 mm/minute, which was sufficient to ensure the tests were in a drained condition for the

coarse tested materials (WHE and WQI). A datalogger was used to record vertical displacement,

horizontal displacement, and shear force at 10 second intervals. Maximum horizontal

displacement was set to 7 mm. Soil specimens were sheared using a vertical stress between

222

75 kPa to 100 kPa. Peak shear stress for the unsaturated sample was recorded at different

predefined matric suction.

As already mentioned, the Vale database and previous laboratory reports have provided

additional laboratory test results used in this research. Only data that could be associated with

anisotropy, bulk density values and geomechanical classification equal to the ones used in the

present study were considered, each associated to a specific material type.

Summing up, the laboratory campaign consisted of tests carried out exclusively for this research,

results extracted from Costa (2009) and Zapparoli et al. (2007) and those from the Vale database

as shown in Table 6.1. Used test results are available at Appendices VI, VII, and VIII.

16 Table 6.1 Testing summary table

Laboratory tests n (Vale database) n (tested) n (total)

Thin sections 16 15 25

PSD – 23 23

ATT – 17 17

K20 – 26 26

UIDST – 52 52

IDST 23 60 83

CU 24 91 115

SWCC – 18 18

6.6 RESULTS

6.6.1 Fabric and mineralogical thin section overview

From the microscope studies of completely to highly weathered material it was possible to verify

mineralogy, visual total porosity, and textural features characteristics as described in the

following sections. Some of these micro-features are responsible for the geomechanical

behaviour observed in these weak material types.

• Weak hematitite

The weak hematitite (WHE) is basically composed of hematite and martite growing over

a granoblastic fabric (0.02 mm to 1.4 mm), as seen in Figure 6.11A and presents strong

heterogeneity when it occurs associated with lamellar iron minerals in the shape of

specularite (0.07 mm long and 0.01 mm wide), or moderate heterogeneity when

223

associated with martite and hematite micro-plates. In both cases the original

metamorphic fabric is obliterated by the tectonic setting.

This metamorphic banding induced by mineral orientation exhibits a relatively small

amount and size variation (0.005 mm to 0.05 mm) of pores, and a visual total porosity (Øb)

varying from 8% to 10 %. The granular hematite portions presented Øb from 10% to 25%,

as presented by Costa (2009), and shown in Figure 6.11A and 6.11B. These porous layers

are interconnected in some areas (due to increased leaching or alteration), defining

weaker zones, depending on the weathering intensity, as shown in Figure 6.11A.

(A) (B)

61 Figure 6.11 A (left), large crystals of granular hematite, surrounded by hematite

micro-plates and interconnected porosity filled by resin (write) (Horta &Costa

2016). B (right), granular and hematite macro-plates defining a moderate

visual total porosity band (Horta &Costa 2016)

• Weathered argillaceous itabirite

WAI is mineralogically comprised of granular and micro-plates of hematite, ochreous

goethite, goethite and granular quartz, for which the total porosity is very high.

Hematite-rich layers exhibit a lower percentage of pores (10%) as defined by Zaparolli

et al. (2007) due to the secondary cementation of the pores, as can be seen in Figure

6.12A. In contrast, for layers rich in quartz, goethite and gibbsite, the Øb can reach 30%.

The anisotropy is defined by the orientation of tabular and granular hematite, with

grains larger than 1.08 mm, and heterogeneity is provided by the alteration of specular

hematite, and gibbsite layers that occur as accessory minerals.

Ochreous goethite is more pervasive, but hematite can also occur as cement, filling voids

between hematite crystals, as shown in Figure 6.12A. Goethite usually occurs with an

anhedral shape (orange-brownish coloured), but aggregates are also present in smaller

224

concentrations and grain size. Large granular hematite (1 mm) occurs at levels defined

as weathered bands of granular hematite, exhibiting a brecciated texture (Figure 6.12B).

(A) (B)

62 Figure 6.12 A (left), microphotography of micro-plates of hematite, martite in sub-

euhedral larger crystals with the presence of ochreous goethite (brownish)

(Horta &Costa 2016). B (right), brecciated texture with large pores (black) in a

fragment of hematite surround by goethite, micro-plates of hematite,

gibbsite, and a quartz matrix (Horta & Costa 2016)

• Weathered goethite itabirite

The WGI material type is composed of an interlayering of goethite, hematite (granular

and macro-plates), granular quartz and ochreous goethite. Goethite has a dark orange

colour, and is anhedral, with few crystals. The hematite occurs as micro-plates and small

granular crystals; larger granular crystals are also present.

Hematite banding is still preserved and is defined by the remaining hematite crystals,

interbedded with goethite-quartz bands as shown in Figure 6.13B. The Øb is high (7% to

14%), slightly lower than WQI, with the intergranular pores usually associated with iron-

rich bands and larger granular crystals of hematite (Figure 6.13A). The secondary

porosity (fractures) is partially or totally filled by hematite (Figure 6.13B).

500 μm

225

(A) (B) 63 Figure 6.13 A (left), ochreous goethite (red), quartz (yellow) and gibbsite cementing

crystals of hematite (black) (Horta &Costa 2016). B (right), ochreous goethite

and goethite (red and orange) cementing the micro-plates of hematite (Horta

& Costa 2016)

• Weathered quartz itabirite

WQI is typically composed of granular and tabular hematite, goethite, and bands of

ochreous goethite, interbedded with quartz, goethite and minor ochreous goethite

bands. Kaolinite and gibbsite occur as accessory minerals. It exhibits high Øb varying

from 16% to 26%, with pore sizes varying from 0.02 mm to 0.15 mm (Figure 6.14A).

Anhedral goethite usually occurs as large crystals (0.4 mm) and the remaining tabular

hematite is concentrated at the boundaries of the iron bands, as indicated in

Figure 6.14B. Intragranular pores are connected and associated with granular hematite

(Figure 6.14B), often filled by goethite.

(A) (B)

64 Figure 6.14 A (left), band with granular hematite (large cream crystals), pores (black),

quartz (light grey) and hematite micro-plates (small cream crystals) (Horta &

Costa 2016). B (right), micro-plates of hematite layer with pore concentration

highlight at white dotted box (Horta & Costa 2016)

1 mm

500 μm

500 μm 1,000 μm

226

Table 6.2 shows summary results for Øb and mineralogy content evaluated in 31 thin sections.

This table also presents the mineral percentage and average crystal size. For porosity, the

intragranular pore percentages correspond to the percentage of pores inside crystals that might

or might not be interconnected. Intergranular pore percentage correspond to pores between

crystals and is generally interconnected. Secondary pore percentage refers to fracture void

percentages.

Visual total porosity is the sum of the three pore type percentages and is presented as maximum

total percentage. In order to evaluate the influence of fractures, the minimum total pore

percentage consider just the intergranular and the intragranular pores, and maximum total

pores consider all three pore types.

17 Table 6.2 Sum

mary table of the porous percentage for all typologies

Lithotype G

ranular

hematite (%

)

size (mm

)

(min-m

ax)

Tabular

hematite

(%) size

(mm

)(m

in-

max)

Specular

hematite

(%)

size (mm

)

(min-m

ax)

Goethite

(%) size

(mm

)(m

in-

max)

Ochreous

goethite (%)

size (mm

)

(min-m

ax)

Quartz (%

)

size (mm

)

(min-m

ax)

Gibbsite/

kaolinite

(%)

size (mm

)

(min-m

ax)

Intragranular

pores (%)

Intergranular

pores (%)

Secondary

pores (%)

Total porosity

(%)

min- m

ax

Size (mm

)

(min- m

ax)

n

(samples)

WAI

29

(0.002–1.08)

26

(0.002–1.0)

3

(0.04–0.12)

9

(0.002–1.1)

15

(0.04–1.5)

7

(0.006–0.3)

–8

2 9

10–19

(0.002–0.16)

9

WG

I 16

(0.01–1.5)

24

(0.01–0.08)

–30

(0.01–2.0)

4

(0.05–2.0)

16

(0.02–0.45)

3

(0.03–

0.10)

3 4

7 7–14

(0.005–0.11)

3

WH

E 55

(0.02–1.4)

30

(0.05–0.1)

–2

(0.005–0.08)

3

(0.04–0.3)

– –

5 3

2 8–10

(0.005–0.05)

16

WQ

I 28

(0.03–2.0)

25

(0.002–0.08)

–6

(0.002–0.04)

2

(0.05–0.15)

20

(0.01–0.120)

3 8

8 10

16–26

(0.002–0.15)

3

227

228

In general, under the microscope, Øb values are close to 20% for WQI and WHE and lower values

(close to 10%) for WAI and WGI. Considering void sizes, the largest pores were observed for

WHE, while the other types showed close to average pore sizes.

A summary from Zaparolli et al. (2007), and Costa (2009), results in visual total porosity averages

of WHE (31%), WQI (28%), WAI (15%), and WGI (14%), upon comparing with Table 6.2 it is noted

that test results presented lower average pore percentages for WHE (9%), WQI (21%), and close

to mean from WGI (11%) and WAI (15%). This difference is verified due to several reasons: e.g.

number of tests, sample site, and sample tectonic setting.

From this study, considering cited references and evaluated thin sections, WAI and WGI were

considered as lower Øb types (less than 15%) and WHE and WQI as higher Øb types (higher than

15%).

6.6.2 Unified soil classification system

The use of the USCS as outlined in ASTM 2487 – 11 (ASTM 2011), represents a soil characteristics

identification capable of distinguishing between the weathered BIF types based on particle size

distribution curves and the plasticity index.

Figures 6.15A, 6.15B, and 6.15C show the PSD curves obtained for WAI (green curves), WQI/WGI

(blue curves), WHE (red curves), respectively; the mean curve is included in each graph as a black

line.

229

(A)

(B)

230

(C)

65 Figure 6.15 A (top), WAI PSD curves – green. B (centre), WHE PSD curves – red. C (bottom),

WQI and WGI PSD curves – cyan. Black curves are the mean value for each type

A certain amount of dispersion for each group is noted; this could be attributed to heterogeneity

caused by different layer thicknesses of iron or non-iron bands that could result in physical and

chemical dispersion for the same material type. Due to the small number of WGI test results and

similarities with WQI results, these two types were grouped for the next evaluations. Table 6.3

presents the percentage content for each particle size distribution (PSD) and textural USCS soil

classification in accordance with ASTM D2487-11 (ASTM 2011).

Table 6.3 also presents the Atterberg limits results for fractions below 425 μm for each material

type and the ρb values. Empirical Atterberg limits describe how moisture content and intrinsic

soil characteristics interact to convey their elasto-plastic behaviour. These tests allow

classification and identification of the fine soil fraction and complete the USCS classification.

18 Table 6.3 Sum

mary table w

ith main soil characterisation param

eters

Sample ID

Field

description (lithotype)

PSD (total sample)

Atterberg limits (<425 m

icron) U

SCS classification from ASTM

D2487-10

Clay %

Silt%

Fine sand %

M

edium

sand%

Coarse sand %

Gravel

%

LL PL

PI SL

USCS classification of

sample fines only from

the plasticity chart

USCS of

sample

USCS

description of sam

ple

USCS description

of lithotype group

SP20 W

AI 2.6

47.9 15.8

16.9 16.8

0.0 35

21 14

17 CL

SC Clayey sand

Loam - sand-silt,

medium

to non-uniform

, well

graded (SM)

S5 2.5

53.8 31.8

10.3 1.6

0.0 24

17 8

15 M

L SM

Silty sand

S12 2.3

42.5 34.1

14.0 6.7

0.4 25

19 6

17 M

L SM

Silty sand

BL_1 4.5

40.6 13.3

8.1 19.1

14.4 40

34 6

– M

LSM

Silty sand

TAM-BL-01

(10388) 0.5

20.9 13.4

14.7 23.5

27.1

TAM-BL-02

(10389) 0.8

33.4 11.3

17.5 28.6

8.5

7 3.0

54.2 37.2

3.4 2.2

0.0 37

15 22

– CL

CL Inorganic clay

Mean

2.3 41.9

22.4 12.1

14.1 7.2

32 21

11 16

WHO

W

HE 1.5

22.7 18.5

34.4 16.6

6.3 22

19 3

19 M

L M

L Loam

- sand-silt, non-uniform

, well

graded (SM)

BL3 1.6

44.9 16.8

13.3 17.5

5.9 19

12 7

11 M

L M

L

BL18 2.9

42.4 33.1

11.2 8.1

2.3 29

16 13

14 CL

CL

TAM-BL-3

(10390) 2.3

48.0 16.3

13.7 15.3

4.4

TAM-BL-08

(10395) 0.1

29.4 8.4

14.7 29.4

18.0

231

TAM-BL-17

(10425) 1.8

41.6 9.0

15.0 19.9

12.7

TAM-BL-18

(10426) 1.3

62.4 23.2

6.4 5.1

1.6

TAM-BL-20

(10428) 1.1

30.7 5.1

10.7 27.2

25.2

Mean

1.7 40.4

17.0 14.6

17.0 9.3

23.5 16.0

7.5 14.4

BL12 W

QI - W

GI 10.0

46.1 5.8

2.7 14.2

21.2 38

17 21

13 CL - O

L Loam

- silt-sand w

ith gravel, non-uniform

, gap graded (SM

) BL16

6.2 45.0

7.9 5.1

11.4 24.4

39 12

27 9

OL

TAM-BL-05

(10392) 0.0

37.7 9.1

11.2 24.9

17.1

TAM-BL-11

(10419) 0.6

71.6 6.4

8.1 11.0

2.3

TAM-BL-12

(10420) 0.9

58.8 5.3

8.7 10.8

15.5

TAM-BL-13

(10421) 3.1

32.6 3.7

19.1 24.4

17.1

TAM-BL-15

(10423) 2.0

25.4 5.6

11.0 16.2

39.8

TAM-BL-19

(10427) 0.6

49.7 6.3

6.1 18.4

18.9

Mean

2.9 45.9

6.3 9.0

16.4 19.5

38 14

24 11

PSD: particle size distribution; USCS: soil classification; LL: liquid index; PL: plastic lim

it; PI: plastic index; LS: shrinkage; CL: clay; SC: sand with clay; M

L: silt; SM: sand w

ith silt; OL: organic soil.

232

233

Considering the PSD, results of the WAI and WHE have similarities: a lower percentage of gravel (less

than 10%) and higher fine and medium sand percentages (higher than 30%). WQI and WGI have a

higher relative gravel percentage (20%) and lower fine and medium sand percentages (15%).

Based on USCS classification and the shape of PSD curves, WAI and WHE (SM) exhibit a similar

classification to ’fine-grained soils’ or ‘silt-sand soils‘, with curves well graded and non-uniform.

WQI/WGI (MS) were classified as gravel soil gap graded, mixed grained soil or silt-sand soils,

with a gap in the size of fine to medium sand. In general, all curves were shown to be

non-uniform and with a higher individual percentage of silt (a variation between silt and sand

fraction content is observed for each material).

WAI displays a high dispersion in the plasticity chart; however, it is mainly silt (ML) with a low

plasticity except for one sample showing high plasticity (sample WAI 3). The sample WAI 3 result

came from a single layer of high clay content and low activity (gibbsite and kaolinite). WHE and

WQI/WGI exhibit a dual classification with a group plotted in silt (ML) with low plasticity and

another group with intermediate plasticity (CL).

The clay size fraction percentage varies from 2% to 3% in average and can reach 10% in some

samples (WAI 3). This clay content is in part related to kaolinite and gibbsite (low activity

minerals) and another part (percentage not evaluated) is composed of clay size minerals

composed basically of non-clay minerals such as iron oxides and hydroxides, including goethite

and ochreous goethite.

234

6.6.3 Bulk density

Bulk density results obtained from PSD samples is presented in box plot graph pretence in Figure 6.16.

66 Figure 6.16 Box plot graph for bulk density results from PSD tests, presenting for each

weathered BIF type the mean line and the inclusive median. Dotted lines

represent the limits of each type

Figure 6.16 shows that WHE presented higher ρb with a median (which is at 50% of results) of

4.5±0.7 t/m3, WQI/WGI presents median ρb of 3.8±1.3 t/m3 and the WAI of 2.7±0.5 t/m3.

An overlapping is noted between WHE and WQI/WGI above ρb ≥ 3.5 t/m3 and for WQI/WGI and

WAI below ρb ≤ 3.2 t/m3. This is e explained by the iron content and mineral composition of

WHE, WQI and WGI, mainly constituted of iron oxides and hydroxides, and quartz with higher

iron content and ρb. WAI presents a high amount of clay minerals (kaolinite) and other

low-density mineral as gibbsite. It is important to note that kaolinite is a non-expansive mineral.

6.6.4 Coefficient of permeability

Table 6.4 presents the coefficient of permeability at 20°C (k20) obtained from constant and falling

head laboratory tests, in three anisotropic directions, with additional information on initial

moisture content (natural moisture) and bulk density of tested specimens.

3.2

2.2

4.7

2.6

5.2

235

19 Table 6.4 Coefficient of permeability (k20), initial moisture content, bulk density, and

anisotropy direction

Sample ID Lithotype Head test Anisotropy

(°)

Initial moisture

content (%)

Bulk density

(t/m3)

k20 (cm/s)

TAM_BL_1_FH_A WAI Constant 0 9.4 2.33 4.47E-07

TAM-BL-01-10388 WAI Constant 0 34.7 2.23 7.60E-06




TAM_BL_18_FH_B WAI Constant 90 9.7 2.75 1.45E-06

AM-8 WAI Falling 90 1.0 2.66 1.09E-05


AM-13 WAI Falling 90 1.0 3.13 8.06E-06

TAM_BL_1_FH_B WAI Constant 90 9.1 2.27 1.98E-06

WHO WAI Falling 90 1.9 2.73 1.79E-05

Mean WAI

9.9 2.69 6.57E-06

TAM-BL-02-10389 WHE Constant 90 6.8 3.69 1.50E-04


WHE7_Fh WHE Falling 45 2.6 3.44 1.03E-03


TAM_BL_3_FH_B WHE Constant 90 5.3 3.24 7.26E-06


Mean WHE

6.5 3.46 3.12E-04

TAM-BL-08-10395 WQI Constant 0 12.1 3.42 3.20E-05


TAM_BL_12_FH_A WQI Constant 0 7.1 2.43 1.92E-07

TAM_BL_16_FH_A WQI Constant 0 5.1 2.69 1.98E-06



TAM_BL_12_FH_B WQI Constant 90 7.2 2.44 2.32E-06

TAM_BL_16_FH_B WQI Constant 90 5.1 2.59 8.27E-06

TAM-BL-05-10392 WGI Constant 90 12.4 3.00 3.10E-05


Mean WQI

8.3 2.68 9.14E-05

The K(20) obtained are in accordance with the USCS classification obtained for each type

presented in Section 6.6.2 and agree with typical k20 values for permeable soils. From WHE with

low permeability (3.1×10-4 cm/s) and through WQI – WG with low permeability (9.14×10-5 cm/s)

in accordance with sand silts with median to low permeability. The clayey WAI with very low

permeability (6.51×10-6 cm/s), with similar agreement with USCS classification and Øb, as noted

236

in Section 6.6.1 presents K(20) of consistent with low permeability silt sands. This relative higher

k20 is attributed to the higher clay content observed for this type.

6.6.5 Saturated hydraulic conductivity and unsaturated functions

This important parameter is constant for a saturated state but varies for an unsaturated state in

accordance with the saturation level defined by the degree of matric suction or volumetric water

content.

The degree of the matric suction was obtained from the SWCC curves evaluated from several

tests of each type.

All material types had SWCC determined using curve fitting and are presented in Figure 6.17.

The soil types are separated by colour, with different lines textures for each sample. The fitting

parameters of the normalised SWCC were obtained using the methodology proposed by Zhai

& Rahardjo (2012) (Method C) and are summarised in Table 6.5.

67 Figure 6.17 Shows the SWCC adjusted fitting curves for all evaluated typologies as

proposed by Zhai & Rahardjo (2012)

237

Table 6.5 Fitting parameters from SWCC adjusted curve for all material types

SWCC fitting for Zhai & Rahardjo, 2013 – Method C

Sample ID Typology Bulk

density

(t3/m)

Air entry value Inflection Residual

ψa (kPa) VWCsat

θsat (%)

ψi (kPa) VWCi

θi (%)

ψr (kPa) VWCr

θr (%)

BL 1 A WAI 2.33 20 78 67 37 136 0

BL 1 B 2.27 17 74 45 37 79 0

Sample 20 1.55 141 44 569 44 1,547 4

Sample 5 3.09 25 44 160 37 447 3

Sample 12 2.52 39 62 212 37 519 3

WHO WHE 2.67 39 92 108 44 230 2

BL 3 A 3.47 7 48 36 37 89 1

BL 3 B 3.45 11 42 39 37 82 0

BL 18 A 2.86 16 59 379 40 67 0

BL 18 B 2.99 11 53 32 38 60 0

WHE 6 3.97 57 10 79 50 109 2

WHE 2 3.11 43 28 86 41 138 1

BL 12 A WQI 2.27 19 26 68 40 154 1

BL 12 B 2.32 22 21 57 43 113 2

BL 16 A 2.75 11 30 39 37 78 0

BL 16 B 2.76 6 24 30 36 74 1

AM 8 WGI 2.65 16 38 48 37 89 0

AM 13 3.13 8 29 21 37 37 0

e: base of natural logarithm; θsat: saturated volumetric water content; θ: calculated volumetric water content; ψa: matric suction

under consideration (kPa); a: fitting parameter related to the air entry value of the soil (kPa); n :fitting parameter related to the

maximum slope of the curve; m: fitting parameter related to the curvature of the slope; ψr: fitting parameter related to the residual

suction of the soil (kPa); ψi: fitting parameter related to the inflection suction of the soil (kPa); θ: inflection volumetric water content.

From Figure 6.17 it is possible to note that WHE, WQI and WGI present similar curve shapes with

steep slopes (dark blue, green, and red curves), and that WAI presents smooth curves (light

blue). The WHE, WQI and WGI SWCC are typically sand materials, while the WAI is sandy-silt

material.

From the WAI tests, Sample 20 is an outlier, which resulted from a silt-sand with more clay

content, as easily identified by its lower ρb (clay layer) as already mentioned in a previous

section.

To define he experimental unsaturated parameters were obtained from SoilFlux SoilVision

Software (SoilVision System LTD. 2009) that allow the use to select the unsaturated hydraulic

conductivity estimation functions developed by Fredlund and Xing (1994) and others.

238

Using the parameters for each of the individual samples (as presented in Table 6.5), a

representative (or adopted) SWCC was developed for each weathered BIF typology. These

representative curves for each typology are summarised in Table 6.6.

20 Table 6.6 Fitting parameters from SWCC adjusted curves for all material types from

SVFlux software using the Fredlund & Xing (1994) fitting curve

Typology WAI WQI-WGI WHE

Saturated VWC 0.98 0.96 1.00

Specific weight 2.55 2.65 3.22

af (kPa) 808 72 104

nf 1.42 2.13 3.28

mf 3.72 4.06 4.35

hr (kPa) 1,837 118 140

error (R2) 0.999 -1.559 0.996

Residual VWC 0.0047 0.0008 0.0001

Residual percentage (%) 0.48 0.08 0.01

Within point (W/C) 0.1437 0.0003 0.0001

AEV (kPa) 156 23 45

Max slope 0.92 1.38 1.94

Saturation suction (kPa) 0.1 0.1 0.1

af: fitting parameter, a function of the air entry value; hr: fitting parameter, a function of the suction which residual water content

occurs; nf and mf: fitting parameters related to particle size diameter; AEV: air entry value

The relationship between permeability and suction can be patterned with an unsaturated

function using SWCC and permeability; the range obtained is in accordance with sand silt soils

and sand silty clayish soils. For this research, the Fredlund and Xing (1994) method of fitting

curve was used.

The methodology followed to determine the unsaturated permeability is referred to as the

statistical approach and is outlined in Zhai & Rahardjo, 2012. The Leong & Rahardjo, 1997

formula used in this approach is shown in Equation 6.18.

𝐾𝐾𝑢𝑢𝑚𝑚𝑢𝑢𝑎𝑎𝑢𝑢 = 𝐾𝐾𝑠𝑠

�1+𝑙𝑙𝑚𝑚((𝑆𝑆𝑎𝑎)𝑏𝑏)�𝑐𝑐.𝑝𝑝 (6.18)

239

where:

kunsat unsaturated coefficient of permeability.

Ks saturated coefficient of permeability.

S degree of saturation.

‘a’, ‘b’, ‘c’ and ‘p’ constant determined by curve fitting the permeability data.

A representative WAI sample set of SWCC fitting parameters was chosen based on the

evaluation set out in previous sections. These were used to calculate the SWCC statistical

midpoint values which are shown in Figures 6.18A and 6.18B and the corresponding kunsat values

and their best fit using the Leong & Rahardjo (1997) formula as shown in Figure 6.18B.

(A) (B)

68 Figure 6.18 A (left), best fit curve for volumetric water content used to determine the WAI

sample SWCC statistical midpoint. B (right), best fit curve for permeability

function

From the best fit adjusted curve, it is possible to obtain parameters using Leong & Rahardjo

(1997) equations for WAI obtained from adjusted curves presented at Figure 6.18A. Obtained

parameters are a = 506.25, b = 1.2, c = 3.42 and p = 3.33.

A simple case study, evaluating the factor of safety (FoS) resulted from usual approach using

saturated effective and using unsaturated parameters obtained in this chapter is presented and

discussed in Appendix II.

6.6.6 Saturated shear strength characteristics

To define the saturated shear strength characteristics of the materials being studied, two types

of tests were undertaken: single stage direct shear tests under consolidated drained conditions,

240

and consolidated undrained triaxial. Each type of test contributed to the saturated shear

strength characterisation.

Single stage intact direct shear test under consolidated drained conditions

Drained shear strength is usually used for soils where pore water pressures can dissipate

relatively quickly (relatively high permeability), or where the soil is over an unsaturated

condition and hence pore water pressures are negligible, permitting the definition of total and

effective shear strength parameters.

For all tested materials, typical shear stress versus shear displacement and normal versus shear

displacements were determined for drained conditions under three different normal stress

levels tested in a single stage for each stress level: Stage level 1 equal to 100 kPa, Stage level 2

equal to 400 kPa and Stage level 3 equal to 800 kPa.

Typical IDST characteristic curves are presented in Figures 6.19, 6.20 and 6.21 in order to

evaluate the main physical and geotechnical characteristics, anisotropy effects are not

considered in this stage and all text presented were undertaken at β0°.

• WAI

(A) (B)

69 Figure 6.19 A (left), WAI – normal displacement versus shear displacement (BL-1).

B (right), WAI – effective shear stress versus shear displacement.

Stage 1 = 100 kPa, Stage 2 = 400 kPa and Stage 3 = 800 kPa

In Figure 6.19, the WAI specimens mobilised the shear strength in a similar way for all stages,

reaching the peak shear strength at the end of the test (8% horizontal displacement), with higher

final values according to the increase in the normal stress. The initial shear stiffness is shown to

increase with the increasing normal stress.

241

• WHE

(A) (B)

70 Figure 6.20 A (left), WHE – normal displacement versus shear displacement (BL-3).

B (right), WHE – effective shear stress versus shear displacement. Stage 1

= 100 kPa, Stage 2 = 400 kPa and Stage 3 = 800 kPa

Figure 6.20 shows the IDST results for the WHE specimens. Different results are observed for

shear development when compared to WAI mainly related to brittle behaviour more evident for

WHE typical curves. All curves initially exhibit elasto-plastic behaviour, but higher peak shear

strengths were obtained, except for the 100 kPa normal stress. It is also seen that peak shear

strength is reached towards the end of the test for specimens tested at 400 kPa and 800 kPa,

but at lower displacement for the 100 kPa normal stress. The displacement results demonstrate

that compression only occurs during shearing at intermediate and higher normal stresses, but

that a dilatant behaviour is detected during shearing of the lower confined specimen. This

overall behaviour is typical of intermediate to dense sand soils where the inherent dilatant

specimen behaviour at low confinement stress is inhibited with the increase in the normal stress.

All curves initially exhibit dilation. However, for higher levels of strain the volume of the

specimen tends to decrease. This could be due to slipping and rotating of particles according to

Wu et al. (2008) or due to crushing of particles and rearrangement of particles to become more

densely packed. For low stress levels (100 kPa) an expansion (compaction) is noted after 1 mm

due to the interlocking state change toward a more stable state.

• WQI/WGI Due to the reduced number of available test results, it was not possible to determine WGI

characteristics alone; for this reason, WGI and WQI were evaluated together due to the

similarities observed between these two materials, as discussed previously.

242

(A) (B)

71 Figure 6.21 A (left), WQI/WGI – normal displacement versus shear displacement (BL-12).

B (right), WQI/WGI – effective shear stress versus shear displacement. Stage 1

= 100 kPa, Stage 2 = 400 kPa and Stage 3 = 800 kPa

The WQI/WGI IDST results in Figure 6.21 show some differences in behaviour when compared

with those for WAI and are very similar to WHE. Peak shear strengths are mobilised at different

horizontal displacements; 3% to 4% for 100 kPa normal stress, around 7% for 400 kPa, and close

to 6% for 800kPa. Differing from the previous studied types, in this case there is a slight decrease

of shear strength towards the end of the test. Regarding normal displacement, at low normal

stress (100 kPa) there is high expansion, while the 400 kPa and 800 kPa specimens experience

slight compression (around 0.1 mm). Thus, this material type also shows dilatant behaviour at

low normal stress and slight contraction with increasing normal stress. This overall behaviour is

also similar to medium to dense sands, however, present a contractive behaviour during

shearing at low normal stresses and has the lowest normal displacement.

Summing up, WAI, WHE and WQI/WGI show general elasto-plastic behaviour, with lower plastic

behaviour for WAI and maximum shear strength resistance mobilised by WHE. Contractive

behaviour could be verified for WAI during shearing and for the WQI and WGI materials at

intermediate and higher normal stresses, while a dilatant behaviour was observed for WHE and

WQI/WGI specimens at low normal stress.

Overall, WAI specimens behave more like loose sands and WHE and WQI/WGI exhibit similar

behaviour to medium to dense sands. In addition, for all types it was noted that stiffness

increases with the normal stress level.

Consolidated undrained triaxial compression test

This is the test commonly used to evaluate shear strength parameters and undrained behaviour

considering porewater pressure effects, a series of CIU tests were undertaken.

243

Typical effective and total shear stress versus displacement curves for anisotropy angles equal

to 0° and 90° and effective p-q’ stress space using three confinement levels (stages) have been

presented for each type. The σ3 confinement stresses used were 100 kPa, 400 kPa and 800 kPa.

For proper comparison maximum strain was limited to 8%.

Typical CIU characteristic curves for each type are presented below in order to evaluate the main

physical and geotechnical characteristics, anisotropy effects are also considered in this stage.

• WAI

Typical CIU WAI sample result curves are presented in Figures 6.22A and 6.22B; Figures 6.22C

and 6.22D present shear stress versus axial strain (deformation), while Figure 6.23 presents p´

versus q curves.

(A) (B)

(C) (D)

72 Figure 6.22 A (upper left), CIU shear stress versus strain curve for WAI at β0°. B (upper

right), effective porewater pressure versus axial strain for WAI at β0°. C

(bottom left), CIU shear stress versus strain curve for WAI at β90°. D (bottom

right), effective porewater pressure versus axial strain for WAI at β90°. Sample

– BL 1 – 10388

CP I (100 kPa) -----

CP II (400 kPa) -----

CP I (100 kPa) -----

CP II (400 kPa) -----

244

73 Figure 6.23 Effective stress path curve (p-q’) for 0° and 90° WAI (Sample – BL 1 – 10388)

• WHE

Typical sample result curves are presented in Figures 6.24A and 6.24B, Figures 6.24C and 6.24D

present shear stress versus axial strain (deformation), while Figure 6.25 presents p´ versus

q curves.

245

(A) (B)

(C) (D)

74 Figure 6.24 A (upper left), CIU shear stress versus strain curve for WHE at β0°. B (upper

right), effective porewater pressure versus axial strain for WHE at β0°.

C (bottom left), CIU shear stress versus strain curve for WHE at β90°. D (lower

right), effective porewater pressure versus axial strain for WHE at β90°. Sample

– TAM BL-08 -10395

CP I (100 kPa) -----

CP II (400 kPa) -----

CP I (100 kPa) -----

CP II (400 kPa) -----

246

75 Figure 6.25 Effective stress path (p-q’) curve for 0° and 90° WHE. Sample – TAM BL-08 -10395

247

• WQI/WGI

Typical CIU WGI/WQI sample result curves are presented in Figures 6.26A and 6.26B; Figures

6.26C and 6.26D present shear stress versus axial strain (deformation), while Figure 6.27

presents p´ versus q curves.

(A) (B)

(C) (D)

76 Figure 6.26 A (top left), CIU shear stress versus strain curve for WQI/WGI at β0°. B (top

right), effective porewater pressure versus axial strain for WQI/WGI at β0°.

C (bottom left), CIU shear stress versus strain curve for WQI/WGI at β90°.

D (bottom right), effective porewater pressure versus axial strain for

WQI/WGI at β90°. Sample – BL 12 – 10420

CP I (100 kPa) -----

CP II (400 kPa) -----

CP I (100 kPa) -----

CP II (400 kPa) -----

248

77 Figure 6.27 Effective stress path (p-q’) curve for 0° and 90°WQI. Sample – BL 12 – 10420

From the previous figures it is possible to note that:

• All tested material types at anisotropy of 0° exhibit peak of shear resistance at around

5% deformation.

• At confinement of 100 kPa all tests showed no clear peak strength and a gentle

post-peak softening (ductile behaviour).

• All materials show a brittle behaviour at higher confinement stresses (400 kPa and 800

kPa).

• Clear peak resistance and post-peak softening occur for all tests at high normal stresses

(800 kPa).

• WAI and WHE exhibit lower differences for 0° or 90° even for higher stress levels.

• WAI shows typical contractile behaviour not observed in other types.

• WAI shows clear peak and moderate strain softening, WHE exhibits no softening and

WQI, shows a clear peak with very high strain softening.

• Considering the effective stress path (q-p’ curve), WAI presented a moderately

contractive curve showing an increase of porewater pressure, except at 100 kPa

normal stress.

• WHE and WQI-WGI exhibit dilative stress path curves for all stress levels, showing a

decrease of porewater pressure.

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Strength anisotropy evaluation

The samples’ strength anisotropy was evaluated according to three directions: βangle equal to 0°

(parallel to the metamorphic banding), 45° (diagonally to the metamorphic banding) and 90°

(perpendicular to the metamorphic banding).

Due to the reduced number of available test results for WQI and WGI these types were

evaluated as a single group due to the similar characteristics observed in previous sections.

The figures 6.28 to 6.30 shown MC linear regression curves presented in σ1 (major principal

stress) and σ3 (minor principal stress) space. To build these curves were used the available

results from each test using RocData 5.0 (Rocscience 2021) to obtain the equivalent HB UCS

adjusted value for each material in three different βangle and an isotropic condition for total and

effective parameters.

To be able to properly compare, the test results were plotted with the same configuration

(scale), and MC envelope was used as the strength criterion, applying a custom envelope model.

For fitting curve analyses, a linear regression algorithm was used, considering the vertical error

simulation, and absolute error.

The following graphs show the major versus minor principal stress for each material (WAI,

WQI/WGI and WHE) separated by βangle as presented in Figures 28A and 28B, 29A and 29B, and

30A and 30B respectively.

(A) (B)

78 Figure 6.28 A (left), WAI linear regression (best fit) lines in σ1σ3 stress space for total

strength values. B (right), linear regression (best fit) lines in σ1σ3 stress space

for effective strength obtained from RocData 5.0 (Rocscience 2021) triaxial CIU

tests considering different anisotropy directions (45° – blue, 0° – red, 90°

– green, and isotropic – black)

250

(A) (B)

79 Figure 6.29 A (left), WHE linear regression (best fit) lines in σ1σ3 stress space for total


effective principal stress obtained from RocData 5.0 (Rocscience 2021) triaxial CIU

tests considering different anisotropy directions (45° – blue, 0° – red, 90° – green,

and isotropic – black)

80 Figure 6.30 A (left), WQI/WGI linear regression (best fit) lines in σ1 σ3 stress space for total


for effective strength obtained from RocData 5.0 (Rocscience 2021) triaxial CIU

tests considering different anisotropy directions (45° – blue, 0° – red, 90°

– green, and isotropic – black)

To evaluate the anisotropic effects related to shear strength values interpreted from the

analysis, the Ramamurthy (1993) method was applied to calculate the anisotropy ratio (Rc) for

UCS fitted parameters based on the CIU tests. Table 6.7 shows the results for all material types.

251

21 Table 6.7 CIU shear strength parameters for WHE, WQI/WGI and WAI considering each

βangle results


β (°)

Strength RocData

fitted UCS

(kPa)

Samples

(n)

Rc

total

Rc

effective

Class

WAI 0 Total 117 11 1.4 1.2 Low

Effective 354 11

45 Total 343 7

Effective 343 8

90 Total 158 7

Effective 425 6

WHE 0 Total 413 10 1.2 1.1 Isotropic

to low

Effective 340 11

45 Total 386 14

Effective 307 13

90 Total 482 10

Effective 340 11

WQI/WGI 0 Total 283 19 1.6 1.7 Low

Effective 118 10

45 Total 252 14

Effective 307 14

90 Total 394 16

Effective 205 9

From Figures 6.28, 6.29 and 6.30, and Table 6.7 it is possible to conclude that:

• Highest UCS values were obtained at β90° for all material types.

• WAI and WQI-WGI showed low anisotropy ratios, while the WHE presented

isotropic ratio is equivalent to isotropic for the total and effective parameters.

To evaluate the differences between total and effective strength parameters Table 6.8 shows

for each material and anisotropy direction the equivalent MC effective and total shear strength

parameters obtained from RocData 5.0 (Rocscience 2021).

252

22 Table 6.8 Fitting MC effective and total strength parameters obtained from RocData 5.0

(Rocscience 2021)

Lithotype Anisotropy – β (°) c (kPa) ɸ (°) c' (kPa) ɸ' (°) Samples (n)

WAI 0 28 39 96 33 11

45 101 29 95 32 7

90 36 41 138 24 7

Isotropic 55 36 100 31 25

WHE 0 122 33 85 37 10

45 107 32 75 38 14

90 139 30 83 38 10

Isotropic 116 32 81 37 34

WQI/WGI 0 72 36 30 36 19

45 67 35 80 35 14

90 107 33 51 37 16

Isotropic 93 33 56 36 49

Although, the MC parameters obtained from software adjusted curves could present some bias

associated with the linear adjusted curve method, the resulted parameters are useful for general

consideration used in this research; however, they cannot necessary represent the average

parameter of each soil type.

From Figures 6.27, 6.28 and 6.29, and Table 6.8 is possible to conclude that:

• Considering isotropic (all samples together), the highest differences between

effective and total strength parameters is obtained for WAI noted for the friction

angle and cohesion. For WHE and WQI/WGI the differences are more relevant for

the cohesion.

• Considering the total cohesion, the WHE presented the highest values and effective

cohesion WHE presented the lowest values.

6.6.7 Unsaturated shear strength characteristics

Unsaturated shear strength test

To evaluate shear strength parameters considering matric suction effects, a series of UIDST tests

were undertaken. The peak undrained shear stress and settled matric suctions equal to the

SWCC and obtained variables were presented in Section 6.6.5 and all used datasets are

presented in Appendix VII.

253

In this section, the corresponding matric suction of each test under a constant normal load from

the UIDST tests was plotted to establish the unsaturated failure envelope’s typical curve and

discussions related to the unsaturated behaviour of each type.

The following figures show the behaviour of each material type under unsaturated conditions.

During shearing, the matric suction of each test was kept constant under a net normal stress of

75 kPa, established previously as the typical result for unsaturated shear strength.

Typical UIDST characteristic curves are presented below and in order to evaluate the main

physical and geotechnical characteristics; however, as noted above, the anisotropy effects are

not considered. All of the tests presented represent shearing parallel to the banding (βangle equal

to 0°).

The effects of shear displacement and unsaturated shear stress under different matric suctions

and constant vertical (normal) pressures are presented in Figures 6.31A, B and C for WAI, Figures

6.32A, B and C for WHE and Figures 6.33A, B and C for WQI/ WGI.

• WAI

(A)

(B) (C)

8 Figure 6.31 A (top left), WAI normal displacement versus shear displacement. B (bottom

left), shear stress versus shear displacement. C (right), water discharge versus

shear displacement for Sample BL_1 (β0°), with matric suction levels: Stage 1

– 0 kPa, Stage 2 – 20 kPa, Stage 3 – 70 kPa and Stage 4 – 150kPa

254

WHE

(A)

(B) (C)

82 Figure 6.32 A (top left), WHE normal displacement versus shear displacement. B (bottom

left), shear stress versus shear displacement. C (right), water discharge versus

shear displacement. Sample BL_3 (β0°). Matric suction levels: Stage 1– 0 kPa,

Stage 2 – 20 kPa, Stage 3 – 70 kPa and Stage 4 – 150kPa

WQI/ WGI

(A)

(B) (C)

83 Figure 6.33 A (top left), WQI/WGI normal displacement versus shear displacement.

B (bottom left), shear stress versus shear displacement. C (right), water

discharge versus shear displacement. Sample BL_12 (β0°). Matric suction

levels: Stage 1 – 0 kPa, Stage 2 – 20 kPa, Stage 3 – 70 kPa and Stage 4 – 150 kPa

The stress versus strain relationship for all types, in general, did not exhibit a strong post-peak

decrease behaviour (strain softening), except under matric suctions of 20 kPa for WQI/WGI,

30 kPa for WHE and 150 kPa for WAI when a very gentle stress reduction occurred post-peak.

255

The volume change/normal displacement associated with each test exhibited dilatation for all

WHE curves, and contraction at the beginning and subsequent expansion for all WQI/WGI

curves, and expansion for WAI at the low suction levels and contraction for the highest suction

level (150 kPa).

As expected, high permeable material such as the WHE specimen showed intense discharge up

to 2 mm of shear displacement, after which there was a reduction in the discharge intensity. For

WAI the lower suction levels (0 and 20 kPa) showed no discharge, after which higher suction

levels resulted in a constant discharge rate until 1.5 mm shear displacement when material

stopped draining at a suction level of 70 kPa and started wetting, or still kept discharging until

the end of the test when the suction level was 150 kPa.

The same occurs with WQI/WGI, which presented no discharge for a suction level of 0 kPa; for

a suction level of 20 kPa, it discharged until 1 mm of shear displacement and from that point it

started wetting, with no equilibrium until the end of the test. In contrast, for a suction level of

70 kPa, it exhibited a wetting process up to 1 mm of shear displacement and from that point

onwards a constant discharge rate until equilibrium was reached after 4 mm displacement. At a

suction level of 150 kPa, it started with fast discharge until 4 mm displacement and from this

point onward it continued with a constant discharge rate with no equilibrium.

Figures 6.31, 6.32 and 6.33 show that peak shear strength increases as matric suction increases

in nonlinear form. Although the well-known shear strength equation developed by Fredlund et

al. (1978) is linear, many experimental results from Gan et al. (1988b), Escario & Jucá (1989) and

Wheeler & Sivakumar (1992) show nonlinear variations of shear strength with respect to matric

suction when tested over a wide range of matric suction.

It is noticed that all lines tend to curve as the matric suction approaches a specified value. This

value was found to be within the range of 50 kPa to 250 kPa for WAI, 40 kPa to 100 kPa for WHE,

and 20 kPa to 70 kPa for WQI-WGI, respectively. This could be associated with studies carried

out by Fredlund & Rahardjo (1993) concluded that this value corresponds to the AEV of the soil.

It is clear from Figures 6.31, 6.32 and 6.33 that envelope lines are almost parallel, and that for

higher values of matric suction there is an increment in the shear strength for the first part of

the curves.

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6.7 DISCUSSION

Considering each completely weathered BIF, based on USCS soil classification, the WAI is

characterised as fine sand silt, with medium plasticity even with a low clay percentage (less than

5%) in which kaolinite and gibbsite (weathered minerals), are responsible for a low activity

behaviour and the uniform PSD that typically results in a low permeability (6.6×10-6 cm/s).

The presence of kaolinite and gibbsite are also responsible for the lowest shear strength and low

anisotropy ratio. Heterogeneity is defined by interlayers of silt clayish and hematite minerals,

resulting in the highest mineralogical hardness contrast for the completely weathered BIF.

The soil classification of WHE is as a sandy-silt material, with low plasticity and characteristics of

non-cohesive soils as noted in Section 6.6.6. Heterogeneity is defined by granular and micro-

plates of hematite with variable visual total porosity layers resulting in a high permeability of

3.1×10-4 cm/s. Due to this high Øb, mineral orientation seems to have a secondary importance in

defining anisotropy, resulting in an isotropic to low anisotropy ratio. Different to other

weathered itabirites the WHE genesis is associated with mimetic supergenic process as defined

by Morris (2002 and 2002a); arguing that the higher Øb levels are associated with previous non-

iron layers that have been totally leached, generating highly porous layers (partially filled by iron

oxides and hydroxides near the surface). This modifies the original bimodal minerology (non-

iron and iron bands), forming a monomineralic hematitic material, in which the original

heterogeneity was obliterated by layers of iron minerals with variable Øb and a very low strength

when compared to the parent rock fresh itabirites.

Secondarily, the lattice weathering (Morris 2002 and 2002a), induces iron oxidation and

reprecipitation as iron oxide or hydroxy cementing the pores and increasing the shear strength.

WQI is classified as silt-sand with gravel material with a non-uniform size distribution and

permeability of 9.1×10-5 cm/s. The heterogeneity defined by the partial leaching is clearly visible

and is defined by iron layers (higher ρb and lower Øb) and quartz layers (lower ρb and higher Øb).

In general, this type presents a typical granoblastic crystal orientation; however, the fabric did

not play an important role for shear strength results compared with non-oriented types. The

low anisotropic ratio obtained for this type is mainly defined by the mineral composition (band

thickness), iron cementation and Øb that seems to equally contribute to this behaviour.

WGI differs from WQI due to the higher presence of goethite and ochreous goethite cemented

bands. Additionally, the WQI presents higher Øb when compared with WGI induced by iron oxide

and mainly hydroxide cementation. For WGI, even with the typical heterogeneity preserved, this

257

material presents low anisotropy ratio and higher shear strength due to cementation compared

with the WQI.

Completely weathered BIF presents, in general, a very high ρb and a well-structured high visual

total porosity mineral skeleton; and a relatively high shear strength when compared with low

dense sand silt soils as sandstones or limestones, as postulated by Costa (2009) and Martin &

Stacey (2018). Otherwise, BIF with lower ρb and high total porosity (e.g. WAI) represents the

weaker types with lower shear strength seams normally associated with the presence of clay

minerals (gibbsite and kaolinite) as resulted from weathering.

Saturated tests indicate that WHE, WQI and WGI behave as a brittle, dense sand silt soils above

a 400 kPa stress level, and below this level they behave more like ductile, loose sand silt soils.

This behaviour could in part be attributed to the high particle densities typical of BIF. However,

the bulk density is in turn partially controlled for the most part by the total porosity, although

not enough to fully control this behaviour. An additional petrophysical characteristic that should

influence weak BIF behaviour is the mineral composition associated with each material type

(quartz, gibbsite, kaolinite and different iron oxides) and, to a lesser extent the fabric (mineral

orientation).

The loose sand silt behaviour below 400 kPa results in slip between particles where only the

friction between the crystals must be overcome. As the stress increases, the samples show a

continuous decrease in volume, i.e. contraction. Conversely, dense sand silts interlocking

between the particles will require additional effort to allow shearing, requiring an increase in

(dilatancy) volume for it to occur. Dense sand reaches a maximum shear stress (peak) then starts

softening rapidly reaching a residual constant shear stress value.

In summary, for individual layers in the BIF units, mineral composition, total porosity, and

associated fabric are essential components that influence the behaviour and shear strength

parameters. An example of this is that thin layers of clay minerals that could exhibit matric

suction values from tens to hundreds of kilopascals (kPa), e.g. in WAI. When comparing matric

suction effects in weathered BIF units, WGI also involves a high percentage of clay size mineral

content (maximum of 10%). However, it does not exhibit the important content of clay minerals

and does not exhibit high matric suction effects. On the other hand, WAI, even with low (3%

average) clay minerals content, shows the highest matric suction effects. This can be attributed

not only to the clay percentage, but also to clay activity as the WAI clay fraction is mainly

comprised of kaolinite and gibbsite (low activity minerals), whereas the WGI clay fraction is

comprised of goethite and ochreous goethite. In addition to the clay percentage content the

258

contribution of clay activity to the matric suction must be considered as the main clay mineral

(kaolinite) is a non-active clay mineral.

The compositional metamorphic banding defines a heterogeneity easily recognised in all highly

to completely weathered BIF and induces a low anisotropic behaviour, as obtained from CIU

tests shown in Table 6.7. From this table WHE shows isotropic to low anisotropic ratio and the

heterogeneity is defined by layers of sandy-silt iron minerals with higher visual total porosity,

interlayered with sand silt iron mineral bands with low visual total porosity. WAI and WQI/WGI

show a low anisotropy ratio and WAI are constituted of sandy-silt layers of iron minerals

interlayered with non-iron minerals silty-sand and varying in percentages of clay minerals

(kaolinite and gibbsite). WQI is characterised by silty-sand with gravel iron mineral layers that

have a lower visual total porosity interlayered with bands of silty-sand iron minerals and quartz

with higher Øb. WGI differs from WQI due to a higher percentage of iron hydroxide minerals and

gibbsite, with more pores filled (cemented), resulting in a relatively lower Øb. From this study,

WAI and WGI were considered as the lower Øb types (less than 15%) and WHE and WQI the

higher total porosity types (higher than 15%).

From the weathering, it is recognised that increases in total porosity and mineral alteration are

important changes that produce an effective softening of the original fresh rocks. For BIF, this

softening is produced by the partial or total leaching of non-iron minerals, mainly iron dolomite,

but also quartz; oxidation of iron minerals; and chemical alteration of amphibole and other

minerals. In extreme cases, the remaining iron oxidises, and non-iron bands (mainly quartz)

exhibit a high total porosity and low grain apparent cohesion (loss of contact between grains).

This results in a weak soil material that is occasionally cemented by secondary iron oxides and/or

hydroxides (WGI), which impose some additional strength due to the total porosity reduction.

These results outlined above generally agree with studies by Gupta & Rao (2001) for crystalline

rocks, Marques et al. (2010) for Australian and Brazilian metamorphic rocks and Leão et al.

(2017) for Brazilian phyllites.

The geological events responsible for the control of the total porosity are the low-grade regional

metamorphism (mainly for bedrock), supergene and hypogene events, and weathering.

Considering each event, the metamorphism promotes granuloblast texture, with small crystals

oriented according to banding. Supergene and hypogene events transformed the bedrock

crystalline phases of iron minerals (hematite, magnetite and martite), and partial or total

leaching of non-iron minerals (quartz and iron dolomite) providing a volumetric reduction which

is responsible for increased intergranular porosity (Pimenta 1992). Finally, the iron enrichment

259

and weathering provide more leaching and alterations that increase the total porosity and

breaks the crystals contacts. Where the drainage of weathering fluids is not effective (or in

shallower depths), this alteration promotes an increase of goethite and iron hydroxides, sealing

(partially or totally) existing pores, as described by Dana & Hurlbut (1960) as shown for WGI.

Conversely, when the drainage is efficient, iron crystal (magnetite and hematite) preservation

does not promote efficient sealing of the pores, as can be seen in WQI.

The heterogeneity and anisotropy are closely related to the total porosity and consequently the

bulk density due to the size of individual bands. Where banding presents thicker centimetric

layers of non-iron minerals (e.g. iron dolomite or quartz bands), there is relatively less dense,

and proportionally more porous rock. Also, the intact rock strength will be lower than other

specimens with more iron bands (thicker iron layers), which increase the iron content, reduce

total porosity, and increase the bulk density.

Due to higher permeability observed in WHE/WGI and WQI samples, it is expected that these

materials exhibit high effective drainage in mining conditions, which means that during the dry

season unsaturated behaviour can be neglected at least for short-term slopes. At contrary, WAI

has exhibited the lower permeability, and in this case, unsaturated behaviour could be

important for slope stability studies mainly for short-term slope stability analysis.

Considering the Brazilian climatic conditions (high rainfall volume and a long rainy season) for

the shallower portion in the mines, saturated behaviour is prevalent for natural slopes and post

mining conditions. For this reason, unsaturated conditions for short-term mining operations, as

well as saturated behaviour for long-term and post-mining conditions must be addressed for

slope stability. The use of consolidated drained tests (CIU) for determining saturated strength

properties, even considered conservative, is appropriate to properly represent in situ

conditions. However, this test will not allow evaluation of transient or cyclic loads

(e.g. earthquakes) that could induce an instantaneous pore water pressure increase, which

could lead to skeleton collapse of WAI. For these conditions other tests are necessary to fully

establish the matric suction effects on BIF.

Table 6.9 summarises the most important characteristics evaluated in this chapter for each type

and support the discussion presented in this chapter.

260

23 Table 6.9 Weathered BIF characteristics summary table

Lithotype WAI WHE WQI/WGI

Geology • Results from weathering of dolomitic itabirites, authigenic breccias or BIF interlayers with phyllites or intrusive sills and dikes (Suckau et al. 2005 and Zaparolli et al. 2007.

• Results from effective superimposed on supergene or hypogene BIF enrichment superposition by weathering.

• Results from superimposed supergene and weathering on amphibolitic and quartzitic Itabirites.

Anisotropy and heterogeneity

• Medium heterogeneity. • Low anisotropy ratio. • Foliated clayey BIF.

• Low heterogeneity. • Isotropic to low anisotropy ratio. • Bands defined by total porosity content.

• Higher heterogeneity. • Low anisotropy ratio. • Bands define by mineralogical composition and total porosity layers.

Macro features • Clayey sandy-silt BIF. • R1- to R0. • W6.

• Sandy-silt BIF. • R1+ to R0. • W5 to W6.

• Sandy-silt BIF. • R1+ to R0. • W5 to W6.

Micro-features • Lower relative Øb (≤15%). • Iron minerals, quartz, kaolinite, and gibbsite. • Loam – sand-silt, medium to no-uniform, well graded. • (ML – CL), LL = 33, PL = 23, and PI = 10 • k20 = 6.6×10-6 cm/s • ρb = 2.7±0.5 t/m3

• Higher relative Øb (≥15%). • Iron minerals and quartz. • Loam – sand-silt, no uniform, well graded. • (ML – CL), LL = 27, PL = 19, and PI = 8 • k20 = 3.1×10-4 cm/s • ρb = 4.5±0.7 t/m3

• WQI higher relative Øb and WGI lower relative total Øb. • Iron minerals and quartz, goethite, and gibbsite. • Loam – silt-sand with gravel, no uniform, gap graded. • (ML – CL), LL = 29, PL = 15, and PI = 14 • K20 = 9.1×10-5 cm/s • ρb = 3.8±1.3 t/m3

Saturated shear strength

• Typical loose silt/sand material type. • Ductile behaviour for low confinement (400 kPa) and moderate strain softening above. • Moderately contractive curve with porewater pressure increases above 200 kPa.

• Intermediate to dense sand material type. • Ductile behaviour for low confinement (400 kPa) and weak strain softening above. • Dilative stress curve for all stress levels and a porewater pressure decrease.

• Intermediate to dense sand material type. • Ductile behaviour for low confinement (400 kPa) and strong strain softening above. • Dilative stress curve for all stress levels and a porewater pressure decrease.

Unsaturated shear strength

• Exhibited expansion at the low suction levels and contraction for the highest suction level (150 kPa). • SWCC typically sand-silt material.

• Exhibited contraction. • SWCC typically sand material.

• Exhibited contraction at the beginning and subsequent expansion. • SWCC typically sand material.

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6.8 CONCLUSION

The use of combined soil terminology and characterisation is appropriated for soil-like BIF

materials similar to the types evaluated in this chapter. The classification proposed for these

types ranges from very stiff to hard clay, through extremely weak to very weak rocks that ranges

from highly (W4), to completely weathered (W5) rocks and to residual soils (W6). These

materials, in general, behave like soils and follow soil mechanical approaches, and inherent relict

anisotropy of the original intact rock may have some influence on slope stability.

From the original fresh parent BIF, mainly the supergenic leaching effectiveness and weathering

promotes alteration processes (oxidation and leaching) inducing a significant increase in voids

and resulting in a reduction of the ρb for some types, e.g. WQI and WAI. Conversely, for WHE

and WGI, the same process produces the same changes but with iron minerals reconcentration

(iron enrichment) resulting in a relative bulk density increase. The physical characteristics

variation noted for each type is attributed to a superposition of the tectonic, supergene and

hypogene events occurred before weathering that contribute to the formation of multiple BIF

materials. This superposition is reduced in depth as supergenic enrichment and weathering

reduce their influence in depth.

However, for shallower depths, these changes are not restricted to void increase or ρb variations.

These multiple weathering processes also impose a mineral alteration, increase in clay minerals

(mainly gibbsite and kaolinite), resulting in a relative plasticity increase inducing some negative

porewater pressures (matric suction), as noted for WAI and in some extent to WGI. Also, for

WGI the iron minerals oxidation and hydration promote mineral alteration responsible for

cementation increase and a relatively higher shear strength for typical sand-silt materials with

low plasticity.

Close to surface, is characterising highly to completely weathered BIF as immature residual soils,

i.e. as materials which are formed in place from mineralogical changes, can properly describe

these BIF types. In general, all types studied exhibited distinct soil behaviours, but can be

grouped using USCS classification. The groups are characterised by the low (WQI, WGI and WHE)

to intermediate (WAI) plasticity index, and as sandy-silty soils, with clay and very low

permeability (WAI) and silt-sandy soils with gravel with low to moderate permeability (WQI, WGI

and WHE).

The original compositional metamorphic banding (noted at great depths) works as a preferential

path for all deleterious processes (weathering, supergene or hypogene) and defines anisotropic

characteristics for weathered BIF varieties. This anisotropy is defined for WQI by the alternation

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of bands with quartz/goethite with high total porosity and bands of iron minerals with low total

porosity, which induces a low anisotropy ratio and high heterogeneity. Considering WGI, the

anisotropy is similar to WQI, and the shear strength increases due to iron oxide and hydroxide

cementation. For WHE, the anisotropy ratio can vary from an isotropic to low anisotropy and is

basically defined by the presence of pores layers. The compositional metamorphic banding is

less evident as the WHE is almost a monomineralic type and the heterogeneity is defined by the

alternation of bands with lower and higher total porosity.

For WAI, the anisotropy ratio is low, and the heterogeneity is defined by interlayers of quartz

and clay mineral bands with higher total porosity in contrast with iron mineral bands with lower

total porosity. For all BIF residual soils, the heterogeneity and anisotropy ratio are also controlled

by the thickness of these individual bands what is associated to sample scale effect.

Considering the heterogeneity and the anisotropy from these soils, mineralogy represents the

most important characteristic. However, the fabric, pores percentage and distribution, and bulk

density may contribute to the full understanding of the multiple behaviours of the weathered

BIF material, and better define the geological categorisation as an immature to residual soil.

Studies presented in this section show that highly to completely weathered BIF share some

common petrophysical characteristics representing dense sand silt materials with relatively high

shear strength parameters, compared to typical residual soils. However, single characteristics as

clay layers concentration for WAI, mineral orientation for WHE, clay layers concentration, and

pore layers concentration for WGI and WQI could induce local anisotropy effects and are

responsible for changes in the common behaviour, so resulting in multiple types.

For soil types with high total porosity as obtained for BIF residual soils, once the cohesion is

destroyed the load is transmitted to the soil skeleton, thereby mobilising the particle friction,

and this could induce sudden collapse during particle rearrangement. This collapse will occur

preferably at more pore layers or in the heterogeneity planes contacts as previously described

for Oolithic iron deposits by Cuccivillo & Coop (1997) and BIF by Martin & Stacey (2018). In this

matter, mineral and pore orientations can subsidiarily induce collapsed behaviour depending on

the stress level and material type. At low stress levels, the more isotropic types (WAI and WHE)

are more prone to ductile collapse than the low anisotropic materials for which the pore and

mineral orientations do not allow brittle failures since the fabric (skeleton) reorganisation redistributes the strain. As a result, the majority of evaluated failure envelopes for low

anisotropic materials are linear, showing no post-peak reduction. However, for isotropic types

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(WHE) at higher stress levels a granular rearrangement can induce sudden collapse, as observed

in the brittle post-peak resistance loss (strain softening) in the majority of the test result curves.

The results presented in this chapter and the case study presented in Appendix II show an

increase in unsaturated shear strength mainly for WAI, reflecting the matric suction. Thus, there

is a potential benefit in using unsaturated behaviour for slope designs and stability analyses,

considering the influence of negative porewater pressures, particularly during the dry season

for temporary slopes.

Multiple evaluations demonstrated that what is defined as weak, highly (W4) to completely

weathered (W5) BIF represent a transition zone from saprolite horizon to residual soils, i.e. weak

rock, or an immature residual soil.

Sand, silt and secondary clay and gravel contents define different behaviours under applied

stress levels. It was noted that for low stress level (below 400 kPa) the BIF behave like loose

sandy soil, while for stress levels above 400 kPa they behave like dense sandy material. However,

in addition to total porosity, ρb, the mineralogy can induce dense behaviour even at low stress

levels, or loose behaviour under high stresses.

Typical BIF fabric associated with the low stress and green schist metamorphic grade zones

noted in the east of the Iron Quadrangle is responsible for fine overall crystal sizes and plays a

secondary, but not negligible, role in BIF shear strength. Additionally, the high ρb associated with

iron minerals content plays an important role that can induce a higher shear strength and

anisotropy ratio in some specific cases. Additionally, with the proposed soil characterisation it is

possible to reduce the variance normally noted in laboratory test results. Scale effect associated

with layer thicknesses and multiple geological features must also be evaluated and considered

during sample preparation, since they can induce different behaviour and increase the

dispersion of strength results.

The historical approach used in Vale mines, based on a saturated approach, using effective and

total strength parameters, and not considering negative porewater pressure for WHE and

WQI/WGI, is shown to be reasonable and suitable for long-term and post-closure slope stability

analyses. However, for WAI short-term slope stability analyses, this approach has shown to be

conservative. This may also be true for the other weathered BIF types (WGI), but more studies

are required to establish conclusive results.

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For this reason, the influence of unsaturated soil mechanics on slope stability is becoming

important in current mining engineering and taken unsaturated soil mechanics into account

must become the best engineering practice.

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CHAPTER 7. WEATHERING PROFILE, INTACT ROCK STRENGTH AND

ELASTIC CHARACTERISTICS OF BRAZILIAN BANDED IRON FORMATIONS

This chapter presents the fourth unpublished manuscript.

ABSTRACT

Brazilian iron ore deposits exhibit weathering profiles that can reach over 400 m in depth. As a

result, shallow open pit mines are excavated in sequences ranging from residual soils to

moderately weathered rocks. In these mines, the rocks exhibit soil-like behaviour and the slope

design approach and failure mechanisms are largely based on applying classical soil mechanics

for slope stability analyses. While this approach has been satisfactory, there are nevertheless

several key geotechnical issues that are still not well understood, and there have been continued

instances of large slope failures which have resulted in significant disruptions to the mines. In

deeper mines, slopes consist of the full range of the weathering profile, from residual soil to fresh

rocks.

In this profile the fresh hard rocks present different challenges for slope designs and failure

assessments due to rock mechanics approach.

It is recognised that the increases in weathering grade induce intact rock strength reductions,

and this softening is easily recognisable by bulk density reduction, induced by mineral alteration

(e.g. oxidation) and total porosity increase (e.g. leaching). Differently, for Brazilian banded iron

formations (BIF) due to the iron enrichment, in some instances, the bulk density increases, and

total porosity decreases associated to the iron cementation causing intact rock strength to

increase in some types. This ambiguous behaviour is investigated in this study through geological

and geotechnical approaches which investigate intact rock strength parameter changes through

the complete weathering profile for different compositional BIF from the Iron Quadrangle of

Brazil.

Site investigations and laboratory tests from 15 different mines indicate that physical and

chemical changes (weathering) imposed on different compositional BIF are responsible for two

types of weathering profiles, characterised by differences in the thickness of the moderately

weathering degree rocks at saprorock and saprolite horizons. These weathered zones can be

defined by petrophysical characteristics, notably intact rock strength and elastic parameters,

supported by typical geomechanical behaviour.

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To support these statements a program of extensive geological/geotechnical field assessments

and laboratory testing was conducted for the three main compositional itabirites (quartzitic,

dolomitic and amphibolitic) and hematitite (rich ore) in four different weathering horizons (fresh,

saprorock, saprolite and residual soil). Additionally, regression best fit linear curves were

established for all materials in each horizon to compare and establish the influence of the

weathering and iron enrichment on changes to intact rock strength parameters.

Laboratory tests undertaken to support the studies included: uniaxial compressive strength

(UCS), bulk density, indirect unconfined tension (Brazilian test), triaxial Hoek & Brown cell, triaxial

consolidated undrained tests (CIU), and empirical correlation equations were used to define

strength and static elastic parameters and establish correlations. An extra challenge to

determine these parameters and correlations is placed by the anisotropy observed for these rock

types.

The investigations showed that weathering profiles for quartzitic and amphibolitic itabirites form

a complete gradation from fresh (bedrock horizon) to moderately weathered (saprorock

horizon), to highly weathered material types (saprolite horizon) and finally to completely

weathered degree (residual soil horizon) while the dolomitic itabirite presents an abrupt contact

between fresh and completely weathered variants.

Bulk density and total porosity variations are induced by changes in mineral composition

(mineralogical alteration related to iron mineral oxidation/hydration and quartz and iron

dolomite leaching) and iron content variation are the major factor in intact rock strength changes

in the weathering continuum.

The anisotropy effects in fresh itabirites and hard hematitites (bedrock) are not as important as

have been supposed, except for fresh dolomitic itabirites, which exhibit a low to medium

anisotropy ratio due to its mineralogical composition. However, saprorock horizons moderately

weathered rocks (quartzitic and goethitic itabirites), display a medium anisotropy ratio due to

the bimodality resulting from porous bands rich and poor in iron content interlayered with iron

rich bands of low total porosity. For saprolite, the highly weathered materials (quartzitic and

goethitic itabirites, and weak hematitite), vary from isotropic to low anisotropy ratios, and the

overall shear strength is comparable weak rock (soil-like materials). The completely weathered

types are typical in situ residual soil material.

All BIF residual soils behave like a sand silt materials with high bulk density and permeability

unless for the completely weathered argillaceous itabirite, due to the presence of clay bands,

that can induce suction for unsaturated condition.

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Results show that the development of variations in the degree of weathering leads to distinct

geomechanical behaviour and characteristics, affecting intact rock strength, as well as elastic

parameters. These variations ultimately control overall slope stability, not only for long-term

excavations but also for temporary slopes for all depths of open pit iron ore mines.

7.1 INTRODUCTION

In the Brazilian open pit iron ore mines operated by Vale, the first 400 m of depth are often

comprised of residual soil and weak rock material zones that exhibit soil behaviour. In these

zones of residual soil and highly to completely weathered rocks, typical failure mechanisms and

the resulting slope stability approach have reflected soil mechanics principles, as demonstrated

by Costa (2009), Marques et al. (2017) and Martin & Stacey (2018). Although this approach has

largely been satisfactory, there are several geotechnical issues that are still not well understood,

and there have been continued large slope failures, which have resulted in significant

disruptions to the mine operations.

Studies by Innocentini (2003) Costa (2009) and Sá (2010) on Vale iron ore mines suggested that,

for these zones, some failure mechanisms in weak leached materials have specific characteristics

for which the use of classical soil mechanics principles do not allow the failure mechanisms to

be fully understood, while rock mechanics concepts were also not entirely applicable.

For mines deeper than 400 m, some walls consist of the full range of the complete weathering

profile, being composed of materials varying from residual soil through fresh rock. For these

slopes, there is no doubt that slope stability at the toe, in fresh BIF, is controlled by structural

factors, in particular the shear strength of the discontinuities and rock mechanics principles are

fully applicable. However, for moderately weathered BIF sectors belonging to saprolite and

saprorock horizons involving a mixture of hard and weak rocks, the intact rock strength

reduction imposed by the weathering can change the controls that involve rock mass strength

as well as discontinuity strengths. Characterising the rock mass in these sectors will therefore

be essential for defining the boundary between soil and rock material behaviour and for

adopting the proper geomechanical approach to the slope designs.

After defining boundaries and geological and geomechanical characteristics of each horizon and

zone for the different BIF types, it is possible to provide an overview of the full range of the

mechanical behaviour of the weathering profiles. This results in better definition of intact rock

strength and static elastic parameters to ultimately support proper slope stability analyses and

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failure mechanism evaluations, thereby establishing reliable information essential for safety and

mining operations.


The aim of this chapter is to evaluate intact rock strength, elastic parameter values and

geomechanical characteristics based on laboratory tests and geological/geotechnical field

evaluation for the full range of BIF occurring in different weathering profiles. The main research

statements are summarised below:

• Show how and if the weathering affects, in different ways, each compositional BIF,

resulting in different weathering profiles.

• Evaluate and determine the intact rock strength and elastic parameters, and

geomechanical characteristics for BIF in different weathering profiles and horizons.

• Determine the importance of the anisotropy (heterogeneity), defined by the

compositional metamorphic banding, for the intact rock strength, elastic properties,

and characteristics over the weathering continuum.

• Describe and correlate the linear regression (best fit) lines in σ1σ3 stress space for

different weathered horizons and zones for all BIF types through the weathering

profile.

In this research, the term BIF is used as a general lithological term to define a group of four fresh

types with different mineral composition (dolomitic, amphibolitic and quartzitic itabirites), and

hematitite. A group of three highly to moderately weathered types, namely partially weathered

quartzitic and goethitic itabirite; and a group of four completely weathered to residual soil

namely, weathered quartzitic, goethitic and argillaceous itabirites, and weak hematitite.

This chapter is focused on the intact rock properties, assessing the effect of physical

heterogeneity and anisotropy to establish practical estimates and correlations which could

address a lack of intact rock strength parameters. Such correlations must consider mechanical

and elastic rock properties as a function of mineralogical composition, fabric, total porosity, bulk

density, iron content and other petrophysical characteristics of the intact rock.

This chapter aims to determine the intact rock strength and static elastic parameters based on

Vale´s laboratory tests database and a considerable number of tests carried out to provide a

dataset able to support correlations between petrophysics and rock mechanics proprieties.

Therefore, UCS, bulk density (ρb), indirect unconfined tension (Brazilian test), triaxial Hoek &

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Brown cell, and triaxial CIU, tests were undertaken. Additionally, thin sections were described

to provide a micro characterisation of the rock fabric.

For this study, a PhD research project was conducted by the lead author, sponsored by Vale S.A.

at the Australian Centre for Geomechanics, School of Civil, Environmental and Mining Engineering

at The University of Western Australia. The main aim of the thesis was to investigate the complete

weathering profile geomechanical characteristics, from fresh and hard to weak and completely

weathered to residual soil banded iron formation materials from several mine sites.

The mines studied are part of Vale’s southern ferrous division, which comprise fifteen mines

located in the centre of Minas Gerais state, Brazil, as shown in Figure 7.1. The mines are: Águas

Claras (MAC), Mutuca (MUT), Mar Azul (MAZ), Capão Xavier (CPX), Tamanduá (TAM), Capitão

do Mato (CMT), Abóboras (ABO), Galinheiro (GAL), Sapecado (SAP), Pico (PIC), Córrego do Feijão

(CFJ), Jangada (JGD), João Pereira (JPE), Alto Bandeira (BAN) and Fábrica (FAB).

84 Figure 7.1 Mine locations and Iron Quadrangle geological settings (Modified from

Morgan et al. 2013)



The focus area is located at the western part of the Iron Quadrangle on the southern border of

São Francisco Craton. The mines are situated on Moeda and Don Bosco Synclines and Curral

Homocline range (Figure 7.1).

- Rio das Velhas Supergroup - Archaean - Minas Supergroup - Proterozoic

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The structure is delineated by a roughly quadrangular arrangement, with Paleoproterozoic BIF

of the Minas Supergroup, as proposed by Dorr (1969). This supergroup is composed of hundreds

of metres of iron rich metamorphic rocks belonging to the Itabira group/Cauê Formation. The

Minas Supergroup comprises, from the bottom to the top: Caraça, Itabira, Piracicaba and Sabará

groups; a sequence of psammitic and pelitic rocks, also defined by Dorr (1969); all of which are

overlain by the Itacolomi group. Below this sequence are the Archean greenstone belt terrains

of the Rio das Velhas Supergroup and domes of Archean and Proterozoic crystalline rocks

(Machado & Carneiro 1992; Machado et al. 1989 and Noce 1995).

The Cauê Formation (Itabira Group), which hosts the BIF, is a marine chemical sequence 350 m

thick, dated 2.4 ± 0.19 Gyr by Babinski et al. (1995).

The regional structure is the result of two major deformational super-positional events, as

proposed by Chemale Jr et al. (1994). The first event produced the nucleation of regional

synclines in the uplift of the gneissic domes during the Transamazonian Orogenesis (2.1–2 Gyr);

and the second is related to an east–west verging thrust fault belt of Pan African/Brazilian age


structures and was mainly responsible for the deformational gradient. Hertz (1978) described

an eastward increase in the metamorphic grade from green schist to lower amphibolite facies

following the deformational gradient. The structures and geological settings are also shown in

Figure 7.1.

The mines which have been studied are located in the western low strain domain and green

schist metamorphic zone, formed in the Transamazonian event. The main trend of the synclines

is north–south, but the structure has also been deformed around Bação dome. It is also

interconnected with Dom Bosco Syncline to the south and partially truncated by Engenho Fault.

To the north, it is continuous with the Serra do Curral (Curral Homocline). At the junction of

these two regional structures a northwest-verging asymmetric anticline and an interference

saddle due to the refolding were developed (Rosière et al. 1993).

The Moeda Syncline has been partially affected by the younger Brazilian tectonic event, mainly

on the eastern limb, with local development of ductile brittle to brittle shear zones. Several

strike–slip faults cut across the structure, dividing it into several segments.

The Serra do Curral hills represent the overturned south-eastern limb of a truncated northwest-

verging syncline–anticline couple, which was highly deformed and rotated by the right–lateral

movement of the northeast–southwest trending inclined ramp of a reginal thrust fault as

described by Chemale Jr et al. (1994). In this segment, the inverted northwestward limb of a

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syncline is also truncated near the contact of the Minas Supergroup with the underlying Rio das

Velhas Supergroup by shear zones related to the thrust.


Brazilian BIF are Paleoproterozoic, metamorphic and heterogeneous banded rocks presenting a

millimetre to centimetre rhythmic alternation (banding) of iron minerals (hematite, martite and

magnetite), and non-iron minerals (quartz, dolomite, or amphibolite). Initially termed itabirites

as defined by Dorr (1969), these iron deposits are classified as Superior Type according to Gross

(1980).

The origin of itabirites and associated high-grade orebodies (hematitite) remains controversial,


For genesis of the friable orebodies, some authors agree on a supergene process and residual

itabirite enrichment, with leaching of gangue minerals by surface waters. For these orebodies,

Spier (2005) and Spier et al. (2006) suggest that the weathering and the mineralisation period

occurred between 61.5 ± 1.2 Myr to 14.2 ± 0.8 Myr, reaching the peak process in 51 Myr. This

dating indicates a tertiary mineralisation, and after this period the further weathering may not

have substantially affected the weathering profile.

For this type of iron deposit Dorr (1969) defined two main primary lithologies: hematite or

hematitite, the high-grade ore (Fe ≥ 62%), and the low-grade ore itabirite (30% < Fe < 62%), with

three compositional lithotypes: quartzitic, dolomitic and amphibolitic. The tectonic,

metamorphic, and weathering have changed these proto-ores in different ways, resulting in

multiple sets of iron ore variants (typologies), as described in the following sections.

As a metamorphic (green schist grade) rock, itabirites present a visible heterogeneity that may

induce a transversal anisotropy as described by Ramamurthy et al. (1993) and Appendix I defined

by mineralogical composition (banding), mineralogical orientation (alignment of minerals),

porosity induced by the original composition or weathering and different bulk density in each

layer. This variation may have been a function of the original sedimentary bedding, the tectonic

setting, metamorphic grade, hydrothermal or supergene processes, and had suffered influence

from recent weathering. Further, the superposition of these processes causes partial or total

mineralogical and textural changes.

Regardless of the controversy about the supergene (groundwater leaching or weathering) or

hydrothermal (hydrothermal water leaching) genesis for these large, rich and heterogeneous

iron ore deposits, in this chapter supergene concentration and subsequent weathering are

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considered to be the main events responsible for the chemical and physical changes that induce

reduction of the original itabirite strength, creating the deep soil and weak rock profiles that are

often seen in iron ore mines. Furthermore, discontinuity characterisation and correlation with

rock mass strength are not considered in this chapter, which focuses on intact rock strength.

Nevertheless, the importance of the structural characteristics for slope stability behaviour and

analyses is recognised.

Itabirites are also sub-divided based on the rock hardness or rock strength rated by Vale’s

laboratory crusher tests. The crusher test, which simulates the industrial process, consists of

crushing a known sample weight to less than 31.5 mm, then sieving it at 6.35 mm. This results

in three main designations: hard (more than 50% above 6.35 mm); medium (50% to 25% above

6.35 mm); and weak (less than 25% above 6.35 mm). This test has been used as a guide to

identify both the rock strength and the weathering grade, resulting in the classifications of hard

(fresh to a slightly weathered), medium (moderately to highly weathered) and weak (completely

weathered to the residual soil). This classification system is also used in association with field

intact rock strengths as presented in the ISRM (1981) tables as suggested by Costa (2009) and

Cruz (2017). In this way field strength characterisation tables and crusher tests can be used to

support a limited number of UCS tests.

Other geomechanical classifications for Brazilian BIF, have been proposed by several authors

such as Zenóbio (2000), Zenóbio & Zuquete (2004), Araujo et al. (2014) and Castro et al. (2013).

These authors have adapted traditional geomechanical classifications using, for example,

specific field procedures and tests, calibrated with laboratory tests, as proposed, and applied by

several authors as presented by Martin & Stacey 2018.

Rich ores (hematitites)

Typically, itabirites display poor mineralogical variation. The most prevalent iron minerals are

hematite, martite, magnetite, specularite, goethite and ochreous goethite. Quartz,

iron-dolomite (ankerite and dolomite), gibbsite and kaolinite as a weathering minerals are the

main gangue minerals; talc, chlorite and pyrolusite are the main accessory minerals (Rosière et

al. 1993, 1996 and 2001). Those studies also describe the mineralogical and textural correlation

with geological association for several Iron Quadrangle mines.

The main geological and geotechnical characteristics from studied lithotypes and typologies, are

described as follows.

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• Hard hematitite

The hard hematitite ore (HHE) is a strong rock with strength varying from R4 to R6 according to,

ISRM (1981). The genesis is a subject of controversy and has been attributed to hipogenetic or

tectonic iron remobilisation in fold axes or the result of initial concentration in rich sedimentary

bedding (fewer common bodies), or in the tectonical planes intersections. Minor bodies are

correlated to discrete shear zones, exhibiting strong anisotropy due to millimetre tectonic

foliation defined by the presence of specularite. Each variant presents a typical fabric defined

by Varajão et al. (2002) as the following types:

• Massive, containing no banding or foliation, constituted by granoblastic hematite or

martite.

• Banded, with centimetric bands, formed by tabular hematite/martite with occasional

specularite.

• Foliated, with thin foliation, formed by tabular hematite/martite and with a significant

presence of specularite.

HHE typically has the highest intact rock strength, taking the form of a dark grey metallic

homogeneous rock with the highest ore grade; it is composed of granular martite and

microplates of hematite as the main iron minerals, followed by magnetite, quartz, and goethite.

A typical dark metallic colour is observed for the massive types and more opaque bands are

observed in banded types where a higher percentage of voids are concentrated, as shown in

Figure 7.2A.

In shallower depths or in fractured zones, the weathering results in only minor mineralogical

changes in this material type, where it is responsible for very low concentrations of oxidation

(goethite and ochreous goethite) on open discontinuities or increase in porosity levels. The

weathering degree vary from W1 to W2 in accordance with ISRM (1981) at the surface or in very

fractured deep zones as shown in Figure 7.2B. Varajão et al. (2002) have found for these

materials in Capitão do Mato mine a visual total porosity almost to 11% for slightly weathered

banded HHE (W2), whereas for fresh HHE (W1) this could be less than 2.5% (massive ore). The

typical grain size varies from 10 μm to 30 μm for hematite and martite granular crystals and is

equal to 1 μm for microplates of hematite. It is suggested that primary micro-pores vary from Å

to 1 μm, and the most important secondary porosity is associated with martite crystals varying

from Å to 5 μm. The average bulk density is 4.4 t/m3 for an iron content higher than 64% and

the natural moisture content shows an average of 1% as presented (Santos 2007).

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(A) (B)

85 Figure 7.2 A (left), HHE at microscope view with granular crystal of hematite (light grey

– B1) and smaller crystal of hematite microplates (dark grey – B3) (Horta

& Costa 2016). B (right), outcrop of fractured HHE at Capitão do Mato mine

• Medium hematitite

This medium material type (MHE) occurs at the contact between weak and hard hematitite, or

as interlayering of the two types, and is generally found surrounding large hard hematitite

bodies or in fold axes. As such, it exhibits more discontinuous surfaces comprised of a mixture

of W3/R2 to W4/R3 (ISRM 1981a) materials.

Under the microscope it shows varying characteristics inherited from the weak and hard

hematitite. MHE exhibits lower-strengths, compared with HHE. Further, there is no correlation

with the weathering profile, since the strength relates to the mixture of HHE and weathered

hematitite (WHE), as shown in Figure 7.3. Due to the low spatial distribution this type was not

evaluated in this chapter.

86 Figure 7.3 MHE outcrop folded in a Tamanduá pit face showing typical interlayer

between HHE and WHE

1,000 μm

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• Weak hematitite

Weak hematite (WHE) is the main supergene ore type, being the most extensive and richest. It

consists of thin (millimetre to centimetre) opaque/dark grey coloured layers of hematite and

martite, with low apparent cohesion (friable) and high total porosity, alternating with more

cohesive layers of dark metallic bands of hematite and martite with higher relative strength and

lower total porosity. The visual total porosity varies significantly from 25% to 30%, according to

Costa et al. (2009), or even higher, from 29% to 37%, according to Ribeiro (2003). This lithotype

exhibits heterogeneity and anisotropy, as defined by Costa (2009) and represents the lowest

strength hematitic ore at less than R2 (ISRM 1981a). In most cases it exhibits a tectonic foliation

but can also preserve the original banding in the form of porous layers.

Typically, these materials exhibit a weathering degree varying from W4 to W6, and a field intact

rock strength grade ranging from R0 to R2 (ISRM 1981a). Associated UCS values are lower than

5 MPa, generally ranging from 1 to 2 MPa, with an RMR class of IV or V, according to the

Bieniawski (1989) system. The bulk density is around 3.5 t/m3.

This lithotype is associated with synclines, as a halo around HHE in the fold limbs, in the lower

surfaces above the itabirites (brown ore) or in shear zones and brittle failures involving

specularite, as described by Costa (2009). Authigenic breccia is quite common and can represent

a load-deformation structure associated with the collapse resulting from the leaching process

and an associated reduction in volumes, as described by Ribeiro (2003).

Grain size and shapes vary with tectonic settings. Larger hematite crystals are granoblastic and

smaller grains are microplates, while lepidoblastic texture is present in the specularite. In size,

these iron minerals vary from 0.005 mm to 0.5 mm, with a maximum of 1 mm, as presented in

Rosière (2005), Figure 7.4A.

Costa (2009) defines three main types of WHE, namely:

• Banded: is the focus of this research, presents a centimetric banding layers with less

apparent cohesion intercalated with more cohesive layers and larger lateral extent. This

type is found in fold limbs, above the itabirite, and in low deformation areas; this is the

main type in terms of extent in the open pits, Figure 7.4B.

• Brecciated: associated with non-tectonic collapse structures and brittle faults, being

isotropic with very low apparent cohesion and high total porosity.

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• Foliated: showing millimetric foliation, strong anisotropy due to specularite content and

a low apparent cohesion. This type usually has a limited lateral extension and is

associated with shear in high deformation zones.

(A) (B)

87 Figure 7.4 A (left), microphotography of brecciated WHE showing granular larger

hematites crystals (light grey) and cement of microplates of hematite (Costa

2009). B (right), WHE at slope scale showing typical banding

Poor ores (itabirites)


In fresh dolomitic itabirite (FDI) heterogeneity is defined by the millimetre to centimetre pink or

white iron dolomite (ankerite and siderite) with lower percentage of quartz bands, interbedded

with dark grey iron bands constituted by tabular hematite, martite and martitised magnetite.

Sericite, talc, and chlorite are the most important accessory minerals. The non-iron mineral

crystal varies in size from 2 µm to 15 µm and iron crystal size varies from 5 µm to 20 µm, as

shown in Figure 7.5A.

FDI exhibits a very low visual total porosity (less than 5%), and moisture content of 5%. ρb = 3.16

(±0.36) t/m3 as described by Santos (2007).

Fresh FDI presents a weathering grade of W1 and are strong rocks (R4 to R6, according to ISRM

1981a), and shown in Figure 7.5B. Slightly weathered levels (W2) are rarely observed in mine

sites.

277

(A) (B)

88 Figure 7.5 A (left), microphotograph of FDI showing typical banding of dolomite and

quartz (light colour), (Horta & Costa 2016); B (right), shows typical hand

sample of fresh folded FDI

• Weathered argillaceous itabirite

The completely weathered argillaceous itabirite (WAI) is characterised by the dark brown colour

determined by high goethite and clay mineral (gibbsite and kaolinite) content. Spier (2005)

argues that WAI was formed by the leaching of dolomitic itabirites. However, according to

Zaparolli et al. (2007), the WAI origin can be also associated to tectonical and authigenic BIF

breccia with high total porosity and in some cases, with talc associated to the banding.

The rock has a fine laminated structure formed by layers of hematite microplates, granular

hematite and goethite alternating with layers of microplates of hematite, goethite, quartz,

gibbsite, kaolinite, and ochreous goethite. Some manganese minerals such as pyrolusite and

cryptomelanite are commonly found cementing the rock voids, as can be seen in Figure 7.6A.

For WAI, when the iron content is greater than 62% it is considered rich and is termed rich

weathered argillaceous itabirite or argillaceous weak hematite. The average ρb = 2.7 t/m3 and

the natural moisture content has an average value of 10%. Total porosity is mainly associated

with the iron bands and with the authigenic breccia texture (Zaparolli et al. 2007).

Typically, this material can be classified as W5 with a rock strength varying from R0 to R1,

according to ISRM (1981) as shown in Figure 7.6B.

500 μm

278

(A) (B)

89 Figure 7.6 A (left), WAI under the microscope showing bands of microplates of hematite

and specularite (light grey) and bands of smaller crystals of gibbsite and

ochreous goethite (light brown) (Horta & Costa 2016). B (right), WAI in an

exposure

• Fresh amphibolitic itabirite

Fresh amphibolite itabirite (FAI) exhibits heterogeneity defined by layers of hematite, martite

and goethite alternated with quartz, goethite, and amphibole (grunerite, tremolite, actinolite

and others) bands (Figure 7.7A). The original mineralogy with amphibole preserved occurs only

at very high depths, where a typical brownish-green colour is present.

General band textures are lepidoblastic to granoblastic, with an average crystal size of 30 µm.

The material exhibits very low visual total porosity (less than 5%) and a ρb = 2.84 (±0.46) t/m3,

as determined by Santos (2007).

As a result of the weathering, amphibole minerals easily alter to fibrous goethite and it is difficult

to obtain fresh FAI with amphiboles (W1) at surface or at low depths. For this reason, in this

research, FAI is mainly composed by the slightly weathered FAI (W2), where some weathering

and discoloration (to dark yellow or brown) can be observed in fractures and banding layers,

with the rock strength varying from R4 to R6 (ISRM 1981a), as illustrated in Figure 7.7B.

Considering the intact rock strength, these strong materials are as strong as the W1 fresh

itabirites.

1,000 μm

279

(A) (B)

90 Figure 7.7 A (left), microphotograph of FAI, illustrating the presence of fibrous goethite

and acicular amphibole crystals (dark fibre minerals) included in quartz bands

(Horta & Costa 2016); B (right), typical slope of folded and fractured FAI (W2)

in Jangada mine

• Moderately weathered goethite itabirite

The moderately or partially weathered goethite itabirites (PWGI) have a mineralogical

composition similar to FAI and FQI, deferring from these types by a significant reduction of intact

rock strength, total porosity increases and presence of goethite (oxidation of iron minerals) as a

constitutive mineral and ochreous goethite, with weathering grade and rock strength levels

varying from W3/R2 to W4/R3 (ISRM 1981a) The ρb = 2.72 (±0.45) t/m3 as described by Santos

(2007).

PWGI is characterised by its yellow to orange colour which is caused by the high content of

goethite occurring as bands, cement or involving quartz, replacing partially the original iron

bands, as shown in Figure 7.8. Can exhibit interlaying or grading (several metres) from FAI to an

ochreous itabirite as residual soil (W6).

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91 Figure 7.8 Typical PWGI slope in Jangada mine

• Weathered goethite itabirite

Mineralogically, the highly completely weathered goethitic itabirite (WGI), differs from the other

highly weathered BIF types due to a higher content of iron oxides and hydroxides (goethite and

ochreous goethite), Figure 7.9A. The average ρb = 2.4 t/m3 and the weathering degree varies from

W5 to W6, with field intact rock strength varying from R3 to R0, according to ISRM (1981).

WGI is characterised by its orange-brown and yellow colour which is caused by the high content

of goethite and ochreous goethite occurring as bands, cement or involving quartz, and replacing

almost all original iron bands, as shown in Figure 7.9B. This type is found at surface or along

fractures in subsurface.

A further increase in weathering can produce a residual soil (ochreous goethitic itabirite), being

composed basically of ochreous goethite and quartz. The abundance of gibbsite, kaolinite and

other deleterious minerals and high void ratio induces a very low shear strength to this material,

which is not evaluated in this study as its type is considered a soil material. The increase in

weathering will reduce the shear strength and increase the total porosity to W0 (residual soil)

defined as ochreous itabirite, not evaluated in this research due to the low spatial distribution.

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(A) (B)

92 Figure 7.9 A (left), WGI under the microscope showing goethite (orange) and eroded

quartz (yellow) (Horta & Costa 2016); B (right), A slope at Tamanduá mine

showing folded WGI interlayered with WHE


Fresh quartzitic itabirite (FQI) is the most common itabirite type. Heterogeneity is defined by

the alternation of non-iron and quartz (originally chert crystal) bands and iron bands composed

of hematite, martite and martitised magnetite.

The fabric of the quartz bands is granoblastic in general, typical from green schist metamorphic

zones, and lepidoblastic in discreate shear zones. The hematite bands have tabular and granular

shapes and grain sizes range between 6 µm to 80 µm (Figure 7.10A) and presents low total

porosity (less than 5%), and in a ρb = 3.06 (±0.29) t/m3, as presented by Santos (2007).

This rock has a high intact strength (R6 and R5), especially for the fresh materials (W1) as shown

in Figure 7.10B. Where some initial weathering occurs, especially in fractures or banding

contacts, its strength can reduce to R5 for slightly weathered (W2) materials ISRM (1981).

FQI is commonly found at great depths, but it can also be exposed in surface at unfractured

zones.

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(A) (B)

93 Figure 7.10 A (left), typical banding under the microscope showing hematite bands (light

grey) and quartz bands (red), (Horta & Costa 2016); B (right), typical FQI slope

at Tamanduá

• Moderately weathered quartzitic itabirite

The moderately or partially weathered quartzitic itabirite (PWQI) has a mineralogical

composition similar to FQI, except for the extensive presence of goethite, resulting from a typical

weathering of iron mineral, with weathering and rock strength levels (ISRM 1981a) varying from

W3/R2 to W4/R3. The quartz bands exhibit a higher void ratio and, in some instances, can be

disaggregated as a result of differential leaching (some layers can reach visual total porosity of

40%) as shown in Figure 7.11.

Leaching intensification further increases the void ratio, iron content and the percentage of

goethite and ochreous goethite, with a decrease in the intact rock strength, especially in quartz

bands when compared with the unweathered rock, FQI.

500 μm

Quatz band

Hematite band

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94 Figure 7.11 An example of leaching in a typical PWGI slope at Tamanduá mine

• Weathered quartzitic itabirite

Highly to completely weathered quartzitic itabirite (WQI), is the weathered variant of FQI, and

the second most common type in terms of spatial distribution. When the iron content is greater

than 62% it is considered rich and is termed rich WQI. The average ρb = 2.2 t/m3 the weathering

degree varies from W5 to W6 and the field intact rock strength varies from R3 to R0 (ISRM

1981a). UCS values can reach 5 MPa for less weathered R2 samples; however, in general, WQI

exhibits a UCS of lower than 2 MPa.

As described by Rosière (2005) the WQI occurs with normal banded fabric, composed of granular

hematite with fewer anhedral grains of martite, or rich in oriented tabular hematite, which

define a banding. The rock fabric of WQI is very similar to the original FQI, differentiating by the

increase in total porosity due to silica leaching and iron band oxidation. It can also have typically

eroded quartz crystals due to the intense weathering, as shown in Figure 7.12A.

Macroscopically, these rocks display friable, dark metallic grey coloured bands of hematite and

martite, and white to yellow friable quartz bands with a minor amount of goethite, as shown in

Figure 7.12B.

The original mineralogy is retained with the addition of remobilised microplates of hematite,

goethite, and gibbsite as a result of the weathering. However, there is a significant increase in

iron content driven by silica leaching. Quartz bands form highly leached (friable) layers, in which

the visual total porosity easily reaches 40% (Ribeiro 2003).

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(A) (B)

95 Figure 7.12 A (left), WQI under the microscope, showing quartz (grey) and

microplates of hematite (light grey) (Horta & Costa 2016); B (right),

WQI slope detail at Tamanduá mine

7.3.3 Banded iron formation weathering profiles

The vertical and horizontal changes in the BIF materials resulting from the weathering form what

is termed a ‘weathering profile’, formed by horizons and zones with different petrophysical

characteristics. The main characteristic of this profile is the general retention of the original

parent rock fabric, although in zones close to surface new fabrics are developed by collapse,

consolidation, and the formation of secondary structures. In extreme cases the original fabric

and mineralogy may be totally obliterated, forming what are called residual soils (Martin

& Stacey 2018).

As discussed by Ribeiro & Carvalho (2002), Ribeiro (2003) and Spier (2005), supergene iron

enrichment and weathering are directly responsible for the reduction of the original rock

strengths. Other geological processes such as tectonic events (e.g. shearing and brecciation)

could also have an effect, as suggested by Pires (1979 and 1995), as well as hypogene processes

and metamorphism, as expounded by Spier et al. (2008).

In this research, the supergene concentration and weathering sequence are considered to be

the main phenomena responsible for chemical and physical changes that reduced the original

hard, fresh rocks to weak residual soils. The original fresh, high-strength rocks were altered by

weathering, producing partial or complete leaching of minerals and chemical alteration as

1000 μm

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postulated by Dorr (1969), Varajão et al. (1997); Ribeiro (2003); Spier et al. (2006); Ramanaidou

(2009); Ramanaidou & Morris (2010).

In the field of geotechnical engineering, there is a great diversity of terminology used to describe

and classify the various components of a weathering profile, and the terms residual soil and

saprolite are often misused. Many authors have proposed their own terminology to designate

the different layers of a weathering profile. In this research, the weathering profile proposed by

Deere & Patton (1971) is used.

As proposed by Dorr (1969), the effectiveness of the weathering in the Iron Quadrangle (IQ) was

induced by a high temperature range, high pluviometry (rainfall), favourable topography, the

high primary and secondary permeability of BIF rocks structural control as synclines and

anticlines, banding with steep dip angles and extensional fractures. All of these factors facilitate

superficial and groundwater circulation, thereby enhancing chemical and physical weathering

and, thus developing deep weathering profiles that can often reach to over 400 m in depth.

Evaluating the ‘canga’ material origin, Spier et al. (2019), propose a genetic and evolution model

of the weathering profile for itabirites within the IQ, comprising a sequence of chemical

weathering and pedogenic processes. This model allows an independence of the pedolith

(ferruginous duricrust of canga) and saprolite (supergene iron ore) formation processes and the

evolution of weathering profiles developed in itabirite. It also indicates that weathering is less

effective in quartzitic itabirite than in dolomitic itabirite, resulting in different sized ore bodies

and geochemical trends.

As outlined, by Ramanaidou & Morris (2010), supergene iron enrichment and subsequent

strength reduction can be divided into two main processes:

• The supergene mimetic mechanism occurs below the watertable, as described by

Morris (1980) and Morris (2002, 2002a and 2003) and is associated with the presence

of structures, topography and climate. It is also responsible for producing deep and large

iron-rich deposits.

• The supergene lateritic mechanism is the result of gangue minerals dissolution and iron

reconcentration above the watertable. This process produces several weathering

profiles characterised by different relative iron enrichment and gangue mineral

composition.

Mineral changes are not the only noted effect of the chemical weathering process. Total porosity

increases are also recognised as an important change observed in those rocks. Studies by Morris

(2002 and 2002a) and Taylor et al. (2001) argue that the supergene process can reduce the

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thickness of BIFs by 32% to 40% and increase total porosity from 6% to 30% (Mourão 2007). This

information is in accordance with conclusions by Aylmer et al. (1978) when evaluating bulk

density, iron grade and total porosity for Mount Tom Price iron mine. These authors concluded

that iron grade and bulk density have a good correlation. However, this correlation accuracy is

affected by high values and variability of the total porosity.

According to Box & Reid (1976), for iron ore formation from Cockatoo Island, the true specific

gravity could be expressed as a function of iron content. However, due to the complexity and

the influence of multiple factors, the same could not be established for total porosity. Thomson

(1963) concludes that, for iron ore samples from Australia, a theoretical hematite quartz curve

can be used for bulk density definition and can allow an approximate iron content calculation.

The same relationship was identified for South African iron ore by Nel (2007); this study

established that for Sishen deposits total porosity is directly correlated to dry and bulk density,

providing a reliable calculation index.

For Vale’s iron ore mines in Brazil, studies by; Santos (2007), Santos et al. (2005) and Ribeiro et

al. (2014), have evaluated the association between bulk density and iron content for the IQ BIF

and concluded that there is a positive linear correlation between total iron content and bulk

density. Also, these authors argue that the weathering has an important influence on bulk

density and iron content variability as a function of the effective porosity.

7.3.4 Failure criteria evaluation for the BIF’s weathering profiles

In the IQ iron ore mines, due to the predominance of weak rocks and soil material, the

Mohr–Coulomb failure criterion (MC) has often been used to define the strength parameters

required for slope stability analyses, based on direct shear and triaxial shear tests. For soil

materials the Mohr–Coulomb criterion is the simplest and most widely used constitutive model

for determining the normal and shear stresses at failure on a loaded frictional material. Bai et

al. (2010) argue that MC has been widely used in rock and soil mechanics and has good

resolution for materials that fail in the elastic range and under low strain plasticity.

However, with increasing quantities of hard rocks in deeper mines it has become necessary to

perform specific tests for these materials and, due to the popularity of the Hoek–Brown failure

criterion (HB), the unconfined compression test (UCS) has become the most important rock

strength test applicable for hard rocks. In the absence of a suitable number of these tests,

geomechanical field classification and empirical correlations with petrophysical characteristics

were used to provide input for slope stability analyses.

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In Barton & Quadros (2015) it is indicated that the HB was initially proposed for the

determination of intact rock strength and rock mass strength under isotropic conditions.

Nevertheless, in recent years, with some adjustments, this criterion has been also used for the

determination of the anisotropic strengths of rocks. To use this criterion for prediction of

strength in anisotropic, intact rocks, a careful definition of the input parameters (uniaxial

compressive strength, ‘σci’, material constants ‘mi’ and ‘s’) is necessary. Moreover, the

determination of minimum and maximum intact rock strength variation due to rock anisotropy

is important in the selection of strength values for the rocks.

According to Saiang et al. (2014) there are fundamental differences between the two models or

criteria that are rather poorly understood or less well appreciated. First and foremost is the fact

that MC alone is a classic constitutive model, while HB is a failure criterion. This means that the

HB model cannot generally relate stress and strain in the same general way as the MC model.

Thus, plasticity results from the two models cannot be expected to be the same, as is often

mistakenly assumed. The next significant difference between HB and MC is the assumption

regarding the yield and deformation characteristics of the rock mass. HB assumes that the rock

mass is characterised by an elastic-brittle-plastic behaviour, while MC assumes that it is

characterised by an elastic-perfectly plastic behaviour.

Lin et al. (2014) presents an appropriate process that involves balancing the areas above and

below the MC plot. The HB envelope diagram is a curve, while the MC diagram is a straight line

(depending on the level of applied stress), as shown in Figure 7.13, and is divided into three parts

which are marked as Regions 1, 2, and 3, respectively. At normal stress in Region 1 or Region 3,

the equivalent MC strength parameter will overestimate the shear strength of rock mass. When

most the normal stress at the bottom of failure surface concentrates in Region 1 or Region 3,

the strength parameter may be slightly underestimated.

The models will give similar observations up to the point of yield or failure (Region 2). However,

HB will show much larger plastic straining than the MC for the same constant stress levels

beyond yield. For Lin et al. (2014), since HB does not relate stress and strain in the same general

way as MC, the accuracy of the plasticity after the yield is therefore questionable.

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96 Figure 7.13 The three regions for the Hoek–Brown strength curve and the equivalent

Mohr–Coulomb strength line (Lin et al. 2014)

The use of MC failure envelope is established for weak completely weathered types and

laboratory test results has been used to define the intact rock strength as well as the shear stress

as presented in Martin & Stacey (2018). For fresh itabirites, HB strength curve is more

appropriated for brittle rock masses observed in these types. However, for moderately

weathered ones the best criterion must be carefully evaluated as these rocks area a mixture of

material that exhibit soil and or rock strength characteristics.

7.4 METHODOLOGY

To determine the intact rock strength parameters for all BIFs along the complete weathering

profile and evaluate the geological and geotechnical characteristics, a methodology divided into

three phases was adopted, as described below.

The first phase included a reference review and field investigation, where several samples were

collected from surface and drill cores, covering all BIF lithotypes. Geological and geotechnical

information based on ISRM (1981) classifications were described, and outcrops and samples

were photographed for additional visual information, e.g. anisotropy, banding, and physical

condition, particularly used for sample identification.

The second phase focused on petrographic thin section analyses used to evaluate rock

mineralogy, fabric, and visual total porosity, to provide information from a microscopic

perspective for comparison with field macro characteristics obtained in the first phase. This

phase also included all laboratory tests, both execution and result evaluation, used to determine

intact rock strength and static elastic parameters values, as well as correlation between

geological characteristics and geotechnical parameters for each materials type.

The third phase involved data interpretation and the development of this thesis.

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Brazilian and Australian laboratories were used to carried out geotechnical tests and provide

proper results used to support the aims of this research. Data from previous laboratory test

reports (available in Vale’s database) and laboratory test results from other authors were

occasionally used to address the lack of information identified for some types.

The first step in the laboratory test program was to define the appropriate geotechnical tests

for each weathering grade. Due to the high-strength variations noted in the weathering profile

and the difficulty in preparing samples with high-strength heterogeneity, mainly for partially

weathered types, the rock strength estimation table from Martin & Stacey (2018) was used to

define a hypothetical strength limit for the three groups of BIF (fresh, moderately an completely

weathered) and associated laboratory tests, as shown in Figure 7.14.

For hard rocks with high-strength and fresh to slightly weathering grade associated to bedrock

horizon (green dotted line) the unconfined compressive strength (UCS), triaxial Hoek cell

(TRI-HB) and indirect unconfined tensile strength (UTS) were used. For weak rocks with low to

very low strengths and highly to completely weathered associated with saprolite and residual

soil weathering grades (red dotted line), single stage consolidated undrained triaxial

compression test (CIU) was applied. For moderate strength and weathering grade values

associated with saprorock (blue dotted lines), where materials exhibit variable strength

behaviour, tests were selected in accordance with the predominant strength characteristic in

each sample (soil for saprolite or saprorock for rock). For all samples, physical tests were carried

out to determine the bulk density and visual total porosity.

97 Figure 7.14 Weathering grade and estimation of the rock strength table plotted for the three

main weathered groups. The applied laboratory tests for each strength level are

grouped by the green dotted square for hard rocks (bedrock), a blue dotted

square for moderate rock (saprorock) strength and the red dotted square for

weak rock (saprolite) and soil-like material strength (after Martin & Stacey 2018)

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Often large variations in values of the strength of rock samples are attributed to accuracy of

sample preparation or testing procedures. However, high variance also results from intrinsic

characteristics of sample negligence during sample selection and grouping. In order to reduce

variability due to geological effects and defects, the sampling validation approach presented in

Appendix I, which suggests a sample grouping to reduce the variance and provides better

lithotype characterisation, was used. This approach was proposed to check the samples before

carrying out the tests, and considers:

• Samples were grouped by the level of weathering based on ISRM (1981), geological

characteristics (e.g. banding features), transverse isotropy direction (βangle) and ρb.

• For fresh rocks, Vp measurements were used to discard inadequate samples (inclusions

or fractured).

• Geological features such as intense folding, the presence of significant specularite and

chlorite, fracture filling materials with a different degree of weathering, quartz or calcite

veins and others that do not represent the typical BIF heterogeneity were discarded.

• Itabirites samples that did not show typical banding were not evaluated, thereby

heterogeneity defined by the compositional metamorphic banding was included and

qualitatively evaluated. The bands thickness was qualitatively considered to the

sample size in order to avoid the scale effect.

• Extreme outlier results were removed according to the box plot statistical

methodology (Whitaker et al. 2013). This technique identifies the mild outlier’s values

from the quartiles (Qt) determination, based on the Equation 7.1.


QtLower = 1Qt - 1.5(3Qt - 1Qt) (7.1)


Qtupper = 3Qt + 1.5(3Qt - 1Qt) (7.2)


The 3Qt evaluates database dispersion around a central data leaving 75% of data

below the sum and is defined by Equation 7.3:

3Qt = × + 1.5.IQR (7.3)

The 1Qt evaluates database dispersion around a central data leaving 25% of data

below the sum and is defined by the Equation 7.4.

291

1Qt = × - 1.5.IQR (7.4)


and these form what are called outliers and are defined by Equation 7.5:

IQR = 3Qt - 1Qt (7.5)

To provide a basis for anisotropy evaluation, different mathematical approaches were used to

make comparisons between different lithotypes and parameters, depending on the number of

test results. When the number of test results was lower than 10 (n < 10) the arithmetic mean

(mean), obtained by summing the data results and then dividing by the number of samples or

variables, was used. When the number of test results was higher than 10 (n ≥ 10) basic statistical

evaluations (arithmetic mean, standard deviation, coefficient of variance, and maximum and

minimum values) were applied. These values were used with a confidence interval of 95%.

The basic statistical analyses allow a clear pattern of values results and variation and thus the

decision making of the most assertive parameters possible to be used.


property, different results in different directions and the degree or ratio of anisotropy is used to

quantify how far the rock is from being isotropic. For geotechnical rock characterisation, the

main physical property used to determine the anisotropy is the rock strength. The term

heterogeneity is used for rocks composed by layers or bands (scale related) that are different

from one another that could, or could not present, for the same physical properties, different

results in different directions.

For BIF which presents metamorphic heterogeneous banding, it is necessary to determine the

ratio of anisotropy to evaluate the anisotropy behaviour. Estimating the variation of intact rock

strength due to anisotropy effect allows the differentiation of spurious test results induced by

rock intrinsic characteristics which can lead to misleading results and increasing variance. In the

present study, the variation of UCS due to anisotropy was considered and was determined by

the degree of strength anisotropy (RC), as firstly proposed by Singh et al. (1989) with defines the

degree of anisotropy as the variation in compressive strength (measured in uniaxial and triaxial

tests) depending on the angle between the direction of the load applied to the tested samples

and the direction of the anisotropy.

In order to evaluate the influence of metamorphic banding and define an anisotropy in intact

rock strength for all tests, the anisotropy (βangle) as described by Jaeger (1960) was considered,

by testing different angles between banding and the loading direction, varying from 0° to 90°.

However, due to the reduced number of valid results for some typologies, results were grouped

292

in three main βangles ranges: for loading parallel to banding (β0°), all tests results from

0° < β ≤ 30° were considered; for direction of loading oblique to banding (β45°), results from



The anisotropy ratio was defined as presented in Singh et al. (1989) as the ratio between the



perpendicular to the planes of anisotropy and σcmin, the lowest compressive strength value

obtained. The range and classification of the degree of anisotropy established by Equation 7.6

are presented in Figure 7.15 as well as the diagram representing the βangle definition by

McLamore & Gray (1967).

𝑅𝑅𝑅𝑅 = 𝜎𝜎𝜎𝜎 90°𝜎𝜎𝜎𝜎 𝑚𝑚𝑚𝑚𝑚𝑚

(7.6)

where:

Rc = anisotropy ratio.



98 Figure 7.15 Classification based on anisotropic ratio, Ramamurthy (1993) and βangle


Laboratory test results were grouped by rock type and plotted in σ1 σ3 space for intact rock

strength envelopes, based on the Hoek–Brown failure criterion as defined by Hoek et al. (2002),

and the Mohr–Coulomb failure criterion was applied to each dataset using RocData 5.0

(Rocscience 2021). Resulting parameters based on each strength envelope scenario were also

established.

RocData 5.0 (Rocscience 2021) enables the determination of intact rock parameters from

triaxial, uniaxial and indirect tensile laboratory tests. Considering the notes given by Hoek &

Brown (1997), the adequate number of tests necessary for a proper curve comprises carrying

out tests over a confining stress range from zero to one half of the UCS. At least five data points

293

should be included in the analysis. In addition, the variation of σ3 must follow a proportion, as

originally proposed, for a reliable value of σci and mi considering a range of 0< σ3 < 0.5 * σci for

laboratory testing.

As the intact rock strength was assessed, the geological strength index (GSI) was considered

equal to 100 and the HB disturbance factor (D) was considered equal to zero, in order to equally

compare intact rock parameters obtained by the software adjusted approaches.

Fundamentally, rock strength in the Hoek–Brown failure envelope has each type of laboratory

test represented in a relative position, as shown in Figure 7.16. This will guarantee the best

failure envelope fit and, consequently the best Mohr–Coulomb linear regression responsible to

determine all parameters. With this approach it was possible to obtain fitting shear straight and

elastic parameters based on MC and HB fitting curves for all evaluated BIF types, enabling a

coherent method to compare these parameters.

99 Figure 7.16 Relative position of fundamental rock strengths on the Hoek–Brown failure

envelope in principal stress space (modify from Sari et al. 2010)


Uniaxial compressive or unconfined compressive strength test

Recognised as the most common and widely used strength test for rock mechanics approaches,

the unconfined compressive strength (UCS) test is also known for its high variance, mainly

related to sample preparation, imperfections or associated with intrinsic characteristics such as

heterogeneity, anisotropy, inherent discontinuities, and other rock intrinsic characteristics.

294

Selected rock samples from drill cores were sub-sampled, reviewed and prepared according to

ASTM D4543-01 (ASTM 2001). Final preparation of the samples involved the formation of

perfectly parallel ends to obtain perfect coupling.

The first set of UCS tests was carried out at an Australian laboratory following ASTM D2938-95

(ASTM 2002). Samples (cylindrical) were obtained from the original 77.8 mm cores using a

diamond drill bit, which sub-sampled to a 20 mm diameter, with lengths ranging from 40 mm to

50 mm, according to ASTM D4543-01 (ASTM 2001). For better axial and lateral strain records,

simultaneous measurements by two pairs of diametral strain gauges were used, and all samples

were subjected to a constant loading rate of 7 MPa/min or 9 MPa/min.

The second set of UCS tests was carried out at a Brazilian laboratory, also in accordance with

ASTM D2938-95 (ASTM 2002). For all tests, cylindrical samples were obtained from the original

diamond drill core of 50 mm or 76 mm diameter with lengths varying from 110 mm to 205 mm,

again prepared in accordance with ASTM D4543-01 (ASTM 2001). Axial and lateral

measurements were obtained by double dial indicator gauges, and strain was calculated by

taking the average in each direction. It is noted that dial indicator gauges have shown less

efficiency on strain measurement than strain gauges. The average strain was calculated by

taking the average of all strain readings; Young’s modulus and Poisson’s ratio were calculated

from the strain moving average.

In line with standard engineering practice, in this study, the tangent modulus (Estat) was

determined by the slope of the line that tangents the regression curve at a given point

corresponding to a fixed percentage of the ultimate or peak stress (𝜎𝜎u), normally 50%. The

Poisson’s ratio (νstat) which correlates the axial and radial deformation, was obtained in the

stretches of the curve corresponding to the observed limits for obtaining the deformability

moduli.

Due to the difficulty in preparing reliable samples during the extraction process, some types

presented reduced numbers of proper samples, specially FDI at β0°, HHE at β90°, and β45°.This

reduced number of tests could influence evaluations and produce bias in the interpretation.

For all samples prepared and tested with diameter sizes different from 50 mm, the results were

converted to d50mm based on the most commonly referenced methodology, i.e. the empirical

correlation proposed by Hoek & Brown (1980). Equation 7.7 shows the empirical correlation

where σcd is the UCS of a sample with diameter, ‘d’; and σC50 is the UCS of a 50 mm diameter

sample. It is generally accepted by the authors that there is a significant reduction in strength

with increasing specimen size.

295

𝜎𝜎cd = 𝜎𝜎𝑅𝑅50 � 𝑑𝑑50�0.18

(7.7)

where:

σcd = UCS of a sample with diameter.

σC50 = UCS of a 50 mm diameter sample.

d = sample diameter.

Indirect unconfined tensile strength test – Brazilian test

Rock tensile strength (σt) is among the most important rock parameters defined from the

indirect unconfined tensile strength (UTS) test, tensile Brazilian test, which was originally

designed for isotropic rocks. Initially developed to measure tensile strength for brittle rocks, as

proposed by Carneiro & Barcellos (1953), this simple and cheap test is a well-known indirect test

to determine tensile strength. Brazilian tests were carried out on cylindrical 50 mm diameter

(D50mm) samples obtained from core samples in accordance with ASTM D4543-01 (ASTM 2001).

The tests were undertaken in accordance with ASTM D3967-08 (ASTM 2008) for Brazilian and

Australian laboratories.

There are few studies covering the tensile strength of anisotropic behaviour of the BIF, and

correlation equations are still limited due to the lack of satisfactory expressions of the

stress–strain field.

To access the tensile strength of BIF anisotropy, the orientation of the anisotropy angle (βangle)

was defined using the same approach as for the UCS tests.

The indirect tensile strength is typically calculated based on the assumption that failure occurs

at the point of maximum tensile stress (i.e. at the centre of the disc). The suggested formula for

calculating the tensile strength based on the Brazilian test was presented by Alawad (2020), as

presented in Equation 7.8.

σt=2𝑃𝑃

𝜋𝜋∗𝐷𝐷∗𝑢𝑢 (7.8)

where:

P = the load at failure (N).

D = the diameter of the test specimen (mm).

T = the thickness of the test specimen measured at its centre (mm).

296

Hoek–Franklin triaxial cell tests

Originally developed by Hoek and Franklin (1967), the Hoek Triaxial Cell (TR-HB) is designed to

determine the triaxial shear strength and elastic properties, i.e. static modulus of elasticity and

Poisson’s ratio, of hard rocks at different confining pressures.

Triaxial compressive testing was carried out using a Hoek–Franklin cell for samples with 20 mm

of diameter according to ASTM D2664-04 (ASTM 2004) at the Australian laboratory, and for

samples of 76 mm diameter in the Brazilian laboratory. Confining pressures applied were 1 MPa,

5 MPa, 15MPa and 20 MPa.

Single stage consolidated undrained triaxial compression test

For rock-like soils types, single stage, isotropic consolidated undrained test (CIU) were



shear. Which does not allow drainage at all phases of the test, is the most common test used for

weak, porous, and granular (permeable) materials. In view of the assumed high porosity of the

large majority of highly weathered BIF (except for the WAI), this test was considered to be the

most appropriate method for establishing results in terms of total and effective stress (with pore

water pressure measurement). Results are presented as total or effective stress and associated

pore water pressure are determined.

The triaxial tests undertaken in Australian and Brazilian laboratories were in accordance with

ASTM D4767-95 (ASTM 1995) and both used cylindrical specimens with a diameter of 50 mm.

Confinement stresses (σ3) of 100, 200, 400, 600 and 800 kPa were applied. Test stress levels

were limited to 800 kPa maximum due to a historical soil profile depth of 250 m for shallow

mines.

Bulk density

Bulk density was determined according to ASTM 1289.6.4.1 (ASTM 2016) for Australian

laboratories and NBR 6508 (ABNT 1984), for Brazilian laboratories. It was calculated according

to Equation 7.9.

ρb=𝑀𝑀𝑉𝑉

(7.9)

where:

ρb = the bulk density (kg/m3).

M = the mass of the specimen measured prior to testing (kg).

297

V = the volume (m3) of the specimen, calculated from dimensions measured during

sample preparation.


Sample thin sections (perpendicular to banding) were prepared to evaluate crystal and visual

pore size, shape and distribution, mineralogy percentage and microtexture at microscale for all

BIF. A total of 33 thin sections were produced at The University of Western Australia’s

petrography laboratory and evaluated by Horta & Costa (2016). Additionally, an extra 12 thin

sections evaluated by Costa (2009), and another six thin section from a Vale internal report

prepared by Zaparolli et al. (2007) were also used.

All samples were analysed using standard petrographic techniques based on a description table

created for better visualisation of the obtained information. This table contains a classification

of the rock type in the thin section, and important additional information observed during the

inspection. Microphotography were taken to show important features, textures, and

particularities.

The techniques used to determine size in thin sections are highly influenced by the angle at

which the banding is cut. For this reason, some bias is expected for these results. In order to

avoid this problem only samples cut at 90° to the metamorphic banding were evaluated.

All rock tests, separated by rock type, mine, and anisotropy (β), are available in Appendix V and

soil tests are presented in Appendix VI. The total number of each test undertaken for this chapter

is shown in Table 7.1.

24 Table 7.1 Summary of laboratory tests

Laboratory tests n (Vale database) n (tested) n (total)

Thin sections 10 41 51

UCS 23 127 150

UTS 20 61 81

Bulk density 43 188 231

Triaxial HB cell 0 68 68

CIU 86 50 136

UCS: unconfined compressive strength test; UTS: unconfined tensile strength test; CIU: isotropic consolidated undrained test.

7.5 RESULTS

7.5.1 Banded iron formation weathering profiles

The term ‘saprolite’ has been used by several authors including Rosière et al. (2001), Ribeiro

& Carvalho (2002), Spier et al. (2019) and others to define weak BIF resulting from bedrock

298

weathering, while preserving the original fabric. This terminology is supported by the presence

in weathered variants of typical macro and micromorphological characteristics that are also

observed in fresh BIF lithotypes. The main characteristics observed are specific fabric and

mineralogical changes; development of total porosity; presence of kaolinite and gibbsite; iron

oxides accumulation in fissures and voids; amphibolite alteration to an iron oxide pseudomorph;

fissures and evidence of dissolution of quartz. In addition, there can be homogenisation of the

banding heterogeneity induced by dissolution of iron dolomite and quartz and collapse of voids

brecciating the original proto-ore fabric. Ultimately, a residual soil with ISRM (1981) weathering

class W6 (ochreous goethitic itabirite) shows disappearance of the original fabric with

homogenisation of the rock mass, changes in colour and clay enrichment with shrink-swell

shearing behaviour.

Based on these characteristics it was possible to separate various BIF types by mineralogical

composition (lithotype) and the degree of weathering, and subsequently associate the materials

with estimated UCS ranges for different geomechanical classifications, based on Martin & Stacey

(2018) proposed table plotted in Figure 7.17.

Following the weathering classification suggested by ISRM (1981) the materials in this study

have the following grouping associated with the weathering horizons profile as suggested by

Deere & Patton (1971):

• FDI, FAI and FQI: bedrock, represent the typical metamorphic banding with minor

influence of supergenic or weathering processes, defined as proto-ore or fresh to

slightly weathered BIF (W1 and W2), with high UCS ranging from 100 MPa to 520 MPa

(light green dashed square in Figure 7.17).

• PWQI and PWGI: saprocks, partially affected by supergenic or weathering processes,

defined as moderately weathered materials (W3) with intermediate intact rock UCS

ranging from 1 MPa to 200 MPa (blue dashed square in Figure 7.17).

• WHE, WAI, WGI and WQI: weak rocks, totally affected by supergenic or weathering

processes, defining the highly to completely weathered BIF (W4 and W5) with low UCS

ranging from 0.05 MPa to 2.5 MPa (red dashed square in Figure 7.17).

• HHE and MHE are hard to medium strength hematitite material, the genesis of these

lithotypes is primarily associated with tectonic metamorphic events and or hypogenic

iron enrichment. For this reason, they are poorly affected by supergenic and

weathering process and are presented in different columns in Figure 7.17.

299

100 Figure 7.17 Estimated UCS summary for all BIF types, showing in light green the high, in

blue the intermediate and red the low UCS values (after Martin & Stacey 2018)

Weathering, as a continuum and multiple conditioning process, generates a range of materials

with different levels of intact rock strength, displaying a full range from bedrock (with minor

effects) to residual soil for each of the compositional itabirites.

As the weathering affects the different compositional BIF in different ways, the weathering

profile presents specific characteristics, with three-dimensional variability of their nature in size

and depth, and different boundaries between weathering horizons and levels for each BIF group.

For typical BIF, two types of weathering profiles were identified:

Discontinuous weathering profile

It is identified in mines where the main proto-ore is constituted by dolomitic itabirites: where

the geology and topography are suitable for supergene laterite weathering enrichment that can

reach high depths (Ramanaidou & Morris 2010) where its influence is reduced in great depths.

300

It´s notice saprolites at more than 500 metres deep. This is promoted by the leaching process,

which defines a dual weathering horizons with bedrock of fresh dolomitic itabirite (FDI) and

completely weathered types (WHE and WAI).

Since the leaching process is more effective over dolomitic itabirite, due to the high solubility of

iron dolomite (siderite and ankerite), in mines with this proto-ore (e.g. Águas Claras, Capão

Xavier and Mutuca mines) weathering profiles are deeper and weathered weak hematitite is

more homogeneous in terms of iron content and percentage of gangue minerals. In these mines,

the transition between WHE and FDI is abrupt, as shown in Figure 7.18A. The partially weathered

material is very narrow (few centimetres), or is generally missing, as shown in Figure 7.18B.

(A) (B)

101 Figure 7.18 A (left), abrupt contact between weathered hematitite and FDI at CPX mine.

B (right), A rare, decimetre thick slightly weathered zone at FDI contact with

weathered hematitite (WHE) at MAC mine

Continuous weathering profile

It is associated to quartz base types, quartzitic and amphibolitic itabirites (FQI and FAI): twelve

out of fifteen evaluated mines, (including Tamanduá, Capitão do Mato, Pico, and others) exhibit

these proto-ore types. In these rocks, the supergene and weathering processes are less

effective, and the weathering profile displays vertical and lateral variability in the first few

hundred metres below the surface. The profile is composed (from the surface to great depths)

of weak hematitites (WHE) overlapping a weathered goethite itabirite (WGI) resulting from

oxidation above or at the water level interface zone, while below the water level a zone with

WQI defines the alteration.

With the deepening (around 200 m), WQI gives way to moderately weathered materials such as

PWQI or PWGI and depending on the proto-ore this transition zone can extend from 300 m to

400 m, defining the saprocks zone. The thickness of this intermediate zone can vary from several

to a hundred metres, depending on weathering efficacy. These saprocks may, however, contain

301

weathered zones associated with deep faults, shear zones or fractured zones. At greater depths,

the metamorphic banding predominates for fresh FQI and FAI, and slightly weathered materials

can occur in more fractured zones, defining the bedrock zone, which is formed by low permeable

rocks with less fracturing or banding dipping unfavourably for water seepage.

In the same weathering profile, the weathering can affect quartzitic and amphibolitic itabirites

in different ways. Weathering profile boundaries and percentage of moderately weathered

material can vary depending on the geology, depth, geomorphology, subsurface water presence

and tectonical settings. In addition, the broad boundary of these moderately to completely

weathered lithotypes demonstrate different geomechanical characteristics due to the iron and

gangue mineral content, heterogeneity, total porosity, bulk density, and other associated

factors. Weathering profile boundaries are illustrated in Figures 7.19A and 7.19B where a metric

presence of partially weathered quartzitic itabirites (PWQI) is noted.

(A) (B)

102 Figure 7.19 A (left), narrow transitional contact zone, showing metre-scale PWQI, WQI

and FQI at Tamanduá mine; B (right), transitional contact zone, showing

metre-scale of PWQI between WQI and FQI at Tamanduá mine

Figure 7.20 illustrates, in a single typical cross-section, the two weathering profiles controlled by

BIF mineral compositions. In Figure 7.20, the schematic cross-section A shows continuous

weathering profile (FAI and FQI), with a gradational transition from fresh to completely

weathered rocks. The schematic cross-section B shows the discontinuous weathering profile for

FDI proto-ore, where the abrupt contact between fresh to completely weathered lithotypes

occurs even at great depths, and moderately weathered materials are not evidenced.

302

103 Figure7.20 Typical geological cross-section highlighting two different weathering

profiles induced by BIF mineralogical compositions and their interference

patterns. A (top left), cross-section for continuous profile with FAI and FQI

proto-ore presenting moderately weathered materials. B (top right),

cross-section for discontinuous profile with FDI proto-ore and absence of

moderately weathered materials

7.5.2 BIF mineralogical and fabric overview

From the microscope studies of all BIF from fresh to highly weathered material it was possible

to verify mineralogy, total porosity, and textural features characteristics as described in the

following sections. Some of these microfeatures are responsible for the geomechanical

behaviour observed in these material types.

Fresh quarzitic Itabirite

The mineralogy of the fresh quartzitic itabirite (FQI) includes quartz, hematite, tabular hematite,

and goethite. Ochreous goethite, carbonate, kaolinite, and gibbsite occur as accessory minerals.

Microplates of hematite and quartz grains are best observed in the boundaries between the

hematite bands and the quartz bands. The total porosity is very low (Øb = 5%). Grain sizes larger

than 0.80 mm for tabular shapes and 1 mm for granular shape were considered part of the

303

hematite band. Goethite appears as small crystals and, in the quartz rich areas, anhedral forms

can also occur. Bands of quartz have low total porosity, which mainly takes the shape of

intergranular total porosity. However, in the layers where microplates of hematite occur in

larger quantity with quartz, the porosity is usually slightly greater. Hematite grains larger than

1.3 mm were considered part of the banding itself.


The predominant mineralogy in the moderately weathered, denominated partially weathered

quartzitic itabirite (PWQI) are hematite, quartz, ochreous goethite, goethite, kaolinite, and

gibbsite. Hematite is characterised by grains with low intergranular porosity. Some pores can be

partially/completely filled with goethite or ochreous goethite, and goethite can also occur as a

cement between grains. In general, kaolinite and gibbsite appear as a filling in intragranular

pores. Quartz bands and hematite bands can occur, with the preserved hematite bands are

mainly characterised by serriform contact between hematite grains; visual total porosity can

reach high percentages (20% to 30%).

Total porosity is lower where tabular hematite is predominant. Secondary porosity is mainly

characterised by fractures varying in width between 0.01 mm to 0.24 mm.

Weathered quartzitic itabirite

The highly to completely weathered quartzitic itabirite (WQI) is composed of granular and

tabular hematite, goethite, and ochreous goethite bands interbedded with quartz, goethite,

ochreous goethite kaolinite and gibbsite bands. WQI exhibits a high visual total porosity

normally varying from 25% to 30%, with pore sizes varying from 0.1 mm to 0.25 mm.

Anhedral hematite usually occurs as large crystals (2 mm) and the remaining tabular hematite is

concentrated at iron bands boundaries. Intragranular are associated with granular hematite,

often filled with goethite.


The mineralogy of fresh amphibolitic itabirite (FAI) is mainly defined by quartz, hematite,

pseudomorphs of amphibolite, goethite, and ochreous goethite. The pseudomorphs of

amphibolite are generally altered to fibrous goethite which only occur in some of the thin

sections, where it is very often observed in the boundaries of the iron bands. Goethite usually

occurs in anhedral shape between quartz grains as a cement, but also in a smaller amount as

crystals. Quartz bands are well preserved, and large hematite crystals were interpreted as part

of the preserved hematite bands. The hematite exhibits low visual total porosity (Øb = 5%), as

304

does the quartz. Hematite occurs mainly as granular grains, while the bands show varying

degrees of preservation, from poorly preserved to well preserved. In some cases, the limits

between quartz and hematite bands are not very clear. All the samples exhibit low visual total

porosity.


The moderately weathered type, referred to as partially weathered goethitic itabirite (PWGI) is

characterised by hematite, goethite, and quartz, with ochreous goethite, gibbsite and kaolinite

occurring in smaller percentages. Anhedral crystalline goethite crystals are also commonly

observed in smaller concentrations, while goethite cement is present filling spaces between

grains of quartz and hematite. Hematite is present as grains and microplates, usually with

serriform contact and have a high visual total porosity usually varying between 15% and 30%.

Preserved banding is observed in most of the thin sections. Some quartz bands have a low total

porosity, with goethite as a cement filling the space in between the grains of quartz and

hematite and, less commonly, as small grains (size in between 0.005 mm to 0.02 mm). The bands

of hematite are mainly characterised by grains of hematite with an intergranular porosity equal

to 15%. Tabular grains also occur as microplates of hematite in the boundaries of the bands.

Fractures (with widths of between 0.10 mm to 0.2mm) occur and can be partially filled with

goethite and ochreous goethite.


The highly to completely weathered goethitic itabirite (WGI) is composed of goethite, hematite,

granular quartz and ochreous goethite. Hematite occurs as microplates and small granular

grains, and larger granular grains are also present.

Hematite banding is preserved and is defined by the remaining hematite grains interbedded

with goethite, ochreous goethite and quartz bands. Total visual porosity is high (15% to 30%),

with intergranular pores usually associated with iron-rich bands and the larger granular grains

of hematite.


The mineralogy of the fresh dolomitic itabirite (FDI) is defined by hematite, iron dolomite, quartz

and goethite. Granular hematite is predominant, and its grain size is decreased when iron

dolomites (ankerite and siderite) occur in higher quantities. The iron dolomites occur interlayered

with hematite bands, as well as filling spaces between granuloblastic hematite grains.

305

Carbonate (calcite) occurs as cement and veins, filling spaces between grains of hematite and

iron dolomites; its occurrence is more common at the boundaries of hematite bands where iron

dolomite and hematite grains of smaller size dominate the scene. Anhedral goethite can also

occur in these areas, but its occurrence is limited, and most of the goethite occurs in the shape

of grains in and adjacent to bands of hematite.

Quartz is present in bands with oriented grains and partial grains. Talc and sericite occur as

accessory minerals. Bands of hematite and iron dolomite are still preserved, characterised by

low porosity. When bands are not preserved, hematite occurs as grains, with very fine-grained

iron dolomite as the matrix. The percentage of pores is generally limited, although intergranular

pores size is considerably larger when associated with carbonate. The small presence of

intragranular pores is mostly linked to grains of hematite.


Highly to completely weathered argillaceous itabirite (WAI) is mineralogically composed of

granular hematite and microplates of hematite, ochreous goethite, goethite, gibbsite and

kaolinite and granular quartz and exhibits Øb = 30%. The hematite bands show Øb varying from

10% to 30% as defined by Zaparolli et al. (2007), due to secondary cementing of the voids.

Anisotropy is defined by the orientation of tabular and granular hematite, with grain sizes larger

than 0.5 mm. Specular hematites occur as accessory minerals.

Ochreous goethite is the most common cement, but hematite can also act in that role, filling the

spaces between hematite grains. Goethite usually occurs in an anhedral form, but aggregates

are also present in smaller concentrations and grain sizes. Iron bands can present large granular

hematite crystals (>1 mm).

Weak hematitite

Weak hematitite (WHE) consists of hematite and martite, showing heterogeneity defined by

lamellar iron minerals such as specularite (0.07 mm long and 0.01 mm wide), or of martite and

hematite microplates growing over a granoblastic fabric (0.024 mm to 1.2 mm).

More oriented bands are defined by small tabular hematite (0.05 mm long and 0.01 mm wide)

Granular hematite sections bands show variations in size, with visual total porosity (10% to 25%),

as described by Costa (2009), and shown in Figure 7.7B. These porous layers are interconnected

in some areas that are more leached or altered, defining weaker zones, depending on the

weathering intensity.

306



Also presented are the percentage, and maximum and minimum size (between brackets) of

pores for three different types of visual measures. Intragranular pores correspond with the

percentage of pores inside crystals that can or cannot be interconnected. Intergranular pores

correspond with visual total porosity between crystals and are generally interconnected.

Secondary pores refer to fracture pore percentage. Visual total porosity is the sum of the three

pore percentages and is considered the maximum total percentage. For total porosity in

brackets, the minimum value did not consider the secondary pores (intact rock) and for

maximum value, the secondary pore is included.

25 Table 7.2 Thins section sum

mary table for all BIF (H

orta & Costa 2016)

Lithology

or

Lithotype

Granular

Hematite (%

)

Size (min-

max)

Tabular

Hematite

(%)

Size (min-

max)

Specular

Hematite

(%)

Size (min-

max)

Goethite (%

)

Size (min-

max)

Ochreous

Goethite

(%)

Size (min-

max)

Quartz (%

)

Size (min-m

ax)

Gibbsite/Kaolinite

(%)

Size (min-m

ax)

Amphibolite

Pseudomorph

(%)

Size (min-

max)

Carbonate

(%)

Size (min-

max)

Intragranular

Pores (%)

Intergranular

Pores (%)

Secondary

Pores (%)

Total porosity

(%) m

in – max

Size (%) (m

in-

max)

Num

ber

of thin

sections

FAI 34

(0.002–1.16) –

– 4

(0.005–0.6) –

43

(0.02–0.8) –

14 –

42

5

(0. 005–0.08) 5

FDI 25

(0.020–1.5)

7

(0.009–0.08)

1

(0.09–0.2)

2

(0.005–0.08) 10

17.5

(0.015–0.4) –

– 32

(0.025–0.5) 3

3 6

(0.005–0.18) 5

FQI

26

(0.002–1.9)

7

(0.002–0.8) –

1

(0.002–0.15)

2

(0.005–1.5)

36

(0.01–1.7)

5

(0.005–0.03) –

– 7

4 2

11–13

(0.001–0.11) 13

PWGI

28

(0.01–1.8)

12

(0.005–0.8) –

34

(0.005–0.9)

8

(0.07–1)

24

(0.01–0.4)

2

(0.06–0.2) –

– 6

3 7

9–16

(0.002–0.12) 5

PWQ

I 36

(0.01–1.2)

20

(0.002–0.10) –

8

(0.002–0.45)

4

(0.01–1.5)

18

(0.01–0.9)

2

(0.03–0.1) –

– 5

6 4

11–15

(0.002–0.120) 5

WAI

29

(0.002–108)

26

(0.002–1.0)

3

(0.04–0.12)

9

(0.002–1.1)

15

(0.04–1.5)

7

(0.006–0.3) –

– –

8 2

9 10–19

(0.002–0.16) 9

WGI

16

(0.01–1.5)

24

(0.01–0.08) –

30

(0.01–2.0)

4

(0.05–2.0)

16

(0.02–0.45)

3

(0.03–.010) –

– 3

4 7

7–14

(0.005–0.11) 3

WHE

55

(0.02–1.4)

30

(0.05–0.1) –

2

(0.005–0.08)

3

(0.04–0.3) –

– –

– 5

3 2

8–10

(0.005–0.05) 3

WQ

I 28

(0.03–2.0)

25

(0.002–0.08) –

6

(0.002–0.04)

2

(0.05–0.15)

20

(0.01–0.120) 3

– –

8 8

10 16–26

(0.002–0.15) 3

307

308

As expected, the evaluations of visual total porosity using thin sections demonstrate a direct

increase as the weathering grade increases. The total porosity increment noted in evaluated thin

sections varies from 1.5 to 3 times from fresh to completely weathered BIF.

There is no change in the pore mean size between fresh to completely weathered lithotypes. The

minimum pore size varies from 0.001 mm to 0.18 mm for fresh lithotypes and from 0.002 mm to

0.16 mm for completely weathered lithotypes. This behaviour could be explained by the increase of

goethite and ochreous goethite as cement for the pores in highly weathered materials.

7.5.3 Heterogeneity and anisotropy for BIF weathering profiles, a micro overview

As the weathering operates in different ways in each itabirite compositional group, different

anisotropic behaviours (transverse isotropy) can be expected in each type of weathering profile and

weathering horizon.

It is expected for heterogeneity promoted by typical itabirite compositional metamorphic banding

to be responsible for a transversal isotropy. Analyses of these thin sections indicated three main

characteristics of pervasive fabric responsible for inducing penetrative planes that could represent

heterogeneity and result in rock anisotropy to intact rock strengths and/or to elastic parameter at

different weathering horizons, namely:

Contacts between layers of iron and non-iron bands (heterogeneity)

It is representing a potential weakness surfaces as they exhibit a physical contrast or boundaries

between different mineralogical composition bands. A simple method to consider the heterogeneity

is the use of the Mohr’s hardness scale, which is a quantitative ordinal scale characterising scratch

resistance, to define this heterogeneity. Using this approach, greater hardness differences between

these different mineral bands will provide weaker contact surfaces prone to inducing anisotropy

effects.

Considering the Mohr’s hardness for the main BIF constituent minerals (hematite is 5 to 6, quartz is

7 and iron dolomite is 3.5 to 4), a lower rock strength is expected from quartz based bands as shown

in Figure 7.21A and higher rock strength contrast for iron dolomite bands as shown in Figure7.21B.

Therefore, a higher anisotropy effect is expected for FDI than for FAI and FQI.

Considering completely weathered types, Figure7.21C, the WAI shows contact between bands of

clay minerals (gibbsite and kaolinite) and hematite presenting higher hardness contrast.

309

(A) (B) (C) 104 Figure 7.21 A (left), FQI microphotograph showing the heterogeneity with crystals of

hematite (light yellow) and crystals of quartz (light grey) (Horta & Costa 2016).

B (centre), FDI microphotograph showing ferroan-dolomite crystals and levels of

lepidoblastic hematite and ferroan-dolomite levels (Horta & Costa 2016).

C (right), WAI showing layers of weaker clay minerals (light brown) interlayered

with tabular hematite (light grey) (Horta & Costa 2016)

Mineral orientation

An important BIF mineral orientation is associated with microplates of hematite, and with iron

dolomite (Figure 7.22A) and granular quartz (Figure 7.22B) orientations often seen on these

lithotypes. However, due to the tectonic setting (low strain) and low metamorphic grade, most

constituent minerals have a low to moderate elongation ratio, except for specularite and talc

(accessory minerals) that have a high elongation ratio.

(A) (B) 105 Figure 7.22 A (left), FDI microphotograph showing typical banding of granoblastic quartz

(light blue) and band of tabular hematite and iron dolomite (dark blue) (Horta &

Costa 2016). B (right), FQI quartz band (light blue) with granuloblastic orientation

and some tabular hematite interlayered, and band of hematite (light grey) with

elongated hematite crystals (Horta & Costa 2016)

1,000μm

1,000μm 1,000μm

1,000μm

310

Porous orientation

Similarly, to mineral orientation, pores orientation defines an anisotropic behaviour specifically for

moderately to completely weathered types. For fresh BIF with very low total porosity this

characteristic can be less effective to induce anisotropy. However, it could be important for PWQI

as shown in Figure 7.23A, with pores concentrated in quartz hematite bands.

For moderately to highly weathered types the anisotropy and rock strength is defined by the pore’s

orientation and high percentage as shown in Figure 7.23B. However, persistent porous levels could

be filled by iron oxides and hydroxides as well as gibbsite and kaolinite, generating harder filling

materials responsible for reducing the rock strength and anisotropy.

(A) (B)

106 Figure 7.23 A (left), PWQI showing hematite quartz bands with high pores content (black)

oriented according to the banding. (Horta & Costa 2016). B (right), WHE

microphotograph illustrating the pores orientation along the metamorphic

banding (highlighted by red dotted lines) oblique to a brecciaed layer (Horta &

Costa 2016)

For all weathering horizons, mineral orientation from very elongated minerals (e.g. specularite) and

secondary porosity generated by fracturing or in brecciated types are considered as discrete

discontinuities and for intact rock evaluation, in this research they were not considered.

The heterogeneity and anisotropy defined for BIF can induce different features that are summarised

and grouped in:

• Hard hematitite: the mineralogical heterogeneity (banding) is not present and total

porosity is very low (Øb<5%), hence anisotropic effects are not effective. The pervasive

1000μm

A

311

anisotropic effect is attributed to the presence of mineral orientation (tabular hematite or

specularite) and pores concentration defined as the main heterogeneity characteristic of

this group.

• Fresh itabirites: the heterogeneity and anisotropy are defined mainly by bands with

different mineral composition (hardness and crystal size), and secondarily by pores

orientation according tectonical banding.

• Moderately weathered itabirites: due to the weathering intensification and partial iron

alteration (oxidation and hydration), increasing the percentage of goethite, ochreous

goethite, kaolinite, and gibbsite reducing the original rock strength and increasing the

anisotropy effects. Weathering also induces an increase of the total porosity from the

partial leaching of prone minerals (iron dolomite and quartz) and a loss of apparent

cohesion from the erosion of quartz crystal contacts, as proposed by Ribeiro (2003).

• Completely weathered itabirites: total loss of apparent cohesion, due to the non-iron

mineral dissolution associated with total porosity increase, is the main factor responsible

for the intact rock strength reduction and anisotropic effects increase noted in these types.

At that stage, mineral orientation seems to have higher importance in defining anisotropy.

For example, the WAI heterogeneity is defined by layers of clay minerals and hematite

demonstrates the highest hardness contrast due to the high content of kaolinite/gibbsite

and the presence of tabular hematite or specularite (Figure 7.24A).

The iron reconcentration, filling the pores (cement) as hematite (hard filling), or goethite

and ochreous goethite (weak filling) can partially or totally modify the anisotropy and the

rock strength as shown in Figure 7.24B. The mineral orientation also plays an important

role in these types.

• Weak hematitite: the intense iron enrichment due to the supergenic iron remobilisation

results in the filling of a high percentage of pores, increasing bulk density and modifying

the original bimodal minerology bands (non-iron and iron). For this monomineralic

hematitic rock the heterogeneity is defined by bands of hematite with higher total porosity

interbanding with hematite bands with less total porosity. For this type, the heterogeneity

is defined by total porosity percentage since there are very low mineralogical variations

and, secondarily by the mineral orientation.

312

(A) (B)

107 Figure 7.24 A (left), WAI microphotograph showing tabular and specularite hematite (light

grey elongated crystals) and gibbsite (large dark crystal at centre) (Horta & Costa

2016); B (right), WGI microphotography illustrating crystal of granular hematite

and quartz, and tabular hematite totally covered by goethite filling (light brown)

(Horta & Costa 2016)

7.5.4 Evaluation of the BIF anisotropy throughout the complete weathering profile

The approach used in this section summarises and compares results from intact rock strength tests

(UCS, CIU and BRA), with petrophysical parameter (ρb), static elastic parameters (Estat and νstat) and

microscopy evaluation. Evaluations were grouped by lithotype (mineral composition), anisotropy

direction (βangles) and weathering horizon, in order to compare results from different groupings and

evaluate the anisotropy degree as defined by Singh et al. (1989).

Due to the different genesis of itabirites and hematitite and different level of weathering horizons

and level they were assessed separately.

Fresh to slightly itabirites (bedrock) evaluated by βangle

This group contains the test results from FAI, FQI and FDI lithotypes from bedrock horizon including

W1, W2 and W3 (for FAI) samples.

Tabular hematite

Granular hematite

Granular quartz

Pores filled by goethite

500 μm 1,000 μm

313

26 Table 7.3 Average value (mean) and standard deviation (SD) summary table for each fresh

itabirite (FQI, FDI and FAI), presented by βangle groups

Lithotype Anisotropy (β) Parameter Bulk density

(t/m3) Estat (GPa) νstat UCS

(MPa) UTS (MPa)

FAI

90°

Mean 3.18 95 0.196 162 10

SD 0.17 24 0.033 96 2

n 17 13 14 18 2

45°

Mean 3.20 82 0.165 138 9

SD 0.20 25 0.031 79 1

n 12 11 8 12 4

0°

Mean 3.32 101 0.227 160 24

SD 0.24 36 0.061 102 8

n 18 16 15 18 4

FDI

90°

Mean 3.19 139 0.156 174 10

SD 0.31 76 0.029 54 2

n 10 8 2 10 17

45°

Mean 3.65 76 0.213 122 7

SD 0.21 31 0.063 56 2

n 11 8 8 14 3

0°

Mean 3.49 97 0.271 85 10

SD 0.61 20 0.060 19 3

n 7 5 3 5 4

FQI

90°

Mean 3.34 70 0.232 162 15

SD 0.33 45 0.140 124 1

n 16 16 12 16 9

45°

Mean 3.45 77 0.220 158 16

SD 0.24 44 0.084 87 1

n 25 22 21 25 5

0°

Mean 3.33 81 0.222 143 14

SD 0.27 56 0.111 99 4

n 19 18 15 19 7

Mean: mean value; SD: standard deviation; n: number of tested samples

314

From Table 7.3 it is possible to note that:

• Bulk density results show similar means, with a higher mean value obtained for FDI β45° =

3.65 t/m3 ±0.21 t/m3 and a lower value for FAI β45° = 3.18 t/m3 ±0.17 t/m3, showing means

raging within the SD. Higher ρb values were obtained for FDI, showing also the highest

range between different Bangles.

• Static Young’s modulus means presented the higher mean value for FDI β90° = 139 GPa ±76

GPa and lower value for FQI β90° = 70 GPa ±45 GPa showing the mean of each lithotype is

within the SD range.

• Static Poisson’s ratio presents the higher mean value for FDI β0° = 0.271 ±0.0.06 and lower

value for FDI β90° = 0.156 ±0.029. FAI and FDI present higher means for β0° and FQI showed

similar means for all directions with a higher SD from 0.084 to 0.140.

• UCS tests exhibit the higher mean value for FDI β90° = 174 MPa ±54 MPa and lower value for

FDI β90° = 85 MPa ±19 MPa. The mean of each lithotype is within inside the SD range.

• FAI and FQI present the highest means with lower values for β45°. FDI presented lowest

mean at β0°.

• BRA (UTS) tests show the higher mean value for FAI β0° = 24 MPa ±8 MPa and lower value

for FDI β45° = 7 MPa ±2 MPa. FQI presented similar means for all directions and presented

the highest mean values. FAI and FDI presented lower values at β45°.

Hard hematitite (bedrock) evaluated by βangle

This group contains only HHE with weathering grade equal to W1 and W2. The same approach used

for itabirites was applied for HHE and results are presented in Table 7.4.

315

27 Table 7.4 Average values (mean) and SD summary table for HHE shown parameters results

by βangle

Lithotype Anisotropy (β) Parameter Bulk density (t/m3) Estat (GPa) stat UCS (MPa) UTS (MPa)

HHE

90°

Mean 4.94 78 0.290 70 21

SD 0.10 40 0.066 13 3

n 6 6 3 4 4

45°

Mean 4.94 37 0.307 69 19

SD 0.14 9 0.006 22 7

n 9 7 3 8 4

0°

Mean 5.07 78 0.281 181 21

SD 0.11 75 0.043 127 6

n 19 16 7 19 4



• Bulk density means present low SD and the means of each Bangle are within the SD.

• Static Young’s modulus exhibited the lowest value mean for β45° = 37 GPa ±9 GPa with

lower SD.

• Poisson’s ratio showed a higher mean at β45° with a mean value of 0.307 and an SD of

0.006.

• UCS tests presented the highest mean value of 181 MPa at β0° with a higher SD of

127 MPa.

• BRA testing revealed no significant mean variation.

Moderately weathered itabirites (saprorock) evaluated by βangle

This group contains the PWQI and PWGI including samples with weathering degree varying from W3

to W4. A similar approach was applied for moderately weathered lithotypes and the mean values

are presented in Table 7.5.

Reflecting the reduced number of tests for PWQI and PWGI and due to the similar results, these

lithotypes were evaluated as a single group.

316

28 Table 7.5 Summary table for PWQI/ PWGI presenting evaluated parameters separated by

βangle

Lithotype Anisotropy (β) Parameter Bulk density (t/m3) Estat (GPa) νstat UCS (MPa)

PWQI/ PWGI

90°

Mean 3.23 61 0.202 74

SD 0.02 19 0.055 15

n 4 5 5 6

45°

Mean 3.26 32 0.242 28

SD 0.17 15 0.035 10

n 3 2 2 2

0°

Mean 3.21 78 0.248 69

SD 0.10 28 0.097 30

n 5 4 4 6


Table 7.5 shows:

• Bulk density means are very close, ranging from β45° = 3.26 t/m3 ±0.17 t/m3 to

β0° = 3.21 t/m3 ±0.10 t/m3, the mean of each type is written the SD range.

• Static Young’s modulus shows mean values varying from β45° = 32 GPa ±15 GPa to

β0° = 78 GPa ±28 GPa.

• The Poisson’s ratio involves mean values varying from β90° = 0.202 ±0.055 to

β0° = 0.248 ±0.097.

• UCS tests mean varies from β45° = 28 MPa ±10 MPa to β90° = 74 MPa ±15 MPa.

Highly to completely weathered (saprolite) evaluated by βangle

This group contain the WAI, WHE, WGI and WQI with weathering grade varying from W5 to W6.

Due to the impossibility of conducting UCS or UTS tests on these materials, CIU test results were

used. These tests were in part obtained from previous reports available on Vale’s database and in

part from additional tests carried out for this research.

Based on the σ1 and σ3 test results from CIU tests, the linear Mohr–Coulomb regression (best fit)

lines in this stress space was determined for each material type using RocData 5.0 (Rocscience

2021). With this approach, it was possible to determine the adjusted rock strength from adjusted

317

fitting curves, as shown in Figures 7.25, 7.26, 7.27 and 7.28. This figure shows the major versus

minor principal stress for each type separated by βangle, unless for the WGI at β45°, as a result of the

lack of samples tested in this direction.

To obtain the intact rock strength equally settled for all types evaluated, the GSI was assumed to be

equal to 100 and a ’D factor’ equal to zero was applied for all evaluations.

108 Figure 7.25 RocData 5.0 (Rocscience 2021) linear regression (best fit) lines in σ1 σ3 stress

space for WQI showing fitted strength parameters for each available βangle.

A – top left at (90°), B – top right at (45°) and C – below (0°)

318

109 Figure 7.26 RocData 5.0 (Rocscience 2021) linear regression (best fit) lines in σ1 σ3 stress

space for WHE showing fitted strength parameters for each available βangle.

A – top left (90°), B – top right (45°) and C – below (0°)

319

u

110 Figure 7.27 RocData 5.0 (Rocscience 2021) linear regression (best fit) lines in σ1σ3 stress

space for WAI showing fitted strength parameters for each available βangle.

A – top left (90°), B – top right (45°) and C – below (0°)

320

111Figure 7.28 RocData 5.0 (Rocscience 2021) linear regression (best fit) lines in σ1σ3 stress

space for WAI, WGI, WHE and WQI showing fitted strength parameters for each

available βangle. A – left (90°), B – right (0°)

Table 7.6 presents the RocData 5.0 (Rocscience 2021) output graphs with plotted used data and the

adjusted curve and summary table with the adjusted parameters obtained for each type in different

anisotropy directions.

29 Table 7.6 Summary table for highly to completely weathered types (WHE, WQI, WGI and

WAI) for each βangle

Lithotype WHE WQI WAI WGI

Anisotropy β (°) 0 45 90 0 45 90 0 45 90 0 90

Cohesion (kPa) 111 85 115 63 49 85 61 54 97 140 154

Friction angle (°) 30 38 33 34 29 29 35 30 38 37 36

UCS (MPa) 0.384 0.353 0.420 0.237 0.165 0.287 0.236 0.190 0.397 0.560 0.612

Samples number (n) 14 7 11 15 4 9 8 4 8 4 4

321

Table 7.7 shows that:

• Cohesion is higher for WGI (β0° = 140 and β45° = 154 kPa) and lower for WQI

(β45° = 49 kPa to β90° = 85 kPa). In general, the lowest results were at β45° and highest at β90°.

Additionally, WQI and WAI presented closer lower mean values, and WHE and WGI

presented closer higher mean values.

• Friction angles exhibit very close values ranging from 29° to 38° and it is not possible to

determine any βangle influence. The lowest relative values were obtained for WQI.

• UCS presents the higher values for WGI (β0° = 560 kPa to β90° = 612 kPa) and WHE

(β90° = 420 kPa to β0° = 384 kPa). The higher values were at β90° and the lower at β45° for all

types.

• WGI has the highest UCS values among the weathered group.

Using the UCS results obtained from Tables 7.3, 7.5, 7.6 and 7.7 it is possible to determine the

anisotropy ratio using Singh et al. (1989) for all BIF weathering profile as shown in Table 7.8.

30 Table 7.8 Anisotropy ratio (Rc) summary table for each evaluated lithotype

Lithotype UCS (MPa) β0° UCS (MPa) β45° UCS (MPa) β90° Rc Class

HHE 181 69 70 1.0 Isotropic

FQI 143 158 162 1.1 Isotropic/low

FAI 160 138 162 1.2 Low

FDI 85 122 174 2.0 Low/medium

PWQI/PWGI 69 28 74 2.6 Medium

WAI 0.236 0.190 0.397 2.1 Medium

WHE 0.384 0.353 0.420 1.2 Low

WQI 0.237 0.165 0.287 1.7 Low

WGI 0.560 – 0.612 1.1 Isotropic/low

From Table 7.8 it is possible to observe that:

• Highest UCS values for fresh types ranging from 85 MPa to 181 MPa; moderated

weathered ranging from 28 MPa to 74 MPa; and totally weathered types ranging from

190 kPa to 560 kPa.

322

• The lowest UCS values are concentrated in β45° unless for FQI and FDI where in β0°.

• It is possible to group the anisotropic ratio (Rc) results in two groups (by mineralogical

composition and weathering grade): Isotropic to low Rc (HHE, WHE, FQI, WQI and WGI)

and low to medium Rc (FDI, PWQI/PWGI and WAI).

7.5.5 BIF isotropic approach throughout the complete weathering profile

The anisotropy evaluation in previous sections reveals that, in general, BIF behave as isotropic to

low anisotropy materials; and a second group being FDI and PWQI/ PWGI defined as low to medium

anisotropy ratio. Based on this result, in this section, the BIF were evaluated as isotropic materials

and grouped only by lithotype mineral composition.

RocData 5.0 (Rocscience 2021) was used to support the data manipulation necessary for a general

overview of intact rock strengths and elastic parameters for highly to completely weathered

variants.

Used tables present for each type the mean, SD, covariance (COV), margin of error, maximum values

(Max), minimum values (Min), values of the first and third quartiles (1Qt and 3Qt), lower and upper

inner fences, and number of tested samples (n).

Fresh to slightly weathered itabirites (bedrock) as isotropic material

For this evaluation itabirites were grouped by mineral composition. Table 7.9 shows basic statistical

results for fresh to slightly weathered itabirites (FQI, FDI and FAI).

323

31 Table 7.9 Basic statistical summary table considering total results for each fresh to slightly

weathered lithotype considering as an isotropic material

Lithotype Parameter Bulk density (t/m3) E stat (GPa) stat UCS (MPa) UTS (MPa)

FAI

Mean 3.26 104 0.20 155 9

SD 0.25 56 0.05 93 1

COV 8% 54% 25% 60% 16% Max 4.13 375 0.35 360 11 Min 2.86 40 0.11 33 8

Margin of error 2% 17% 9% 18% 17% 1Qt 3.11 77 0.17 59 8

3Qt 3.32 114 0.23 220 11

Lower inner fence 2.79 21 0.09 0 5

Upper inner fence 3.63 170 0.31 461 12

n 48 42 37 48 6

FDI

Mean 3.42 117 0.22 135 10

SD 0.41 81 0.07 64 2

COV 12% 69% 30% 48% 22% Max 4.66 383 0.34 262 13

Min 2.75 32 0.12 40 5

Margin of error 4% 31% 18% 18% 9% 1Qt 3.10 74 0.17 82 8

3Qt 3.59 118 0.24 169 11



n 31 22 13 31 25

FQI

Mean 3.38 74 0.20 149 15

SD 0.28 48 0.05 99 2

COV 8% 65% 24% 66% 17% Max 4.04 220 0.31 350 18

Min 2.88 4 0.07 7 9

Margin of error 2% 17% 7% 17% 7%

1Qt 3.16 21 0.17 55 13

3Qt 3.60 113 0.23 221 16



n 60 56 44 60 22

SD: standard deviation; COV: covariance; Max: highest value obtained; Min: lowest value obtained; 1Qt: first quartile; 3Qt: third quartile,

n: number of tests.

324

From Table 7.9 it is observed that:

• Bulk density results show low COV between 8% and 12%, with a very low margin of error

of 3%. The higher value was obtained for FDI = 3.24 ±0.41 t/m3 and lower for FAI

= 3.26 ±0.25.

• Static Young’s modulus results showed a high COV from 54% to 69%, with a moderate to

high margin of error between 17% and 31%. The lowest mean value was obtained for FQI

= 74 ±48 GPa and the highest for the FDI = 117 ±81 GPa.

• Poisson’s ratio values vary from FAI and FQI of 0.20 ±0.05 and to 0.22 ±0.07, with a high

COV between 24% and 30% and a low to moderate margin of error varying from 7% to

18%. The highest value occurred for the FDI = 0.22.

• UCS test mean values vary from FDI = 135 ±64 MPa to FAI = 155 ±93 MPa, with a high COV

from 48% to 66% for a moderate margin of error of 18%.

• UTS results range from FAI = 9 ±1 MPa to 15 ±2 MPa with a moderate COV of 16% to 22%

and a low to moderate margin of error varying from 7% to 17%. The higher mean is for FQI.

• For all the evaluated parameters, the means showed no significant variation and are within

the SD.

Hard hematitite (bedrock) evaluated as an isotropic material

The same approach was used for HHE considering all validated tests. Table 7.10 shows basic

statistical results.

32 Table 7.10 Basic statistical summary table for HHE

Lithotype Parameter Bulk density (t/m3) E stat (GPa) νstat UCS (MPa) UTS (MPa)

HHE

Mean 5.01 51 0.28 96 20 SD 0.14 33 0.04 34 5

COV 3% 65% 16% 35% 25% Max 5.21 120 0.35 178 27 Min 4.72 6 0.22 55 9

Margin of error 1% 25% 9% 15% 7% 1Qt 4.91 31 0.24 70 18 3Qt 5.09 72 0.31 114 24

Lower inner fence 4.64 0 0.14 5 9 Upper inner fence 5.36 134 0.42 179 27

n 35 28 14 23 12


n: number of tests.

325


• Bulk density results show a mean of 5.01 (±0.14) t/m3, with low COV = 3% and very low

margin of error equal to 1%.

• Static Young’s modulus exhibits a mean of 51 (±33) GPa, with very high COV = 65% and

moderate margin of error equal to 25%.

• Poisson’s ratio has a mean of 0.280 (±0.040), with moderate COV = 16% and a low margin

of error equal to 9%.

• UCS results have an average of 96 (±5) MPa, with high COV = 35% and low to moderate


• UTS results have an average of 20 (±5) MPa, with high COV = 25% and low margin of error

equal to 1%.

Moderately weathered itabirites (saprorock) as an isotropic material

A similar approach was applied for the moderately weathered lithotypes. Table 7.11 shows basic

statistical results, which were grouped together due to their similarity and the limited number of

results for PWQI and PWGI.

Difficulties were experienced in sampling preparation and laboratory tests executed for Brazilian

tests. Due to the low weathering grade, these tests did not reach the sufficient number and were

not evaluated.

33 Table 7.11 Basic statistical summary table for partially weathered lithotypes (PWQI and PWGI)

Lithotype Parameter Bulk density (t/m3) E stat (GPa) ν stat UCS (MPa)

PWQI/PWGI

Mean 3.21 62 0.23 68

SD 0.07 26 0.07 23

COV 2% 43% 30% 34% Max 3.31 103 0.37 107 Min 3.08 21 0.13 35

Margin of error 2% 29% 20% 20% 1Qt 3.16 42 0.18 52 3Qt 3.26 76 0.27 80

Lower inner fence 3.01 0 0.05 11

Upper inner fence 3.41 128 0.41 121

n 11 11 11 14


n: number of tests.

326


• Bulk density results showed mean of 3.21 ±0.07 t/m3, with low COV = 2% and very low


• Static Young’s modulus is lower than the other itabirite groups at 62 ±26 GPa, with very

high COV = 43% and moderate to high margin of error equal to 29%.

• Poisson’s ratio is similar to the other itabirite groups at 0.23 ±0.07, with high COV = 30%

and high margin of error equal to 20%.

• UCS results showed mean of 68 ±23 MPa, with high COV = 34% and moderate margin of

error equal to 20%.

Highly to completely weathered itabirites (saprolite) as an isotropic material

For this evaluation, CIU test results were used, based on the tests undertaken for this research and

from the available Vale database.

Based on the σ1 and σ3 test results from CIU tests the linear Mohr–Coulomb regression (best fit)

lines in this stress space was determined for each material type using RocData 5.0 (Rocscience

2021). With this approach, it was possible to determine the adjusted rock strength from adjusted

fitting curves, as shown in Figure 7.29 (A, B, C and D). This figure shows the major versus minor

principal stress for each type (WAI, WQI, WGI and WHE) assuming isotropic materials.

To obtain the intact rock strength equally settled for all types evaluated, the GSI was assumed to be

equal to 100 and a ’D factor’ equal to zero was applied for all evaluations.

327

112 Figure 7.29 Mohr–Coulomb linear regression (best fit) lines in σ1σ3 stress space for WAI

(A – top left), WQI (B – top right), WHE (C – bottom left) and WGI (D – bottom

right), showing MC parameters obtained by the adjusted fitting curve

328

Table 7.12 presents the RocData 5.0 (Rocscience 2021) output summary table with the adjusted

parameters obtained for each type.

34 Table 7.12 Summary table for completely weathered lithotypes

Lithotype WHE WQI WAI WGI

Cohesion (kPa) 140 126 101 127

Friction angle (°) 32 30 30 28

UCS (MPa) 0.502 0.441 0.352 0.525

Samples number (n) 29 19 19 6


• Cohesion presents the highest value for WHE = 140 kPa and lowest for WAI = 101 kPa. WGI

and WQI presented similar cohesion value.

• Friction angles have very similar mean values, varying from 30° to 32° for WHE, WQI and

WAI and WGI presented the higher value equal to 38°.

• UCS presents the highest value for WGI = 0.525 kPa followed by WHE = 0.502 kPa and WQI

= 0.441 kPa, and lowest for WAI = 0.352 kPa.

7.5.6 Strength envelope of best fit curve for completely weathered BIF profile

As outlined in previous sections, there are no significant changes on intact rock strength parameters

induced by the anisotropic effects, except for FDI, PWQI and PWGI, which exhibit low to medium

anisotropic ratio. This suggests that, in general, BIF could be treated as isotropic materials.

Based on these assumptions, this section uses the most common isotropic stress failure criteria

(Mohr–Coulomb and Hoek and Brown) to define the linear regression (best fit) lines in σ1σ3 stress

space curves using RocData 5.0 (Rocscience 2021) to establish intact rock strengths parameters

properties by comparing linear and non-linear strength envelopes for all evaluated lithotypes and

weathering grade zones, and to determine a complete characterisation of intact rock strengths for

the complete weathering profile. The results are presented separately by lithology (mineral

composition) and then sub-divided by weathering grade zones.

329

To obtain the strength curves, UCS, BRA, HB triaxial cell and CIU tests were used for bedrock. UCS,

HB triaxial cell and CIU tests were used for saprorock. While saprolite only CIU tests were analysed

to determinate the UCS value from the adjusted fitting curves making it possible to compare all BIF

types under the same numerical basis (intact rock strength value). The resulting graphs show major

stress with minor stress ranges for each type, and the tables present a summary of the shear

strength and intact rock strength.

To evaluate intact rock strength characteristics and establish the same basis for comparison, the

same input criteria were applied in the software, namely: the use of the Simplex adjusted curve for

the best fit, except for HHE and PWQI for which Levenberg-Marquardt best fit adjust curve was

used; GSI was assumed to be 100 and the ‘D’ factor was set at equal to 0 (zero).

To be able to evaluate all weathering profile horizons, for each material, σ1σ3 stress space graphs

plotted the curve fit results for each weathering grade zone. To evaluate the difference between

the strength models Generalised Hoek–Brown (HB) and Mohr–Coulomb (MC) fitted adjusted curves

were plotted. Additionally, a complete table of fit strength and elastic parameters values based on

HB and MC strength envelopes best fit and test results to the dataset is presented.

This table presents the UCS of the intact rock (σci), the material constant for the intact rock (mi), the

minor confining principal stress (σ3max), the HB fitted cohesion (cfit), the HB fitted friction angle (ɸfit),

the rock mass tensile strength (σt), the global rock mass compressive strength (σcm), the rock mass

modulus of deformation (Em), cohesion (c), friction angle (ɸ), the tensile strength (UTS), UCS, and

the number of used tests (n).

Amphibolitic itabirite completely weathering profile (continues profile)

Considering the complete weathering profile for amphibolitic itabirites, the FAI is considered the

bedrock unweathered and hard type, PWGI is the saprorock – moderately weathered grade and

WGI the saprolite – completely weathered grade.

Graph 7.1A shows the HB envelope for FAI and PWGI, represented by the squares and continuous

curve, and the triangles and dashed curve, respectively. Graph 7.1B shows WGI – HB envelope

represented by dots and the continuous curve. Graph 7.1C presents the MC linear regression (best

fit) for FAI and PWGI, represented by the squares and continuous curve, and the triangles and

dashed curve, respectively. Graph 7.1D shows the MC envelope for the WGI – represented by dots

and a continuous curve.

330

(A) (B)

(C) (D)

(C) (D)

0-1 Graph 7.1 A (top left) and C (bottom left), regression strength (best fit) curves in σ1σ3 stress

space for amphibolitic itabirite for HB adjusted best fit curve for FAI, PWGI and

WGI. B (top right) and D (bottom right), strength best fit curve for amphibolitic

itabirite for MC adjusted linear regression curve for FAI, PWGI and WGI

Table 7.13 summarises the fitted parameters resulted from the adjusted curve for Mohr–Coulomb

(MC) and Hoek & Brown (HB) failure criteria for amphibolitic itabirites typical weathering profile.

35 Table 7.13 Summary table for amphibolitic lithotype

Criteria Hoek & Brown Mohr–Coulomb

Parameter/Lithotype

σci

(MPa) mi σ3max

(MPa) cfit

(MPa) ɸfit (°) σt

(MPa) σcm

(MPa) Em

(GPa)

C

(MPa)

ɸ

(°) UTS

(MPa) UCS

(MPa) n

FAI 184 16 2 27 59 -12 175 7.6 24 58 -11 171 11

PWGI 100 17 10 15 54 -6 96 5.6 16 52 nr 96 11

WGI 0.576 11 0.400 0.125 37 -0.052 0.535 0.4 0.127 38 nr 0.525 6

UTS: tensile strength; UCS: uniaxial compressive strength; σci: UCS of the intact rock; mi: material constant for the intact rock; σ3max: minor

confining principal stress; cfit: HB fitted cohesion; ɸfit: HB fitted friction angle; σt: rock mass tensile strength; σcm: global rock mass compressive

strength; Em: rock mass modulus of deformation, c: cohesion; ɸ: friction angle; n: number of used tests.

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By evaluating Graph 7.1 (A, B, C and D) and Table 7.13 it is possible to determine that:

• There is a considerable strength reduction from FAI through PWGI and a severe strength

fall to WGI.

• Comparing MC with HB fitted results there are no considerable differences.

• It is noted a not common higher σ3max value obtained for the PWQI (higher than the FAI) is

attributed to the set of laboratory tests used in this evaluation.

Dolomitic itabirite completely weathering profile (discontinuous profile)

Considering the weathering profile for, the FDI is considering the unweathered and hard itabirite,

WAI the completely weathered grade and the absence of moderately weathered type for this

weathering profile is responsible for the discontinuous terminology.

For the HB regression (best fit) curves in σ1σ3 stress space, Graph 7.2A shows for FDI, represented

by squares and the related continuous curve, Graph 7.2B shows WAI, represented by dots and the

associated continuous curve. The MC linear regression (best fit) lines in σ1σ3 stress space are shown

in Graph 7.2C for FDI, as squares and the continuous curve, and Graph 7.2D for WAI, represented

by dots and associated curve.

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(A) (B)

(C) (D)

0-2 Graph 7.2 A (top left) and B (top right), Hoek–Brown linear regression (best fit) lines in σ1σ3

stress space for FDI and WAI, respectively. C (bottom left) and D (bottom right),

Mohr–Coulomb adjusted for FDI and WAI, respectively

Table 7.14 summarises the HB and MC parameters obtained from the RocData 5.0 (Rocscience 2021)

adjusted curve fitting.

36 Table 7.14 Summary table for dolomitic itabirite completely weathering profile


Parameter/

lithotype

σci

(MPa) mi

σ3max

(MPa)

cfit

(MPa)

ɸfit

(°)

σt

(MPa)

σcm

(MPa)

Em

(GPa)

c

(MPa)

ɸ

(°)

UTS

(MPa)

UCS

(MPa) n

FDI 131 12 10 22 50 -11 122 6.4 18 56 -11 119 13

WAI 0.343 8 0.378 0.107 30 -0.043 0.317 0.3 0.128 28 nr 0.427 20




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For Graph 7.2A, B, C and D and Table 7.14 it is evident that:

• There is a strong decrease in all parameters from FDI to WAI, i.e. as a result of the

weathering.

• Comparing MC with HB fitted results there are no considerable differences.

Quartzitic itabirite completely weathering profile (continues profile)

Similar to amphibolitic itabirite, considering the complete weathering horizons for quartzitic

itabirites, the FQI is considered the bedrock, PWQI is the saprorock and WQI the saprolite.

Graph 7.3A shows the HB regression (best fit) lines in σ1σ3 stress space for FQI and PWQI, as

represented by squares and the continuous curve, and triangles and the dashed curve, respectively,

while Graph 7.3B represents the WQI adjusted HB linear regression curves and plotted dataset.

The MC regression (best fit) lines in σ1σ3 stress space for FQI and PWQI are shown in Graph 7.3C in

the form of the squares and continuous curve, and the triangles and the dashed curve, respectively.

Graph 7.3D shows the linear regression for WQI.

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(A) (B)

(C) (D)

0-3 Graph 7.3 A (top left) and B (top right), represent HB a regression (best fit) curves in σ1σ3

stress space for FQI and PWQI and WQI; C (bottom left) and D (bottom right),

show MC adjusted for FQI, PWQI and WAI

Table 7.15 Summarises the HB and MC parameters obtained from the RocData 5.0 (Rocscience

2021) software using adjusted curve fitting.

37 Table 7.15 Summary table for quartzitic itabirite lithotypes


Parameter/ Lithotype

σci

(MPa) mi σ3max

(MPa) cfit

(MPa) ɸfit

(°) σt

(MPa) σcm

(MPa) Em

(GPa) c

(MPa) ɸ (°)

UTS (MPa)

UCS (MPa) n

FQI 270 17 10 38 58 -16 260 9.2 35 60 -15 256 14

PWQI 83 9 5 15 48 -9 76 5.1 31 22 nr 90 6

WQI 0.381 10 0.786 0.164 28 -0.037 0.353 0.3 0.103 31 nr 0.368 21




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Graph 7.3A, B, C and D and Table 7.15 clearly indicate that:

• There is a considerable strength reduction from FQI through PWQI and a severe strength

fall to WQI.

• Comparing MC with HB fitted results there showed considerable differences for PWQI

possible induced by the low number of tests. It is also noted by the unexpected lower

relative values obtained for this type.

• The cohesion and friction angle resulted for FQI is induced by the dataset composed mainly

by low σ3max CIU tests inducing higher friction angles and lower cohesion for the adjusted

curves.

Hematitite behaviour over weathering profile

The strength change noted in the hematitite cannot be directly correlated to the weathering process

as presented in previous sections. For hematitite evaluation were considered the HHE, the hard

variant and WHE the weak. The moderate strength type was not considered as they are generally

absent or very thin levels.

Graph 7.4A presents HB regression (best fit) curves in σ1σ3 stress space for HHE, while Graph 7.4B

shows the fitting curve HB bet fit curves for WHE. The MC linear regression (best fit) lines in σ1σ3

stress space for the HHE and WHE are shown in Graph 7.4C and 7.4D, respectively.

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(A) (B)

(C) (D)

0-4 Graph 7.4 A (top left) and B (top right), show the HB regression (best fit) curves in σ1σ3 stress

space for HHE and WHE; C (bottom left) and D (bottom right), are the MC adjusted

linear regression curves for HHE and WHE


adjusted curve fitting for the two material types.

38 Table 7.16 Summary table for hematitite lithotypes


Parameter/ Lithotype

σci

(MPa) mi σ3max

(MPa) cfit

(MPa) ɸfit

(°) σt

(MPa) σcm

(MPa) Em

(GPa) c

(MPa) ɸ (°)

UTS (MPa)

UCS (MPa) n

HHE 460 22 5 57 63 -21 456 12.1 50 65 -21 445 5

WHE 0.481 10 0.789 0.182 29 -0.05 0.445 0.4 0.123 33 nr 0.454 30




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From Graph 7.4A, B, C and D and Table 7.16 it is evident that:

• There is a considerable high-strength reduction from HHE through the weathering

profile.

• Comparing MC with HB fitted results there showed no considerable differences.

Complete BIF weathering profile

By grouping all compositional itabirites and hematitites for the specific weathering horizons in the

same graph it is possible to evaluate the linear regression curve for the full continuum of the

weathering profiles. Such groupings resulted in two graphs using the respective most appropriate

linear regression, where for rock-like material (FAI, FDI, FQI, HHE, PWQI and PWGI), using HB

adjusted curve was applied, and for soil-like materials (WAI, WQI, WGI and WHE) the MC adjusted

curve was applied.

Graph 7.5A shows the HB regression (best fit) curves in σ1σ3 stress space for fresh (bedrock)

lithotypes (HHE, FAI, FDI and FQI), represented by coloured continuous curves, and for (saprorock)

moderately weathered variants (PWQI and PWGI) as coloured dashed curves. Graph 7.5B shows the

MC linear regression lines for (saprolite) completely weathered materials (WAI, WQI, WGI and

WHE), where each coloured dotted curve represents a different type.

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(A)

(B)

0-5 Graph 7.5 A (top) A regression (best fit) curves in σ1σ3 stress space for the rock-like BIF with

HB adjusted regression curve for HHE, FAI, FDI, FQI, PWQI, and PWGI. Presenting

the three different grouping from σ3max, ranging from: (1) Saprolite – completely

weathered; (2) Saprorock – moderately weathered, (3) Bedrock – fresh BIF;

B (bottom) Linear regression for soil-like BIF with MC adjusted linear regression

curve for Saprolite to residual soil WHE, WAI, WGI and WQI


adjusted curve fitting for all evaluated BIF.

0.1 0.1

❶ ❷ ❸

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39Table 7.17 Summary table for all evaluated BIF


Parameter

/Lithotype

σci

(MPa) mi

σ3max

(MPa)

cfit

(MPa)

ɸfit

(°)

σt

(MPa)

σcm

(MPa)

Em

(GPa)

c

(MPa)

ɸ

(°)

UTS

(MPa)

UCS

(MPa) n

HHE 460 22 5 57 63 -21 456 12.1 50 65 -21 445 5

FQI 270 17 10 38 58 -16 260 9.2 35 60 -15 256 14

FAI 184 16 2 27 59 -12 175 7.6 24 58 -11 171 11

FDI 131 12 10 22 50 -11 122 6.4 18 56 -11 119 13

PWGI 100 17 10 15 54 -6 96 5.6 16 52 nr 96 11

PWQI 83 9 5 15 48 -9 76 5.1 31 22 nr 90 6

WGI 0.576 11 0.400 0.125 37 -0.052 0.535 0.4 0.127 38 nr 0.525 6

WQI 0.381 10 0.786 0.164 28 -0.037 0.353 0.3 0.103 31 nr 0.368 21

WHE 0.481 10 0.789 0.182 29 -0.050 0.445 0.4 0.123 33 nr 0.454 30

WAI 0.343 8 0.378 0.107 30 -0.043 0.317 0.3 0.128 28 nr 0.427 20



strength; Em: rock mass modulus of deformation, c: cohesion; ɸ: friction angle; n: number of used tests

From Graph 7.5 and Table 7.17 it is possible to determine that:

• Considering the fresh types there is a strong decrease in the strength from HHE, to FQI, to

FAI, and to FDI and partially weathered type PWQI and PWGI presented similar strength

curves although PWGI presents lower tension values compared with the PWQI.

• The lower relative strength observed for FDI can also be associated with the mineral

composition of iron dolomite minerals, mentioned in previous sections.

• A strength reduction is also observed for soil-like materials decreasing from WGI, to WHE,

to WQI and finally with lower-strength values for the WAI.

• The WGI presented a regression curve slightly different from other completely weathered

material. The same is noted for PWGI. This is attributed to the strength gain induced by

iron oxide and hydroxide cementation.

In Table 7.17 it is possible to separate strength and elastic parameters from different weathering

horizon, based on the σci values range. Fresh BIF show σci varying from 460 MPa (HHE) to

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131 MPa (FDI). Moderately weathered BIF show σci varying from 100 MPa (PWGI) to 80 MPa (PWQI).

Completely weathered BIF show σci varying from 0.576 MPa (WGI) to 0.343 MPa (WAI).

Based on Graph 7.5, three groups separated by the σ3max were determined:

• BIF saprolite horizon: Completely weathered BIF, from 0.789 MPa for the WGI to 0.378 MPa

for WAI with a σ3max ≤ 1 MPa;

• BIF saprorock horizon: Moderately weathered BIF, from 10 MPa for the PWGI to 5 MPa for

PWQI with a σ3max raging 1 MPa < σ3max ≤ 5 MPa;

• BIF bedrock horizon: Fresh BIF, from 10 MPa for the FQI to 2 MPa for FAI σ3max > 5 MPa.

7.6 DISCUSSION

Anisotropy and heterogeneity from BIF weathering profiles

The itabirite compositional metamorphic banding is defined by the alteration of iron and non-iron

bands forms the itabirite heterogeneity, which is promoted by its bimodal mineral composition,

mineral hardness, bulk density, fabric, and total porosity, and are controlled by the associated

degree of weathering and iron content. The hematitite metamorphic banding is defined by the

alternation of different iron minerals, total porosity, and fabric, defining a hematitite heterogeneity.

Contact surfaces between heterogeneity of itabirites (iron and non-iron bands) and hematitites

(higher total porosity and lower total porosity bands) define porous layers and mineral orientation.

These weaker surfaces are potentially prone to driving the weathering and to generating individual

discontinuity planes (e.g. ubiquitous joints), or to inducing an increase in the anisotropy ratio for

more weathered BIF. Depending on the weathering horizon and zone, these possible contact planes

generally represent single discontinuities not penetrative for fresh to slightly weathered BIF. They

can also generate penetrative and persistent surfaces that will induce medium anisotropy ratios in

FDI, PWQI and PWGI.

Given the several uncertainties about their geneses, it is possible to assert that hematitite and

itabirite groups present significantly different behaviours, even with similar intact rock strength. It

can be argued that the itabirite group behaviour is primarily induced by mineral composition

(heterogeneity) and secondarily by the total porosity. For this rock the intact strength, and

heterogeneity are highly influenced by the geological and structural settings of the original bedrock.

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The bedrock with typical compositional metamorphic banding, was progressively transformed by

the iron enrichment process and weathering in the different BIF types through moderately

weathered variants to residual soils, resulting in a complex tridimensional weathering profile with

not only vertical but also lateral variation. The profile variations result from the banding, structures

and weathering interaction, defining at least two different types of weathering profiles associated

with the proto-ore mineralogical composition, namely, a discontinuous weathering profile for easily

leached BIF lithology such as FDI, and the continuous weathering profile for other BIF less prone to

leaching, e.g. FAI and FQI.

Such complex variations result in different weathering profile characteristics. When considering

each compositional itabirite, the dolomitic proto-ore (FDI) shows the deepest weathering profiles

taking into consideration the saprolite horizon thickness. At the Águas Claras, Mutuca and Capão

Xavier mines saprolite can easily reach 400 m in depth with completely weathered BIF (WHE and

WAI) showing no transitional (saprorock – moderately weathered) material. For quartzitic proto-ore

(FAI and FQI) the weathering profile is also as deep, as observed at the Tamanduá, Capitão do Mato,

Pico and other mines, where it is defined by a sequential weathering range from fresh to residual

soil in which the thickness of the transitional saprorock (PWQI and PWGI) can vary from tens to

hundreds of metres, depending on intrinsic and extrinsic characteristics as originally proposed by

Dorr (1969).

The iron enrichment based on supergenic or hypogenic genesis, and metamorphic and tectonic

setting, should affect hematitite (hard or weak) total porosity and fabric, (i.e. mineral and pores size,

and orientation), establishing the different levels of intact rock strength consistency and hardness

that could not be divided and associated to the weathering horizons. In these materials, the typical

compositional metamorphic banding and hypogene enrichment for HHE and supergene enrichment

for WHE are the main process and weathering plays a secondary role, being responsible for

hematitite oxidation and secondary total porosity increase mainly for the completely weathered

grades. The weak hematitite genesis is controversial and is a result from hypogene and supergene

process defined by Ramanaidou & Morris (2010) as supergenic mimetic mechanism from previous

itabirites.

Additionally, as in great depths the iron enrichment process were not efficient for itabirites, the

total porosity for fresh BIF is originally imposed by the low-grade regional metamorphism imposing

a granuloblast texture, with small crystals oriented according to a banding. At shallower depths,

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supergene events transformed the crystalline phases of iron minerals (hematite, magnetite and

martite), providing a volumetric reduction which is responsible for increased intergranular porosity

(Pimenta 1992). Finally, the lattice weathering process induce oxidation and other mineral changes

(deleterious minerals) responsible to increase the total porosity and reduce the intact rock strength.

These considerations agree with studies by Rosière et al. (1993), Lagoeiro (1998), Morris (2002 and

2002a) and Costa et al. (2009) which suggest that the origin of high-strength, hard hematite is

associated with original band concentration during depositional events by hypogenic fluids that

were concentrated in fold axes, interception of foliations or banding, and/or near intrusive dikes.

These studies also indicate that at close to the surface, low strength hematitites (brown hematite)

is associated with an ancient supergene origin (mimetic weathering), mainly by the surface

weathering in itabirites.

From the weathering, for the hematitite group it is recognised that increases in total porosity and

mineral alteration are important changes that produce an effective softening of the original fresh

rocks.

For itabirites, this softening is produced by partial or total leaching of non-iron minerals, mainly iron

dolomite, but also quartz; oxidation of iron minerals; and chemical alteration of amphibole and

other minerals. In extreme cases, the remaining iron oxidises, and non-iron bands (mainly quartz)

exhibit a high total porosity and low grain apparent cohesion (loss of contact between grains). This

results in a weak material that is occasionally cemented by secondary iron oxides and/or hydroxides

(WGI and PWGI), which impose some additional strength due to the total porosity reduction. These

results outlined above agree with studies by Gupta & Rao (2001) for crystalline rocks, Marques et

al. (2010) for metamorphic rocks and Leão et al. (2017) for phyllites.

Weathering changes imposed on BIF through the weathering profiles

Supporting geological and geotechnical considerations, laboratory tests undertaken especially for

these studies and results from Vale’s database show a significant reduction of intact rock strength

and elastic parameters from fresh to completely weathered BIF types. These changes are presented

and described below, for each weathering horizon:

• Fresh to slightly weathered (bedrock horizon)

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At great depths, for this horizon, even with a reduced opening of pre-existent fractures promotes

some initial oxidation at discontinuities walls. Additionally, weathering of the band boundaries with

higher pore levels could increase the oxidation also defining slightly weathered types (W2).

At this stage, intact rock strength reduction is very low, and the real changes are observed only in

the strength reductions on discontinuities walls for fresh BIF and the typical compositional

metamorphic banding is preserved. The anisotropy is insipient induced by the heterogeneity for

itabirites and pores layers for HHE.

The intact rock strengths for fresh and hard BIF show a progressive reduction from HHE, through

FQI and FAI to FDI. The lower values seen for FDI is attributed to the presence of weaker minerals

(e.g. siderite and ankerite), and fabric with higher relative grain size variation associated with

weaker banding planes at the contact between iron dolomitic and iron bands. This characteristic is

also responsible for inducing a low to medium anisotropic ratio for FDI (Rc = 2).

Conversely, the higher intact rock strength of HHE is induced by the mono-mineral composition,

with denser and harder iron minerals that induce a higher bulk density and higher intact rock

strength, and an anisotropy ratio of Rc = 1 defined as isotropic.

However, considering anisotropic effects for HHE, the obtained values presented some

inconsistencies mainly due to the low number of test results obtained for β90° due to the difficulty

in extracting reliable samples, which could have induced some bias.

The HHE must be treated separately since they are mainly a product of hypogenic enrichment, for

this reason are not directly associated with the weathering grade continuum.

• Moderately weathered (saprorock horizons)

For this horizon, in the first few hundred metres below the surface, the intense strength reduction

is not restricted to fractures but is also related to the intact rock developing through band contacts

and higher total porous layers increasing the total porosity by leaching of non-iron bands and

oxidation of iron bands in these zones.

In this horizon, the metamorphic banding orientation competes with the new forming weathering

banding that could or not be parallel one with other depending on the topography and metamorphic

banding dip.

344

For FDI, the typical discontinuous weathering profile does not present saprorock horizons. The

supergene alteration occurs in a single stage, inducing a complete leaching of the iron dolomite

minerals and loss of grain cohesion for all bands, thereby resulting in a total change from FDI

to WHE or WAI, depending on geological settings as described by Spier et al. (2008).

Due to the lower solubility of quartz in FQI and FAI, the supergene process is less effective and the

moderately weathered types (PWQI and PWGI) are present, defining a thicker saprorock horizon

ranging from tens of meters to several hundred of meters, normally observed in continuous

weathering profile.

Anisotropic effects are induced in this stage by the intact rock strength reduction and higher number

of open discontinuities, increasing of the total porosity as a result of partial or total leaching, and

iron mineral alteration inducing a loss of apparent grain cohesion, mainly in the quartz bands but

also in iron bands reflecting a medium anisotropy degree (Rc = 2.6) and resulting in a clear

heterogeneity observed in these types.

For moderately weathered itabirites, the σci for PWGI is higher than for PWQI, even with the reduced

number of tests, anisotropic grouping indicates a sensitive reduction of elastic and intact rock

strength parameters at β45°.

• Completely weathered to residual soil (saprolite to residual soil horizons)

At shallower depths to the surface, these horizons present respectively weak rock and soil material

resulted from a total softening, loss of grain cohesion and an increase of total porosity for all bands,

resulting in an accentuated strength reduction. Deep oxidation and hydration of iron minerals are

abundant for the residual soil horizon but are not necessarily found at saprolite horizon.

For BIF saprolite horizon, the anisotropy for WAI, displays a low anisotropy ratio associated with the

presence of clay bands. These bands display very low shear strength levels when compared with

quartz or hematite bands.

WGI is geologically similar to WQI, with the main difference being related to the greater presence

of goethite as cement that increases the weathering degree, but conversely also increases the shear

rock strength due to the porous cementation. For this reason, WGI presents higher intact rock

strength values.

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WQI, with the quartz partial leaching and iron enrichment induces a ρb increase even with total

porosity increase. For the WHE, contrarily, a total leaching of non-iron bands will induce an increase

in the ρb as all the soil is constituted mainly of denser minerals (hematite and martite) even with

high total porosity. Consequentially, the WHE strength is higher at saprolite horizon.

Specially for shallower depths, where all geological processes are more effective, the balance

between intrinsic characteristics such as iron content, total porosity, bulk density, iron cementation

and shear strength, and extrinsic characteristics such as porewater content define a multivariable

correlation difficult to track that controls the BIF weak rock behaviour. This process is presented by

Ramanaidou & Morris (2010), who argue that weathering softening is primarily due to the

dissolution and reprecipitation defined by supergene mimetic iron enrichment, and secondly the

result of the supergene laterite fluids that are mainly responsible for the oxidation of iron minerals:

in other words, ‘weathering’. It was also evaluated, considering the supergene alteration process

that induces sufficient porosity increases to generate subsidence, as proposed by Ribeiro & Carvalho

(2002) and Ribeiro (2003).

Failure criteria through the BIF weathering profile

• MC and HB failure criteria are widely applied to define intact rock strength parameters for

iron ore mines. Each one has been used and defined as more appropriated for evaluate

rock (HB) and soil or soil-like (MC) materials. For isotropic to low anisotropy fresh/hard

lithologies showed HB regression curves with a progressive strength reduction from HHE,

FQI, to FAI, presenting similar curves, indicating that these lithologies display similar

strength behaviour.

• For low to medium anisotropy FDI and PWQI showed lower HB strength curves and PWGI

a particularly distinct envelope, indicating higher strength attributed to a strength increase

resulting from the presence of iron oxide cement.

• For highly to completely weathered BIF, the use of MC criterion indicates a similar friction

angle for all types except for the WGI with the highest value, attributed to the presence of

iron oxide and hydroxy such as cement and WAI with lower MC curve promoted by the

presence of clay minerals that will finally affect the porewater pressure for unsaturated

condition.

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7.7 CONCLUSION

The use of geological and geotechnical criteria allows the elimination of atypical BIF features, and

simple statistical approaches were used to define outliers, these approaches emphasising the

importance of data evaluation to provide best fit results, reducing the natural variance observed at

laboratory test results.

The itabirite compositional metamorphic banding defined by the iron and non-iron band alternation

which configure the itabirite heterogeneity is promoted by bimodal mineral composition, mineral

fabric, bulk density, mineral hardness, and total porosity, that are primary imposed by the

metamorphic grade (greater depths), supergene events, all of which are controlled by the

weathering degree and banding thickness.

For itabirites, intact rock strength and elastic parameter are associated with the weathering horizon

and zones and present similar parameters associated with the itabirite composition grouping, e.g.

quartz-based types (quartzitic, amphibolitic) and iron dolomite-based type (FDI). The hematitic

group, although having different genesis and must considerate with restrictions as a part of the

weathering profile. Among other critical characteristic presents in general higher bulk density and

intact rock strength compared with other itabirites.

The hematitite metamorphic banding is defined by alteration of different iron minerals and total

porosity, and also in texture, and shape at all weathering horizons. Thereby defining a hematitite

heterogeneity that are primarily imposed by the metamorphic grade and hypogene process for HHE,

and metamorphic grade and supergene process for WHE (mimetic supergene enrichment,

Ramanaidou 1989 and 2009), all of which are not (or partially) controlled by the weathering degree.

For the hematitite, tectonic and/or metamorphic conditions are the main factors controlling intact

rock strength reduction. Weathering (laterite weathering enrichment, Ramanaidou 1989 and 2009)

has only a secondary effect on intact rock strength variations of this type by iron oxidation reducing

the strength along bad planes or discontinuities.

At bedrock horizon in great depths, the anisotropy effect is attributed to the compositional

metamorphic banding in typical itabirite heterogeneities, are promoted by minerology. For this

reason, the mineralogical composition is more effective in lithotypes with higher mineral hardness

contrasts, as noted in the FDI, due to the high contrast between iron bands (hematite and martite),

with non-iron bands (ankerite and siderite). Total porosity can cause minor anisotropy effects in

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intact rock strengths for fresh weathering degree but is mandatory to increase the anisotropy for

PWQI/PWGI which show medium anisotropy ratios.

UCS, triaxial HB and Brazilian test data analysed with RocData 5.0 (Rocscience 2021) provides

reliable linear regression (best fit) lines in σ1σ3 stress space, which were used to determine intact

rock strengths and static elastic parameters for different weathered levels. Mohr–Coulomb and

Hoek–Brown criteria were evaluated and compared for all materials, resulting in similar adjusted

parameters.

From the fifteen mines that were studied, two different types of weathering profile were identified.

The discontinuous profile presents no saprorock with moderately weathered materials (W3 and

W4), between the hard and fresh bedrock horizon to weak and highly weathered saprolite horizon,

indicating a discontinuous weathering profile. This type is observed at the Águas Claras, Mutuca and

Capão Xavier mines, where the high leaching-prone proto-ore is mainly composed of dolomitic

itabirites. These profiles present the deeper saprolites horizon level, reaching over 400 m in depth.

The second type includes a transition zone, forming a continuous weathering profile, in which

saprorock (moderately weathered types) can vary in thickness from tens to hundreds of metres.

This profile is generally observed in mines where the proto-ore is less prone to leaching, composed

of amphibolitic and/or quartzitic itabirites, such as in the Tamanduá, Capitão do Mato, and Pico

Mines.

Each profile type is defined by the weathering horizons and zones, and the tridimensional spatial

distribution is highly conditioned by the itabirite compositional metamorphic banding geological

setting (folds and faults), depth, topography and other external characteristics as e.g. pluviometry

and temperature.

Considering these different weathering profiles for itabirite it is possible to group and summarise

the main characteristics:

• Bedrock: fresh itabirites (FDI, FQI and FAI) generally display low values of total porosity

(5% to 10%) and a medium ρb (3.11 t/m3 to 3.41 t/m3). These characteristics are supported

by the basic mineralogy (iron dolomite, quartz, and hematite), and the mineral shapes

(tabular to granoblastic). UCS values range from quartzitic itabirite (UCSFQI = 256 MPa) and

amphibolitic itabirite (UCS FAI = 171 MPa) to the lowest dolomitic itabirite

(UCSFDI = 119 MPa) but with a higher anisotropy rate. Due to the differences in geological

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genesis, the stronger hematitite (UCSHHE = 445 MPa) must be considered in a different

group.

• Moderately weathered itabirites at saprorock horizon (PWQI and PWGI), have

intermediate total porosity (10% to 20%), high ρb (3.6 t/m3 to 3.26 t/m3) and considerably

lower UCS values (UCSPWQI/PWGI = 68 MPa). This high strength reduction is associated with

the increase in anisotropy ratio and total porosity.

• Highly to completely weathered itabirite at saprolite and residual soil horizons present

very high porosity (20% to 30%) and very low bulk density varying from ρb (2.2 t/m3

to 2.7 t/m3) present very low relative UCS values, decreasing from the strongest WGI

(UCSWGI = 0.53 MPa) to moderately weak hematitite (UCSWHE = 0.45 MPa) and argillaceous

itabirite (UCSWAI = 0.37 MPa) and to a lower mean for the WQI (UCSWQI = 0.37 MPa).

• The iron enrichment process, in this stage will produce two different types of saprolite

materials: itabiritic (WQI, WGI and WAI), with the partial leaching inducing a bulk density

and total porosity decrease. For hematitic (WHE), the total leaching (supergenic iron

enrichment) of non-iron minerals induced a higher bulk density (ρb = 3.5t/m3) even

increasing the total porosity. Consequentially, the WHE shear strength is higher than WQI

due to the iron content.

In summary, for BIFs:

• The metamorphic grade and tectonical settings, are responsible for the mineral

composition and fabric are responsible for the original mineral hardness, grain contacts

and mineral orientation (size, shape, and texture) mainly at great depths for bedrock.

• In shallower depths, supergenic and hypogenic iron enrichment processes are responsible

for modifying the original configuration, reducing, or increasing intact rock strength

depending on the lithotype, and inducing iron concentration and ρb increases mainly for

hematitites, but also for rich itabirite lithotypes.

• At sallower depths to surface, the weathering grade increase (mainly total porosity

increase) is responsible for inducing medium anisotropy in PWQI and PWG.

• Mineral composition is responsible for inducing anisotropy for FDI.

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• Since the bulk density is an important parameter responsible for the intact rock strength

behaviour, it is crucial to understand the balance between iron content and the total

porosity through the weathering profile to appreciate the intact rock strength behaviour

of the BIF. Additionally, these characteristics are highly affected by the mineral

composition and band thickness that could induce scale effect in tested samples.

• The evaluations of visual total porosity from thin sections demonstrate a direct increase

as the weathering grade increases. The Øb increment noted in evaluated thin sections

varies from one to four times for fresh to completely weathered BIF.

• Based on the UCS values obtained from HB failure envelopes, strength reductions of 55%

from fresh to moderately weathered, and of 48% from moderately to completely

weathered BIF were noted, illustrating an effective strength softening due to the

weathering.

• Moderately weathered lithotypes are important due to the ambiguous behaviours

observed at saprorock horizon typifying the boundary between fresh (bedrock, controlled

by typical compositional metamorphic banding) and completely weathered BIF (saprolite,

controlled by a mixture of metamorphic banding, supergene and weathering processes).

Such intermediate materials can behave like rock, when the leaching process was not

sufficiently effective to increase total porosity and when iron remobilisation fills the

porosity induced by the weathering at saprorock horizon increasing the intact rock

strength, or like a soil when the iron remobilisation only partially seals the pores leaving,

highly porous zones at saprolite horizon.

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CHAPTER 8. CONCLUDING REMARKS

The research objectives of this thesis were presented and discussed in Chapters 1.2 to 1.4 and the

main findings and their respective contributions are summarised and divided based on the three

following distinct groups of materials, namely:

• Fresh to moderately weathered BIF types and their geological, petrophysical, intact rock

strength and elastic parameters, in association with the geomechanical behaviour

considering the anisotropy and heterogeneity characteristics. These results are presented

and discussed in Chapters 4 and 5, and Appendix I.

• The completely weathered to residual soils BIF types and their geological, petrophysical,

saturated and unsaturated shear strength and elastic parameters, in association with the

geomechanical behaviour considering the anisotropy and heterogeneity characteristics.

These results are presented and discussed in Chapter 6 and Appendix II.

• BIF completely weathered profile geological and geomechanical characteristics,

considering the weathering horizons and zones boundaries, and heterogeneities;

presented mainly in Chapter 7.

A summary of the conclusions along with a critical insight into the current and future work is

demonstrated in the following sections.

8.1 FRESH TO MODERATELY WEATHERED BIF GEOLOGICAL AND GEOMECHANICAL

CHARACTERISATION

Related to the variation obtained in the used dataset for rock materials definitions. In this thesis,

some laboratory test results presented high amplitude (standard deviation), indicating the presence

of a natural variance related to anisotropy, randomness of geological features and scale effect

related to the banding thickness and the sample size. Part of this behaviour is attributed to the

presence of low anisotropy effect and the samples natural heterogeneity even when adopting

coherent groupings and methodologies to eliminate outliers and reduce the natural variation and

others experimental problems presented above. Likewise, the tested materials present geological

features responsible for inducing large dispersions of the results, since they may have different

variations in fabric, rock heterogeneity and band thickness. In this matter, BIF was considered in this

thesis as a typical geological feature of the homogeneous sub centimetric parallel banding. Other

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geological features considered not typical, e.g. intense fold, shear or breeched zones or veins must

be avoided from tested samples.

The field survey provided geological and geotechnical characterisation useful to group and remove

non-suitable laboratory test results from the dataset reducing bias and data variance. This approach

emphasised the role of identifying the geological characteristics that were crucial to define a proper

sample grouping, i.e. to separate the hematitite from itabirite group and better represent mainly

the three different compositional types of fresh itabirites. Based on laboratory tests, it was possible

to evaluate the intact rock strength and its static and dynamic elastic parameters. Thin sections

added important information to explain and show for each rock the fabric behaviour, visual

qualitative total porosity and correlation with results from laboratory tests.

Even with several attempts to reduce data dispersion, some parameters still showed high standard

deviation. This may represent a natural range expected for these properties and/or by the slightly

higher anisotropic ratio (medium anisotropy) observed for FDI, PWQI and PWGI. Even so, it was

possible to verify reliable trends and establish proper correlation for these types.

The techniques used to determine the total porosity, using a qualitative micro thin section

evaluation (Øb) correlated with laboratory tests (Ø), even with inconsistence related to the

measurements techniques, showed to be reliable to define the ranges of each fresh to moderate

lithotype.

Three main petrophysical characteristics in the rock fabric were found to be responsible for the

penetrative heterogeneous bands that could induce the anisotropy ratio for fresh to moderately

weathered BIF:

Heterogeneity: the contact between iron and non-iron mineral bands, which define the

itabirites compositional metamorphic banding.

Mineral orientation: planar or granular minerals oriented along a single or multiple directions

which also define the BIF metamorphic banding.

Pores concentration and orientation: oriented porous layers defining the hematitite


With this starting point and based on anisotropic ratio (Rc) revaluations (based on UCS test results),

the low anisotropy ratio obtained for FAI and FQI are correlated to the banding heterogeneity

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represented by the dual mineral composition (iron and non-iron bands). The isotropic to low

anisotropy ratio for HHE is correlated to the banding heterogeneity represented by the dual pores

concentration (higher and lower total porosity bands).

The low to medium anisotropy ratio obtained for FDI is attributed to the presence of low strength

bands related to iron dolomite minerals, that also induces different correlations from those

obtained for other itabirites. Additionally, moderately weathered BIF (PWQI and PWGI) presented

a moderately anisotropic index induced by the higher total porosity.

The use of dynamic evaluations (anisotropy index - IVP) based on Vp and Vs wave velocities, was used

as a second anisotropy evaluation to confirm the anisotropy effects for fresh and moderately types.

Using this approach, the obtained anisotropy index was fair (low) in accordance to anisotropic ratio

(static method) for FAI, FQI and HHE; however, for FDI it presented an isotropic index. This behaviour

is attributed to the high P and S wave velocities obtained for iron dolomite minerals bands with

values closer to velocities obtained for iron minerals bands. For PWQI and PWGI a moderately

anisotropic index was also identified, mainly caused by the total porosity increase and bulk density

decrease notedly in more weathered types.

Even with variance obtained from UCS test results, this parameter proved to be adequate to

distinguish the intact strength variance, from homogeneous and almost isotropic hard hematitite to

heterogeneous and low anisotropic fresh itabirites, and effective in distinguishing the changes

between different compositional itabirites, even for the moderately anisotropic rocks.

Considering the anisotropy behaviour evaluated from IVp index (P wave velocity) and from Rc ratio

(UCS test results), even representing different physical characteristics, presented consistent ranges

with higher values obtained for FDI (IVp=1.3 and Rc=2) and lower values obtained for HHE (IVp = 1.1

and Rc = 1).

Due to the general low total porosity of fresh BIF (Øb ≤ 5%), bulk density is directly correlated to the

iron content and an increase in the intact rock strength is proportional with the increase of iron

bands in the specimen. This proportionality was not evaluated in this research. However, a linear

positive correlation is suggested and the correlation between UCS and bulk density could be used

to infer the UCS values, as presented at Appendix I.

Coefficients of determination and adjusted regression curves provided reliable empirical equations

mainly for FAI and FQI and made it possible to claim that petrophysical, intact rock strength and

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elastic properties obtained from dynamic and elastic tests, were able to support correlations and

determine these parametres. By contrast, FDI and HHE presented low coefficients of determination,

mainly imposed by high standard deviation and/or intrinsic characteristics such as undetectable

microfractures for the HHE, and the iron dolomites bands with lower strength (hardness) and high

P and S wave velocities. These lower correlations could also be related to the P wave velocity being

more sensitive to elastic modulus than bulk density as proposed by Marques et al. (2010). Even with

low coefficient of determination observed for these types, this represents useful data for describing

their geomechanical behaviour.

The findings from the experimental works and basic statistical analyses showed a simple and reliable

method of predicting the BIF typology, weathering horizons and indirectly establish correlations

between petrophysical characteristics with bulk density, total porosity, and P and S wave velocities.

As previously proposed by Hack & Huisman (2002) and supported by this thesis results, there are

important interrelationship that could be used as a geotechnical index characterisation of BIF. Thesis

results have emphasised the importance of identifying even the simplest geological and/or

structural features in order to have a proper characterisation able to represent and identify different

types of BIF. The research highlights important relationships defined by the correlation between the

iron content with bulk density, total porosity and UCS tests as an important approach to infer the

intact rock strength and elastic parameters and other geotechnical behaviours.

The use of P and S wave velocities datasets to obtain dynamic empirical correlations prove to be a

reliable, non-destructive indicator for total porosity, bulk density, and consequentially rock

weathering degree and, indirectly, the iron content, as presented in Appendix I mainly for fresh BIF

due to the reduced total porosity and effective coupling of the wave velocity transductors. This

behaviour is noted when comparing dynamic with static elastic modules where were obtained a

correlation Edyn/Estat < 1 showing wave velocity attenuation mainly induced by the samples inherent

microcracking (noted for HHE), pores layers, mineral composition and fabric.

The obtained empirical correlations graphs and equations are useful methods to predict elastic and

intact rock strength parameters, petrophysical properties, and to estimate the lithotype directly

associated with the weathering horizon and levels. Additionally, the weathering grade and

heterogeneity were defined as important features that control anisotropy, bulk density, elastic

parameters and, ultimately, the intact rock strength of the BIF.

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It is possible to summarise fresh BIF into two similar behaviour types:

Hematitite group: mostly HHE, is defined as the isotropic type, mineralogically homogeneous

with heterogeneity defined by multiple pore bands. This group is characterised as the

highest bulk density, P and S wave velocity, and higher ‘maximum’ UCS values. For this

type, the empirical correlations between UCS with bulk density and P and S wave velocity,

and static Young’s modulus resulted in reliable correlation equations and graphs.

Itabirite group: includes all the three fresh itabirites (FAI, FQI and FDI) that showed several

similarities in their definition and characterisation. However, some particularities can be

used for individualisation of FDI. A characteristic of this group is the lower relative bulk

density, UCS values and elastic parameters considering just the fresh BIF. The results

dispersion is deeply associated with the itabirite heterogeneity (defined by the iron and

non-iron band alternation), and secondarily by the present low anisotropic ratio.

The itabirite group was divided into two subgroups defined by the main mineral present in the

non-iron band and by the weathering degree:

Quartz rich itabirites FAI and FQI: present weathering degree varying from W1 to W2 with

low anisotropy ratio with higher UCS, Vp and Vs, Edyn and Estat, and ρb values presented

reliable equations for all evaluated correlations. PWQI and PWGI, present weathering

degree varying from W3 to W4 with lower (compared with fresh itabirites) UCS values,

wave velocities, Edyn, ρb, and higher total porosity values and moderately anisotropy index.

Iron dolomite rich itabirite (FDI): present weathering degree varying from W1 to W2 with

isotropic Ivp and low to medium Rc . This behaviour is attributed to the intrinsic mineral

characteristics of iron dolomite minerals with present higher P wave velocity, and lower

band strength related to Mohr hardness scale compared with quartz. This subgroup

showed lower UCS results, and few reliable equations and low correlations.

These fresh groups are easily identified by the high intact rock strength very different from

moderately weathered types. Such differences are associated with the weathering increase for FAI

and FQI grading from fresh (W1) to slightly weathered (W2), and finally to PWQI/PWGI (moderately

weathered – W3/W4) degree. In the same context, this grading is not observed for FDI as, for this

type, the weathering increase results in different weathering horizons – fresh or bedrock (W1) to

rare and very thin layers of slightly weathered zone (W2) running directly to highly to completely

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weathered or saprolite/residual soil horizons (W5/W6). This behaviour is attributed to the efficiency

of the leaching process more prone for FDI than to FQI and FQI. This behaviour is in accordance with

the distinct weathering profiles observed in Iron Quadrangle iron mines, as proposed in Appendix I

and II.

8.2 COMPLETELY WEATHERED TO RESIDUAL SOILS AND WEAK BIF GEOLOGICAL AND

GEOMECHANICAL CHARACTERISATION

The use of combined soil and weak rock classification is adequate for soil-like BIF materials similar

to those types evaluated in this research. The classification proposed for these types ranges from

very stiff to hard clay, through extremely weak to very weak and weak rocks that are classified

according to ISRM (1981) and Martin & Stacey (2018) separated into two main weathering horizons:

highly to completely weathered (saprolite horizon), and completely weathered (residual soil

horizon). These materials, in general, behave like soils and follow soil mechanic approaches, and

inherent relict anisotropy of the original intact rock may have important effects on slope stability

and failure mechanism specially when deeping unfavourable to stability. Multiple evaluations

demonstrated what is defined as weak rock (highly to completely weathered BIF) represent, in

reality, a saprolite horizon, a transition zone to residual soils, i.e. an immature residual soil.

For these types, bulk density and particle size distribution (mineral composition) define different

behaviours under applied stress levels. It was noted that for lower stress level (below 400 kPa) the

BIF soils behave like loose sandy material, while for stress levels above 400 kPa they behave like

dense sandy material. However, the total porosity percentage can induce dense behaviour even at

low stress levels, or loose behaviour under high stresses.

The genesis of BIF saprolitic to residual soils such as WAI, WGI and WQI are mineralogically

associated to the original fresh itabirites. The effectiveness of the supergene and weathering

processes promotes physical and chemical alterations (oxidation of iron minerals, alterations and

partial or total leaching of non-iron minerals) inducing a significant increase in voids, weakening the

crystal contacts, increasing deleterious minerals (e.g. ochreous goethite, gibbsite and kaolinite)

resulting in an intense total porosity increase hence reducing the soil shear strength even with a

bulk density increase resulted from the iron enrichment.

For the WHE, the lateritic weathering process (Ramanaidou & Morris 2010) produces minor iron

minerals reconcentration and the main change is restricted to iron mineral oxidation and pores

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increase. As the mimetic supergene is the main event responsible for the leaching, pores increase.

The same authors suggested this mimetic supergene process is responsible for the ganga mineral

leaching, pore increase and respective intact rock strength reduction, that posteriorly were imposed

by tectonical and metamorphic changes, even though in this research the WHE is positioned at

saprolite horizon, as long as it presents geomechanical characteristics consistent to this horizon.

As the low efficiency of supergene process mimetic for some itabirites, the lateritic weathering is

the main process responsible for reduction in intact rock strength and increase the total porosity

and bulk density. These changes are resulted of mineral alteration increasing in deleterious minerals

(kaolinite, gibbsite and ochreous goethite), imposing a relative higher plasticity index responsible

for inducing some negative porewater pressures (matric suction) mainly controlled by the kaolinite

increase, as noted for WAI and, in some extent, to WGI.

From the kaolinite, it is important to note that it is commonly found in fine fractions of tropical and

subtropical Brazilian soils and must have contributed, in some instances, to increasing the matric

suction in a minor effectiveness due to the non-expansive clay mineral nature.

The use of the ISRM (1981) tables to characterise highly to completely weathered BIF proved to be

adequate to identify the main typologies and defines these soils as the saprolitic horizon as an

immature residual soil, i.e. as materials which are formed in place from mineralogical changes and

leaching. From this table the WQI is defined as the totally weathered degree and WGI as the

transition zone to a completely weathered degree (ochreous itabirite), from original FQI and FAI

proto-ore. WAI is characterised as completely weathered type from original FDI or FAI. In general,

all evaluated soil types could be properly grouped using USCS classification (ASTM 1998). Using this

classification, they were grouped into low (WQI, WGI and WHE) to intermediate (WAI) plasticity

index, dense sandy-silty soils with very low permeability (WAI) and dense silt-sandy soils with gravel

with moderate permeability for WQI, WGI and WHE.

The heterogeneity is defined for WQI by the alternation of bands with quartz/goethite with high

total porosity and bands of iron minerals with low total porosity bands, which induces a low

anisotropy ratio. Considering WGI, the heterogeneity and anisotropy is similar to WQI, and it is

noted that an increase in the shear strength resulted from iron oxide and hydroxide cementation.

WHE is almost a monomineralic type and the heterogeneity is less evident, defined by the

alternation of bands with lower and higher total porosity as described by Costa (2009), and the

anisotropy ratio can vary from isotropic to low anisotropy ratio. For WAI, the anisotropy ratio is low,

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and the heterogeneity is defined by interlayers of quartz and clay mineral bands with higher total

porosity in contrast with iron mineral bands with lower total porosity.

For soil types with high total porosity, as obtained for BIF residual soils, once the cohesion is

destroyed the load is transmitted to the soil skeleton, thereby mobilising the particle friction, and

this could induce sudden collapse during particle rearrangement. This collapse will occur preferably

at higher total porosity bands mainly for WHE and along the different compositional bands contacts

for WQI and WGI as similarly described for oolithic iron deposits by Cuccivillo & Coop (1997) and for

BIF by Martin & Stacey (2018).

In this matter, mineralogical composition (band strength), total porosity, and mineral and pore

orientations are the main characteristics responsible for brittle collapsed behaviour depending on

the stress level and material type. At low stress levels (below 400 kPa), the higher total porosity soils

are more prone to ductile collapse than the lower total porosity soils for which the pore and mineral

orientations are less important and do not allow brittle failures since the fabric (skeleton)

reorganisation redistributes the strain. For high stress levels (above 400 kPa), more brittle post-peak

behaviour for WGI, WQI and WHE is evidenced when compared with WAI. However, for higher

stress levels, a granular rearrangement can induce sudden collapse, as observed in the brittle

post-peak resistance loss (strain softening) in the majority of the test result curves.

Considering the porewater effects in BIF soils, the first attempts proposed for this thesis for BIF soils’

unsaturated behaviour shows an increase in shear strength parameters, due to an apparent

cohesion contribution, reflecting the matric suction effects mainly for WAI. Thus, there is a potential

benefit in using unsaturated soil behaviour for slope designs and stability analyses, considering the

influence of negative porewater pressures, particularly during the dry season or for temporary

slopes.

The historical approach used in Vale mines, based on saturated laboratory tests and effective

parameters, not considering porewater pressure for WHE and WQI/WGI, is shown to be suitable for

long-term and post-closure slope stability analyses. However, for WAI short-term slope stability

analyses, this approach is conservative and the benefit in using porewater pressure parameters for

slope stability analyses with WAI was presented in a case study shown in Appendix II with a

considerable gain in the Factor of Safety using unsaturated soil parameters and unsaturated soil

criteria.

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The use of saturated parameters in the analyses of slope stability may be acceptable in cases, such

as slopes where soil present moderate to high permeability, as presented for WHE and WQI even in

the unsaturated zone. However, for slopes in saprolites or residual soils with low permeability and

large unsaturated zone, unsaturated behaviour must be evaluated as a standard approach.

Meanwhile, it should be emphasised that even though steep slopes may be stable in dry seasons

because of the additional strength provided by apparent cohesion, in this case, the matric suction

can be loosed in rainy seasons, resulting in rain-induced failures. Therefore, it can be concluded that

the use of saturated parameters is likely to lead to an overconservative result, and the influence of

negative porewater pressure needs to be considered in the evaluation of slope stability to a certain

extent as the matric suction cannot always be relied on because of rainfall. The real understanding

of the matric suction contribution for slope stability should make it possible to provide a better

management of open pits. The potentially damaging effect of rainfall can be planned for, and

appropriate measures taken to ensure adequate drainages are provided at all times.

The effect of negative pore water pressure must be considered in slope stability analysis for mines

located in high rainfall areas even for high permeability materials where the pore water pressure is

generally assumed to dissipate eventually, which results in strength reduction and decreases

stability with time.

Finally, the influence of unsaturated soil mechanics on slope stability is becoming more important

in current mining engineering and must be considered as becoming the best engineering practice.

8.3 BIF COMPLETELY WEATHERED PROFILE GEOLOGICAL AND GEOMECHANICAL

CHARACTERISATION

The genesis and BIF and evolution of the iron enriched deposits, due to the multiple events

superposition, are still controversial with several authors supporting different geological

hypotheses. However, for this research the statements from Ramanaidou (1989 and 2009),

Ramanaidou & Morris (2010), Ribeiro & Carvalho (2002) and Ribeiro (2003), provide reliable

geological records able to support geomechanical characteristics correlations with geological

features normally observed in this deep BIF weathering profile.

Supported by these authors it is possible to argue that for BIF, the itabirite compositional

metamorphic banding configures a geomechanical heterogeneity for all weathering horizons and all

compositional itabirites defined by Door (1969). However, the supergene enrichment and

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weathering can generate weathering surface fronts able to produce new banding modifying the

original metamorphic banding. For each horizon (depth) there are specific characteristics controlling

the anisotropic behaviour of each BIF typology. This weathering heterogeneity is an outcome from

intrinsic characteristics such as bimodal mineral composition that will repeat in bulk density, total

porosity, and rock fabric, ultimately determining intact rock, shear strength and elastic parameters.

Differentially, for hematitite the metamorphic banding defined by monomineralic composition,

bimodal total porosity that will repeat in bulk density and rock fabric ultimately determines the

geotechnical parameters.

Considering the depth to define the geological process, it is possible to argue that for bedrock in

greatest depths the typical compositional metamorphic banding controls the geomechanical

behaviour. For saprorock horizon there is a competition between metamorphic banding, iron

enrichment and weathering processes. For saprolite the supergene ad weathering process controls

de geomechanical behaviour and finally for residual soil horizon the weathering dominates and

controls the geomechanical behaviours.

Anisotropy behaviour for itabirites is more effective in compositional BIF with higher mineral

hardness contrasts, as noted for FDI.

Considering the total porosity increase due to the supergenic or weathering process for fresh BIF,

as the total porosity is very low, the anisotropy is managed by the mineral composition imposing an

isotropic to low anisotropy for bedrock BIF typologies (except for FDI presenting a low to medium

anisotropy). For saprorock, as the total porosity is much higher, mineral composition is less

important and total porosity controls the medium anisotropy behaviour. This behaviour is mainly

determined by the contrast between high total porosity bands interlayer with low total porosity

bands. Finally, for saprolites and residual soils the mineral compositional and total porosity act

together to control the generally low anisotropy.

Moderately weathered lithotypes are characterised by ambiguous behaviours typifying the

boundary between bedrock and saprolite horizons that can define as saprorock. Such intermediate

materials can behave like rock (saprorock horizon dominate by typical compositional metamorphic

banding); when leaching, mineral alteration and oxidation processes were not sufficiently effective

to increase total porosity and reduce the mineral hardness; or like soil (saprolite horizon mix of

metamorphic and enrichment processes), when the leaching process is more effective. These

differences were evaluated at fifteen mines that were studied. From these mines, two different

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types of weathering profiles were identified: the discontinuous weathering profile, where the

transition zone constituted by moderately weathered materials (saprorock) is absent, indicating an

abrupt transition between bedrock to saprolite horizons This can be observed at Águas Claras,

Mutuca and Capão Xavier mines, where the high leaching-prone proto-ore is mainly composed of

dolomitic itabirites. The continuous weathering profile includes a transition zone, in which

moderately weathered rocks (saprorock) can vary in thickness from tens of metres to hundreds of

metres, where it is possible to identify all four horizons as proposed by Deere & Patton (1971). This

profile is more common in studied mines and the proto-ore is less prone to leaching, being

composed of amphibolitic and/or quartzitic itabirites, such as in Tamanduá, Capitão do Mato, and

Pico mines. Each weathering profile type presents different zone sizes and spatial distribution.

Using test results and field evaluations, supported by graph and empirical correlations curves and

equations that defined intact rock strength and elastic parameters, it was possible to determine the

boundary between the three main weathering horizons (bedrock, saprorock and saprolite) providing

an adequate definition of the transition zones for continuous and discontinuous weathering profiles.

Additionally, for a complete geomechanical characterisation of the weathering profile the use of

Hoek–Brown to evaluate the elastic brittle plastic behaviour of hard and fresh BIF, and

Mohr–Coulomb to elastic perfectly plastic behaviour of weak and friable BIF. The use of RocData 5.0

(Rocscience 2021) enable this evaluation based on intact rock strength data derived from UCS,

Brazilian, triaxial HB cell and CIU tests. Since the equivalent Mohr–Coulomb parameters are derived

from the Hoek–Brown imputed parameters, the two models are therefore considered compatible

and were used to determine the equivalent intact rock strengths and elastic parameters for different

weathered levels. Using this approach, similar values for adjusted parameters were obtained for

both criteria allowing the use of the more adequate criterium depending on the elastic behaviour

identified for each BIF type at different weathering zone. It is attributed not only from the adjusted

methods used by the software, but mainly from the low level of confinement available in the triaxial

HB dataset, disabling high stress level evaluation that results in the flattening of the HB curve

becoming similar to the MC curve.

In summary, the BIF types can be grouped using weathering horizons and zones based on ISRM

(1989) tables, and the most important geomechanical parameters are:

Bedrock: weathering grade varies from W1 to W2, reaching W3 for FAI, generally display low

anisotropy ratio, the values of Øb = 5± 5% and Ø ≤ 5%, bulk density ranges from 3.11 t/m3

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to 3.41 t/m3. The basic mineralogy is composed by iron dolomite, quartz, and iron minerals, and the

mineral shapes varies from tabular to granoblastic. UCS values range from higher values obtained

for quartzitic itabirite (UCSFQI = 256 MPa), amphibolitic itabirite (UCSFAI = 171 MPa), and the lowest

for dolomitic itabirite (UCSFDI = 119 MPa) with a low to medium anisotropy rate.

For HHE, due to the differences in genesis and characteristics, it represents the higher UCS value

obtained for a single test (UCSHHE = 445 MPa) and the anisotropy ratio is generally isotropic to low

with higher bulk density.

Saprorock to saprolite: varying from W3 to W4 with high total porosity varying from 10% ≥ Øb ≥ 20%,

with an increment in the ρb (3.6 t/m3 to 3.26 t/m3) comparing with bedrock types, form itabirites

present intermediate UCS values (UCSPWQI/PWGI = 68 MPa), and a medium anisotropy ratio.

Saprolite to residual soil: varies from W5 to W6. The granulometry is basic a silt, fine sand to sand

soil with a very high total porosity (20% ≥ Øb ≥ 30%). It presents very low relative UCS values,

decreasing from the strongest weathered goethitic itabirite (UCSWGI = 0.58 MPa) to weak hematitite

(UCSWHE = 0.48 MPa), to the lower values of the weathered quartzitic itabirite (UCSWQI = 0.38 MPa)

and the argillaceous itabirite (UCSWAI = 0.34 MPa).

In addition to all presented characteristics, the scale effect promoted by the iron and non-iron bands

thickness variation on the laboratory tests samples must be considered from all evaluations, that

could induce different behaviour and increase the dispersion and or resulting in bias on parameters

presented above.

In general, the low strain and low metamorphic grade (green schist) observed at great depths along

the western side of the Iron Quadrangle (Rosière et al. 2001), imposed over the relatively simple

mineralogical assemblages of the BIF, a fabric with small size and poorly orientated minerals that

generates fair anisotropy for these heterogeneous rock. This behaviour can be attributed to the

similar mineral hardness (band strength), bulk density, and strong contacts between the individual

crystals defined by BIF fabric.

Finally, the field geological and geomechanical investigations, laboratory tests undertaken during

the research programme, the relationships evaluated, and the adjusted regression curves provided

equations that can be used to estimate and characterise the BIF completely weathering profile

enabled to assess the effect of weathering process, heterogeneity and anisotropy on the

geomechanical properties of the BIF deposits in Brazil’s Iron Quadrangle. These evaluations will

363

strengthen to develop procedures that will optimise pit slope design, promote a better

understanding of potential slope failure mechanisms and ultimately reduce the risk of slope failure

– gains that will help Vale and other iron ore mines in the region to improve the operational

productivity and safety of their iron ore mines.

364

365

CHAPTER 9. RECOMMENDATIONS FOR FUTURE WORK

Although a broad range of topics has been covered in this thesis, there are still several aspects that

still need to be addressed for the range of BIF typologies mainly associated with geomechanical

characterisation and geological aspects that influence their rock and soil mechanical behaviour.

A series of recommendations for future works are summarised below in order to help others to build

on these contributions. The recommendations are provided in the following sections.

9.1 BIF HARD ROCK BEHAVIOUR

To confirm the results of this thesis, related to anisotropy effects for fresh BIF, it is recommended

to increase the number of UCS tests in different directions to anisotropy plan, to support a statistical

correlation achieving the appropriate minimum sample size and define an anisotropic strength

function for each type.

Further studies must be considered for UCS test results correlations with total porosity and iron

content of fresh BIF, which turned out to be the most important parameters for rock strength and

deformability characterisation. The author suggests increasing the number of tests, mainly for HHE

and FDI, to check presented correlations and verify other significant correlations that could be

established. It is also suggested to establish the correlation between dynamic and static parameters.

In this thesis, the importance of the metamorphic bands thickness (heterogeneity and related

sample scale effect) related to the mineral composition or total porosity was qualitatively evaluated.

Further studies are necessary to define the influence of these variables for the intact rock strength,

and elastic parameters and its representativeness in slope scale. After establishing these

relationships for fresh materials, the study is then likely to move forward to moderately and

completely weathered BIF.

Since very high intact rock strength values were obtained for all fresh BIF and a low influence of

anisotropy was defined, it is expected that failure mechanisms in these lithotypes will occur mainly

by shearing along discontinuities. Consequently, the influence of discontinuities and the rock masses

in slope failure mechanisms should be assessed (e.g. using Barton–Bandis approach) to establish

reliable parameters and failure criterion that are able to properly describe the failure mechanism

behaviour.

366

9.2 BIF WEAK ROCK BEHAVIOUR

In a future study, different approaches will be necessary to evaluate BIF soil-like type materials

increasing the number of unsaturated soil tests. The focus of the future studies should be on the

WAI which presented the higher potential for apparent cohesion increase due to the matric suction

effects. However, other types must be considered as the matrix suction benefit could also be true

for the other weathered BIF types such as the WGI, due to the higher amount of clay minerals, but

more studies are required to establish conclusive results.

Seasonal-induced fluctuation of the near-surface matric suction value within situ tests, as well as

the influence of the clay mineralogy must be evaluated to establish robust parameters for

undertaking sensitivity studies and achieving a high degree of reliability in the designs mainly for

this type of BIF soils.

A deeper evaluation of the matric suction effects must be undertaken with a more representative

number of laboratory and in situ water content and permeability tests. The non-linearity of the

failure envelope on the τ - matric suction plane should be defined and evaluated in case studies to

check the effectiveness of this parameter.

9.3 FUTURE APPROACHES FOR THESIS LIMITATIONS

Some limitations presented in Chapter 1.3 should be addressed in future studies. One important

limitation of this thesis is the geographical coverage limits restricted to western low metamorphic

grade side of the Iron Quadrangle. It is suggested to apply the same methodology and evaluation

for the Iron Quadrangle west high metamorphic grade side and for different iron deposits.

It is suggested a deepened evaluation of the correlation between de typical compositional

metamorphic banding with the new formed weathering banding imposing changes to the original


Additionally, this thesis is also limited to the intact rock characteristics while the influence of relict

discontinuities in residual soils and fracture in fresh BIF may occur, and it may have significant effects

on the behaviour of these rock mass. Therefore, in a future study, the effects of discontinuities

should be considered when evaluating the stability of BIF fresh to residual soil slopes. For soil-like

materials, an extra challenge is faced for low intact rock and low-strength discontinuities evaluation

and comparation, becoming difficult to identify the role and importance of each one in cases where

the discontinuities present unfavourable orientation for slope stability evaluations.

367

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APPENDIX I

This paper was presented and published in 2015 at Iron Ore AusIMM and CSIRO conference

proceedings – Perth, Australia

AN OVERVIEW OF THE WEATHERING PROCESS AND PRELIMINARY BULK

DENSITY AND UCS CORRELATIONS FOR FRESH ITABIRITES IN VALE MINES ON

THE WESTERN SIDE OF THE IRON QUADRANGLE, BRAZIL

COSTA, T.A.V.; DIGHT, P.M.; MERCER, K.; AND MARQUES, E.A.G.

ABSTRACT

Brazilian banded iron formations (BIF), known as itabirite, is the main host rock for iron ore in the

Iron Quadrangle mines, Brazil. Their geneses are controversial, and it is agreed that diagenetic,

metamorphic, and tectonic events, as well as weathering processes, were responsible for the iron

concentration. In addition, it is understood that weathering processes are mainly responsible for

reducing the original high intact rock strength, and for generating deep weathered profiles with low

strength rocks commonly observed in Vale’s mines.

A considerable amount of work has been done to correlate the macro and micro aspects of the

itabirites as physical, chemical, and geotechnical characteristics that directly or indirectly affect the

intact rock strength. However, it is not yet clearly understood what effects the weathering processes

have on the strength reduction of these rocks and its associated impact on pit slope stability. This

forms a key focus of the lead author’s current research work.

This paper presents a review of the geological, geotechnical, and weathering characteristics of the

itabirites, and early results for the fresh itabirite evaluations from the PhD thesis research being

undertaken by the Australian Centre for Geomechanics, School of Civil, Environmental and Mining

Engineering, The University of Western Australia, sponsored by Vale S.A.

The results showed that to evaluate the bulk density and unconfined compressive strength (UCS)

correlation, not only the total porosity but also the geological features should be considered, as they

are responsible for data dispersion. However, the presented data suggested a trend that provides

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the confidence to develop advanced studies and data population increment to establish a proper

correlation to support a reliable UCS index method.

1 INTRODUCTION

Vale S.A. Iron Quadrangle mines in Brazil generally exhibit weathering profiles that can reach over

400 m in depth. This means that for shallow open pit mines, less than 400 m in depth, the slopes

will predominantly be composed of completely to partially weathered rocks. However, in deeper

mines, more than 400 m in depth, the slopes will consist of the full range of weathering, from

completely weathered through to fresh rocks.

For these mines, in the weathered profile, the rocks exhibit soil mechanics behaviour. The slope

design approach and the identification of possible failure mechanisms is largely based on applying

classical soil mechanics principals as well as historical trial and error experience. While this approach

has been largely satisfactory, there are nevertheless several key geotechnical issues that are still not

well understood, and there have been continued instances of large slope failures which have

resulted in significant disruptions to the mines.

On the other hand, for the proposed deeper mines, the increase in fresh hard rock at the toes of the

slopes presents new challenges for subsequent geotechnical designs and slope failure assessments,

such as the intact rock mass and the anisotropy behaviour.

A PhD research project is currently being undertaken by the lead author at the Australian Centre for

Geomechanics, School of Civil, Environmental and Mining Engineering, The University of Western

Australia, sponsored by Vale S.A. The main objective of the thesis is to investigate how the

weathering process affects the geological and geotechnical characteristics ranging from hard and

fresh rock to weak and completely weathered rocks. This will facilitate improved optimisation of

final pit slope design and promote a better understanding of potential failure mechanism. This will,

in turn, lead to a reduced risk of slope failure and hence, and improvement of operational

productivity and safety of the iron ore mines.

This paper aims to present the early findings from the author’s PhD research and begins by reviewing

the geological setting of the Iron Quadrangle mines in Brazil and goes on to summarise the key

geological and geotechnical characteristics of the itabirite rocks. The general weathering process

found at the mines is described, followed by a review of the implications of the weathering process

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on current slope stability. A preliminary focus of the author’s research has been to study the iron

formations’ physical and geotechnical index propriety relationships, mainly on the correlation

between the bulk density (ρb) and UCS of fresh itabirite using considerable historical research. The

author’s current findings, together with the correlation between these parameters, are presented

and discussed. The paper concludes by describing the subsequent stages of the research.

2 GEOLOGICAL SETTING

2.1 LOCALISATION

The mines under study are part of Vale’s south ferrous division (DIFL) and comprise of fifteen mines

located in the centre of Minas Gerais, Brazil: Águas Claras (MAC), Mutuca (MUT), Mar Azul (MAZ),

Capão Xavier (CPX), Tamanduá (TAM), Capitão do Mato (CMT), Abóboras (ABO), Galinheiro (GAL),

Sapecado (SAP), Pico (PIC), Córrego do Feijão (CFJ), Jangada (JGD), João Pereira (JPE), Alto Bandeira

(BAN) and Fábrica (FAB). The location of these fifteen mines is shown in Figure 1.

2.2 REGIONAL GEOLOGICAL SETTING

The focus area is in the west part of the Iron Quadrangle located on the southern border of São

Francisco Craton. The mines are located on the Moeda and Don Bosco Synclines and Curral

Homocline ranges.

The structure is delineated by the roughly quadrangular arrangement, with Paleoproterozoic BIFs

of the Minas Supergroup, as proposed by Dorr (1969), composed of hundreds of metres of iron ore

rich metamorphic rocks belonging to the Itabira Group/Cauê Formation. The Minas Supergroup

comprises, from bottom to top, the Caraça, Itabira, Piracicaba, Sabará groups; a sequence of

psammitic pelitics rocks, also defined by Dorr (1969) all superimposed by the Itacolomi Group.


domes of Archean and Proterozoic crystalline rocks as studies of Machado et al. (1989), Machado

& Carneiro, (1992), and Noce (1995) have shown.

The regional structure is the result of two main deformational super-positional events as Chemale

Jr et al. (1994) showed. The first produced the nucleation of regional synclines in the uplift of the

gneissic domes during the Trans Amazonian Orogenesis (2.1–2 Gyr); and the second is related to an

east–west verging thrust fault belt of Pan African/Brazilian age (0.8–0.6 Gyr) described by Marshak

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& Alkmim (1989). This event deformed the earlier structures and was mainly responsible for the

deformational gradient.


deformational gradient to green schist to lower amphibolite’s faces. Structures and geological

settings are shown in Figure 1.

Figure 1 Mine locations and Iron Quadrangle geological settings (Modified from Morgan

et al. 2013)

2.3 ITABIRITE GEOLOGICAL SETTINGS

The Iron Quadrangle Paleoproterozoic BIF are mainly metamorphic and heterogeneous banded

rocks presenting a millimetre to centimetre rhythmic alternation banding of iron minerals

(hematite, martite and magnetite), and non-iron minerals (quartz, dolomite, or amphibolite).

Originally called itabirites as defined by Dorr (1969), these iron deposits are classified as superior

type according to Gross (1980).

The Cauê Formation (Itabira Group) represents the host of BIF and it is a marine chemical sequence

350 m thick, dated 2.4 ± 0.19 GY by Babinski et al. (1995).

Banding is the most typical characteristic that defines a strong heterogeneity and anisotropy. This

variation could be controlled by the original sedimentary bedding, tectonic setting, metamorphic

grade, hydrothermal or supergene processes. However, the superposition of these processes causes

partial or total mineralogical and textural changes.

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Dorr (1969) defined, for this type of iron deposit, two main lithologies: hematite or hematitite – the

high-grade ore (Fe ≥ 62%), and the low-grade ore itabirite (30% < Fe < 62%), with three

compositional lithotypes: quartzitic, dolomitic and amphibolitic. From these proto-ores, the

tectonic, metamorphic, and weathering process changed in different ways resulting in multiple

settings of iron ore lithotypes as described in the following sections.

The origin of itabirites and associated high-grade hematitite orebodies remains controversial and

several works have been produced on this topic, as largely discussed in Spier et al. (2003). For the

friable ore bodies’ geneses, some authors agree on a supergene process and residual itabirite

enrichment, leaching the gangue mineral by surface waters. For these ore bodies, the 40Ar/39Ar

dating of manganese minerals in Vale mines, presented by Spier (2005) and Spier et al. (2006),

suggest that weathering processes and the mineralisation period occurred between 61.5 ± 1.2 Myr

to 14.2 ± 0.8 Myr, reaching the peak process in 51 Myr. This suggests a tertiary mineralisation, and

after this period the weathering process may not have substantially affected the weathering profile.

2.4 THE WEATHERING PROCESS IN IRON QUADRANGLE MINES

Studies by Morris (2002) and Taylor et al. (2001) argue that the supergene process can reduce the

thickness of the itabirites by 32% to 40%, and increase the total porosity from 6% to 30%, as

described by Mourão (2007). Based on that, weathering processes produce an effective softening

from the leaching, and the remaining iron oxide bands present a high void ratio, that could be

cemented by secondary iron oxides (recrystallised hematite or goethite). This material presents

weak strength and the iron enrichment promotes an increase in bulk density due to iron mineral

concentration.

Studies by Ramanaidou (1989 and 2009), Ribeiro & Carvalho (2002), Ribeiro (2003) suggest that

during the first leaching processes there have been no volumetric changes and no quartz to goethite

substitution. Progressively, the volumetric changes start to leach the gangues minerals (more

effective on dolomites, but also in quartz bands) and replace quartz to goethite. This porosity could

be completely cemented by goethite or iron hydroxide in superficial levels, creating a metric hard

crust called canga.

As presented by Spier et al. (2003), the superficial water is the main physical and chemical agent

responsible for the effective dissolution and leaching of carbonates and siliceous minerals, and for

the oxidation and hydration of iron-based minerals.

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Due to the effectivity of the weathering process in tropical climates induced by the high pluviometric

index and the topography featured, the weathering profile can often present for more than 200 m

and can reach over 400 m in depth. In addition, the usual syncline and anticline geological

configuration and the presence of a favourable structural control, defined by the high banding

angles and extensional fractures, can facilitate the superficial and groundwater penetration and

circulation, increasing the weathering profile even more.

The weathering as a continuum and multiple conditioning processes generates a range of different

materials for each compositional itabirite; from fresh to completely weathered rock. This

weathering profile presents specific characteristics and boundaries between the different

weathering levels.

In general, the weathering processes are more effective over dolomitic itabirite due to the high

leaching capability of the dolomitic band. In mines with this proto-ore, the weathered profiles are

deeper (MAC and MUT) and the weathered lithotypes are more homogeneous in terms of iron

content and gangue minerals. In these mines, the transition between the weathered to fresh rocks

is narrow and it is not possible to identify the medium or partially weathered lithotypes (Figure 2)

cross-section B.

As the weathering process is not highly effective over the quartzitic and amphibolitic itabirites, the

weathering profile could be shallower and the presence of partially weathered material more

frequent in mines where these proto-ores dominate (TAM, PIC, CMT and other). Also, the broad

boundary of these partially and weathered lithotypes could have different characteristics due to the

iron and gangue minerals content, apparent cohesion, and other associated characteristics

(Figure 2) cross-section A.

Figure 2 shows a typical cross-section in the surface decametre levels of canga, weathered hematite

and weathered rich itabirites.

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Figure 2 Presents a typical geological cross-section highlighting two different weathering

profiles interference patterns

The first hundred metres below the surface are composed of completely weathered itabirites

(goethite rich), and above are commonly found partially weathered itabirites or even some first

presence of fresh itabirites resulting from differential weathering processes conditioned by low

permeable zones (less fractured or banding direction not favourable to the water percolation). In

this zone, the fresh material behaves like a rock bridge to the overall slope stability, and the

thickness of this intermediary zone can vary from several metres to hundreds of metres depending

on the weathering effectivity. At greater depths, fresh itabirites predominate, interrupted by more

weathered zones associated with deep faults or shear zones.

2.5 WEATHERING PROCESS IMPLICATIONS FOR SLOPE STABILITY

For these weathering profiles the slope behaviour is a mix of fresh, partially, and weathered BIF,

varying in depths. The use of classical rock or soil mechanics must be carefully applied in order to

not under or overestimate the geomechanical parameters and slope stability behaviour.

In shallow mines, the approach to dealing with weak rocks and slope design has largely been based

on adopting soil mechanics principles and precedent experiences. However, slope failures and

recent studies by Costa (2009) and Sá (2010) suggest that some failure mechanisms have specific

characteristics for weak leached materials, and that the application of classical soil mechanics

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principles does not enable the failure mechanisms to be fully understood, while rock mechanics

concepts are also not fully applicable.

The current approach used to develop slope stability analyses in these types of weak rocks is based

on limit equilibrium analyses and, in some special cases, these have been supported by numerical

models. The weak rock shear strength parameters for these designs are normally based on a limited

number of intact rock laboratory tests, such as triaxial and direct shear, and supported by adopted

rock mass classifications. While this approach has been mostly successful, it would appear that it

may be conservative. Despite that, there have been examples of large slope failures not captured

by these design approaches that have had significant negative impacts on mine production.

For future proposed open pit mines, there will be an increase in the percentage of hard/fresh rock

exposed in deeper Vale mines, which poses new challenges to the geotechnical team. They may

need to develop slope designs and evaluate possible failure mechanisms in hard rock by using

classical rock mechanics principles, breaking some paradigms established over past decades

operating mainly in weak rocks.

For these hard rocks, it is crucial to ensure a good evaluation of the intact rock strength, as well as

the geotechnical characteristics of the discontinuities in order to understand the slope deformation

behaviour for quite a complex arrangement of different rock strength materials.

It is also important to determine the geotechnical parameters for the partially weathered rocks

(a mixture of hard and weak rocks) and to develop a method to better define and map the

boundaries along the weathering continuum.

3 VALE ITABIRITES GEOLOGICAL AND GEOTECHNICAL CHARACTERISATION

In Vale mines, itabirites receive a local denomination defined by the iron ore, non-iron minerals

(gangue) contents and by the rock strength. Hematitite has the richest iron content (more than 62%

iron) and itabirite has the poorest iron content (less than 62%). Itabirites are reclassified by the main

gangue mineral found in three main lithotypes: quartz or quartzitic, dolomitic or carbonatic,

amphibolitic or goethitic. Others can be described as specularitic or martitic, but these are not so

prolific in Vale mines.

Hematitite and itabirites are also sub-divided using the rock hardness rated in Vale technological

crusher laboratory tests used to simulate the industrial process. This test consists of crushing a

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known sample to less than 31.5 mm, then sieving it through a 6.35 mm sieve, resulting in three main

typologies: hard (more than 50% above 6.35 mm); medium (50% to 25% above 6.35 mm); and weak

(less than 25% above 6.35 mm). Table 1 presents the main geological characteristics used in Vale to

classify the studied BIF.

In general terms, the actual Vale crusher classification has been used as a guide to identify the

weathering level, where typology is defined as hard, representing the fresh to slightly weathered

material; partially weathered representing moderately weathered material; and weak representing

highly to completely weathered to residual soil. It could also be associated with intact rock strength

as hard representing extremely hard to medium hard material; medium as medium hard to medium

soft; and weak as medium soft to extremely soft material, as presented in the ISRM (1981) tables.

Characteristics for the different lithotypes are shown in Tables 1 and 2.

Not considering the controversy about the supergene (groundwater leaching) or hydrothermal

(hydrothermal water leaching) genesis for these large, rich, and weak Iron Quadrangle ore deposits,

these discussions have provided a large amount of data and field correlations. These studies have

been focused on mine planning, metallurgical applications and geological evolution, and a limited

amount of research has been done using these data for geotechnical purposes.

Using this knowledge and the Vale chemical–physical database, it is possible to associate the

geological information with the geotechnical characteristics.

Table 1 Research lithotypes characteristics and geological classification used in Vale mines

Lithotype Typology Fe% (total) Main gangue mineral Al2O3% Crusher test (% > 3.35 mm)

Hematitite Weak ≥ 62 – < 2.5 < 25

Quarzitic Itabirite

Fresh 30≤ % <62 Quartz < 2.5 ≥ 50

Partially weathered

30≤ % <62 Quartz < 2.5 25 ≤ % ≤ 50

Weathered 30≤ % <62 Quartz < 2.5 < 25

Amphibolitic Itabirite

Fresh 30≤ % <62 Amphiboles/goethite and quartz

< 2.5 ≥ 50

Partially weathered

30≤ % <62 Goethite and quartz < 2.5 25 ≤ % ≤ 50

Dolomitic Itabirite

Fresh 30≤ % <62 Dolomite, siderite, and quartz

< 2.5 ≥ 50

Goethitic Itabirite

Partially weathered

30≤ % <62 Goethite and quartz < 2.5 < 25

Argillaceous Itabirite

Weathered 30≤ % <62 Kaolinite, gibbsite, goethite, and quartz

≥ 2.5 < 25

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3.1 MAIN GEOLOGICAL CHARACTERISTICS

Typically, itabirites present a small mineralogical variety: hematite, martite, magnetite, specularite,

goethite and ochreous goethite are respectively the most important iron minerals; quartz, iron

dolomite, gibbsite and kaolinite are the main present gangue minerals; and talc, chlorite, pyrolusite

are the main accessory minerals. Several studies of Rosière et al. (1993, 1996 and 2001),

Lagoeiro (1998) and Pires (1995) describe the mineralogical and textural correlation with geological

association for several Iron Quadrangle mines.

For the researched lithotypes that represent approximately 75% of all typologies present in Vale’s

Iron Quadrangle mines, the main geological characteristics can be described as follows:

Weak hematitite (WHE) is the main supergene representative lithotype and the most extensive and

richest. It consists of thin (millimetre to centimetre) opaque/dark grey colour bands of hematite and

martite, with low resistance (friable) and high total porosity alternated by more consistent layers of

dark metallic bands of hematite and martite with higher relative resistance and lower total porosity.

The global porosity varies significantly from 25% to 30%, according to Costa et al. (2009), or even

higher, from 29% to 37% according to Ribeiro (2003). This lithotype represents a small

heterogeneity, but a sensitive anisotropy as defined by Costa (2009). In most cases, it represents

the tectonic foliation, but can also preserve the original banding.

Generally, these lithotypes are associated with synclines, and can be found in the lower surfaces

above the itabirites and can also occur in the shear zones and brittle failures presenting specularite

as presented in Costa (2009). Authigenic breccia is very common and can represent a load

deformation structure associated with the collapse. This collapse is imposed by the leaching process

and respective reduction in the itabirite volumes as exposed by Ribeiro (2003).

The grain size and shape of the minerals vary with the tectonical settings. The larger hematite

crystals are granuloblastic, the smaller are microplates, and the specularite presents lepidoblastic

texture.

In size, these iron minerals vary from 0.005 mm to 0.5 mm, with a maximum 0.1 mm as presented

in Rosière (2005).

The natural moisture content is closely linked to the rainy season, presenting an average of 15%.

The typical chemical composition is Fe (total) = 64%, SiO2+Al2O3<1%.

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Fresh quartzitic itabirite (FQI) is the typical banded iron formation and more common itabirite as

originally described. The non-iron band was totally metamorphosed to quartz (originally chert

crystal), and the iron band is composed of hematite, martite and martitised magnetite. The typical

chemical composition presented in Vale mines is Fe (total) = 43%, SiO2 = 36% and Al2O3 = 0.3%.

The banding is generally folded and the microtexture of the quartz layers is granuloblastic to

lepidogranuloblastic. These crystals are euhedral due to the metamorphic level and their sizes range

from 10 µm to 120 µm, and the hematite layers present tabular and granular shapes and grain sizes

between 6 µm to 80 µm. They have a very low void ratio (e < 5%) and their moisture content is 10%

as presented by Santos (2007).

These rocks are commonly found in great depths but can also be observed in lowly deformed and

unfractured zones where it is possible to observe sedimentary structures and the primary bending.

Partially weathered quartzitic itabirite (PWQI) is the moderately weathered quartzitic itabirite,

presenting a mineralogical composition similar to the FQI, except by the extensive presence of

goethite. The quartz layers present a higher total porosity, and, in some instances, the quartz bands

can be disaggregated by differential leaching (some layers can reach e > 40%). Changes in mineral

texture or crystal size are not recognised in these materials. The leaching intensification determines

an increase in the total porosity, iron content and the percentage of goethite and ochreous goethite.

However, the intact rock strength and grain cohesion decreases, especially in the quartz bands.

Weathered quartzitic itabirite (WQI) describes the completely or totally weathered quartzitic

itabirite. With the increase in the weathering process, total porosity also increases, and bulk density

decreases due to the silica leaching.

For this weak rock, the original mineralogy remains with the addition of goethite and gibbsite

resulting from the weathering process. However, there is an important increase in the iron content

driven by the silica leaching. The typical chemical percentages are Fe (total) = 54%, SiO2 = 12% and

Al2O3 = 2%. Macroscopically, these rocks present a friable, dark metallic grey colour band of

hematite and martite, and white to yellow friable quartz bands with a minor amount of goethite.

The quartz bands present highly leaching layers (friable), where the total porosity easily reaches

40%.

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Fresh amphibolitic itabirite (FAI) presents hematite, martite bands alternated with quartz, and

amphiboles (grunerite, tremolite, actonolithe and others) bands. The general band textures are

lepidogranuloblastic to granuloblastic and the crystal size is 30 µm on average.

The standard chemical composition is Fe (total) = 24%, SiO2 = 42% and Al2O3 = 0.4%. The original

mineralogy is preserved just in high depths where a typical brown-yellow colour is present. Usually,

the amphibole minerals change to fibrous goethite.

It is important to note that it is difficult to obtain fresh FAI which has not been influenced by some

mineral degradation specifically associated with the amphibole’s oxidation. The presence of

amphibole minerals is only observed in very high depth. In this paper, FAI is considered fresh if it fits

in the Vale crush test as a hard material and considered as slightly weathered material even if the

original amphiboles have changed to goethite.

Partially weathered goethitic itabirite (PWGI) represents the FAI partially weathered material and

the intact rock mass strength reduces significantly. The colour becomes yellow and it is noted that

there is an important increase in the total porosity and oxidation of iron ore minerals generating

goethite and ochreous goethite. These are shown in the several metres of the band between the

FAI and weathered goethite itabirite (WGI).

Weathered goethite itabirite (WGI) is characterised by the red and yellow colour caused by the high

goethite and ochreous goethite content presented mainly as cement. These also partially replace

the quartz crystal interlayer with friable quartz bands. They represent the totally weathered process

from the itabirites, quarzitic and amphibolitic, and are considered the more terrigenous lithotypes.

They are present in lower surface over WQI and MAI but can also be found in depth in open fractures

and shear zones.

Fresh dolomitic itabirite (FDI) is typically composed of millimetre to centimetre bands of euhedral

iron dolomite, responsible for the red colour and less percentage of iron carbonates and quartz.

These crystals vary from 2 µm to 15 µm. Non-iron bands are constituted of tabular hematite, martite

and martitised magnetite bands with crystal size varying from 5 µm to 20 µm. The most important

accessory minerals are sericite and chlorite. The total porosity is lower than 5% and the standard

chemical percentages are Fe (total) = 32%, CaO = 16% and MgO = 11%.

Weathered argillaceous itabirite (WAI) is characterised by the dark brown colour determined by

the high goethite and other clay mineral content, such as gibbsite and kaolinite. Spier (2005) argues

399

that they are formed by the effective leaching process over the dolomitic itabirites. The typical

banding is millimetre to centimetre composed of very fine crystals of martite hematite and goethite,

and bands of clay minerals. Some manganese minerals are common such as pyrolusite and

cryptomelanite. The total porosity is lower than 15% and the ρb = 2.64 t/m3. Chemical percentages

presented by WAI differ from the other weathered materials by the percentage of Al2O3 higher than

2.5%. However, the values of loss of ignition (LOI) (3.5 ≤ LOI < 5%) is the most important parameter

used to separate this typology from WGI (LOI ≥ 5%) and WQI (LOI < 3.5%).

The main visual characteristics of all evaluated lithotypes are presented in Figure 3, and Table 2

shows the main geotechnical characteristics.

Ta ble 2 M

ain geotechnical characteristics presented by studied material

Lithotype Typology

Weathering grade

Iron content (%

) Field intact rock strength grade

Total porosity (%

) Bulk density

(t/m3)

UCS (M

Pa)

Hematite

Weathered

W5-W

6 64

R0-R2 15-35

3.2 U

CS < 5

Quarzitic

itabirite Fresh

W1-W

2 40

R5-R6 7

3.4 150 < U

CS < 450

Partially weathered

W3-W

4 40-50

R3-R4 7-15

2.7-3.4 1 < U

CS < 150

Weathered

W5-W

6 50

R0-R2 15-35

2.7 U

CS < 5

Amphibolitic

itabirite Fresh

W1-W

2 30-35

R5-R6 10

3.0 150 < U

CS < 450

Partially weathered

W3-W

4 30-40

R3-R4 10-15

2.6-3.0 5 < U

CS < 150

Dolomitic

itabirite Fresh

W1

30-40R5-R6

53.1

150 < UCS < 450

Goethitic itabirite

Weathered

W5-W

6 52

R0-R2 15-35

2.6 U

CS < 5

Argillaceous itabirite

Weathered

W6

52 R0-R1

<15 2.6

UCS < 1

400

401

Figure 3 Itabirites visual macro characteristics and weathering profile association

3.2 PHYSICAL INDEX AND UCS RELATIONSHIPS

Slope stability in fresh itabirites is generally controlled by the shear strength of the discontinuities.

However, in the weathered itabirites the reduction in the intact mass strength imposed by the

weathering process can change this setting, and slope stability would become controlled by the

intact rock strength and associated heterogeneity. For these rocks, the weathering limit where the

intact rock strength reduces to shear strength, equal or lower than the discontinuities (structures),

remains unknown.

What is understood is that, in terms of leaching, the weathering process takes place differently for

each compositional rock type. During the weathering process, there is a critical reduction in the

intact rock strength as a result of three components, namely: mineralogical changes, increase in the

total porosity, and reduction in the grain face contacts. All three have been shown by Gupta and

Rao (2001) and others to directly influence intact rock strength and, consequently, the UCS test

results.

Partially weathered

quartzitic itabirite

Partially weathered

goethitic itabirite

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UCS test results are largely used for fresh rock geotechnical evaluations. Several techniques have

been used to determine the intact rock strength. Some are based on laboratory (UCS), point load

test (PLT) or field tests (Schmidt hammer), and others are based on tactile visual or qualitative

estimation and its comparison with characteristic tables. Hack & Huisman (2002) and Vale’s

experiences suggest that, in many cases, the use of characteristic tables have proved to be more

than adequate and reliable to correlate with intact rock strength than the UCS values. However,

these simple estimation methods must be calibrated using laboratory testing in order to associate

the field strength estimation with the intact rock strength value.

The establishment of a standard characteristic table for Iron Quadrangle BIFs has been proposed by

several authors (Zenobio 2000; Zenobio & Zuquete 2004; Castro et al. 2013; and Araujo et al. 2014).

These authors generally adapt classical geomechanical classifications using, for example, specific

field procedures and tests, calibrated with laboratory tests.

For fresh rock in Vale mines, adapted strength field characterisation tables, crush tests and a limited

number of intact rock UCS tests have been used to determine the intact mass strength and guide

slope stability evaluation. Although this approach has been satisfactory, the increase of fresh rock

in deeper mines demands a higher level of information and more reliable data. Based on this

demand, several studies were initiated using the Vale geological/geotechnical database in order to

improve the data used for geotechnical purposes. The main approach has been to correlate the

available chemical and physical parameters from the different itabirites to the geotechnical index

proprieties. Proper correlations could be also used as a guide to predict BIF geotechnical

characteristics added to the existing techniques.

Index propriety interrelationships have been established worldwide for several rock types. Gupta

and Rao (1998 and 2001) and Irfan & Dearman (1978) mainly focused on granitic rocks and

established correlations between the physical proprieties of the rocks to the weathering process for

geotechnical evaluations. Even assuming geological differences between studied rocks and BIF,

these correlations are a useful guide for first consideration.

Specifically, for BIF, a similar access was established by Aylmer et al. (1978) evaluating the bulk

density, grade and porosity for Mount Tom Price iron ore and concluded that the iron grade and the

bulk density have a good correlation. However, the accuracy was largely affected by the high and

variable porosity.

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Also, according to Box & Reid (1976) for the iron ore formation from Cockatoo Island, the true

specific gravity could be expressed as a function of iron content. However, due to the complexity

and multiple factors, the same influence could not be established for porosity. Thomson (1963)

concluded that for iron ore samples from Australia, a theoretical hematite–quartz curve can be used

for bulk density definition and can present an approximate iron content calculation. Nel (2007)

established for Sishen South iron deposits that porosity is directly correlated to the dry and bulk

density providing a reliable calculation index. These works are based on the same correlation and

present similar findings for different worldwide BIFs deposits.

In Brazil, studies by Ribeiro et al. (2014), Santos et al. (2005) and Santos (2007) evaluating the

association between bulk density (ρb) and iron content for Vale’s iron formation concluded that

there is a linear positive correlation between the total iron content and the bulk density. Also, they

argued that the weathering process has an important influence on the bulk density and iron content

dispersion imposed by the total porosity. Figure 4 illustrates the correlation between the bulk

density and iron content for fresh (blue diamonds) and weathered itabirites (red squares). These

studies find a similar correlation for Iron Quadrangle itabirites, as presented in worldwide iron

deposits.

Based on historical research and findings, available data in Vales’ database and the availability of

laboratory tests, bulk density was selected for preliminary studies searching for interrelationships

with UCS values initially for fresh itabirites.

Figure 4 Bulk density and iron content correlations for fresh and weathered itabirites from the

Iron Quadrangle, as presented by Santos (2007)

To establish this correlation, some drawbacks were considered and evaluated:

404

• The ρb dispersion between the three main fresh itabirites presented a small range (2.7t/m3

to 4 t/m3).

• The ρb is highly influenced by porosity and this relationship is still not determined.

• The difficulty in obtaining fresh samples that do not present any type of weathering

change.

• The iron content variability due to the sample size.

In addition, as proposed by Hack & Huisman (2002), the use of UCS tests to determine the strength

of the material could present three main problems that could influence the results:

• The sample could include small discontinuities or flaws.

• The samples tend to have better mechanical characteristics associated with the sampling

technique (i.e. the best samples are selected).

• For anisotropic material, the results are sample orientation dependent.

To this list, one could add some geological features, such as intense folding, fill material, weathered

levels, veins, and other features that do not represent the typical rock mass material resulting to

invalid or biased results.

To avoid these drawbacks and in order to have more reliable results some assumptions were made

during the methodology development.

3.3 METHODOLOGY USED

In order to verify the correlation between the UCS values and the ρb of fresh itabirites, several

samples of FDIs, FQIs and FAIs were sent to Brazilian and Australian laboratories to undertake UCS

tests and ρb determinations. The laboratory UCS tests were undertaken in accordance with ASTM

D2938-95 (2002). Bulk density determination was based on the instantaneous water immersion

technique, while natural and dry mass per volume calculations were made using the samples’

dimensions.

For the test work undertaken in Australia, 15 samples were obtained from original 77.8 mm

diamond drill core subsampled for 20 mm diameter with lengths varying from 40 mm to 50 mm. For

the test work undertaken in Brazil, seven samples were used comprising the original 77.8 mm drill

cores with diameters varying from 50 mm to 76 mm and lengths from 110 mm to 205 mm.

405

To characterise the anisotropy and heterogeneity influence on the UCS test, the samples were

orientated in three directions, referred to the angle (βangle) and measured from the loading direction

to the banding dip. Loading parallel to banding (β = 0°); loading perpendicular to banding (β = 90°);

and banding situated at an oblique angle (β = ±45°) to the loading. Table 3 lists the samples tested,

summarises the results and presents global and partial averages. For samples with β value of 90°, a

better correlation was expected due to the angle of anisotropy which was considered unlikely to

influence the test results. In addition, at this angle, the highest intact mass strength was expected

to be obtained.

Table 3 Summary of test results for all fresh itabirites

ID Mine Lab Lithology UCS

(MPa)

Bulk

density

(t/m³)

Average

UCS

(lithology)

Average

bulk

density

(lithology)

Average UCS

(lithology-β)

Average bulk

density

(lithology-β)

1 CMT AUS FAI (0°) 351 3.24

250 3.42

288 3.63 2 CMT AUS FAI (0°) 145 3.68

3 CMT AUS FAI (0°) 341 3.77

4 CMT AUS FAI (0°) 313 3.81

5 CMT AUS FAI (90°) 173 3.04

199 3.15 6 CMT AUS FAI (90°) 256 3.42

7 CMT AUS FAI (90°) 169 3

8 MAC AUS FDI (0°) 68 3.08

99 3.16

79 3.06 9 MAC AUS FDI (0°) 85 3.05

10 MAC AUS FDI (0°) 83 3.06

11 JGD BR FDI (45°) 69 2.71

94 3.19 12 JGD BR FDI (45°) 126 3.19

13 JGD BR FDI (45°) 86 3.68

14 JGD BR FDI (90°) 174 3.34 174 3.34

15 ABO AUS FQI (0°) 166 3.5

234 3.56

184 3.63 16 ABO AUS FQI (0°) 202 3.76

17 JGD BR FQI (45°) 128 3.25 113 3.32

18 JGD BR FQI (45°) 97 3.39

19 JGD BR FQI (90°) 359 3.37

320 3.65 20 ABO AUS FQI (90°) 379 3.79

21 ABO AUS FQI (90°) 288 3.69

22 ABO AUS FQI (90°) 253 3.76

406

The average values used in Vale for iron content, ρb, and total porosity (presented in Table 2) were

used as standard for these index propriety evaluations.

Considering the total porosity influence on bulk density results, studies of Box & Reid (1976), Aylmer

et al. (1978), Ribeiro (2003) and Santos et al. (2005), argued that the dispersion observed in the

results could be associated with this parameter variation. In this research stage, the influence of

total porosity was not evaluated.

However, for fresh itabirites, a minor total porosity variation is expected. This statement is

supported by Ribeiro et al. (2014) which argued that for FQI from the Conceição Mine, the total

porosity obtained by regression correlations presented values varying from 6.1% to 6.8%, and the

ρb varied from 3.3 t/ m3 to 3.9 t/ m3. These fit into the same range observed in the tested samples.

These values were confirmed by Moraes et al. (2014) who verified for the FQI from the Abóboras

Mine that the total porosity was 7.4% for a ρb = 3.07 t/ m3; for the Galinheiro and Sapecado Mines,

the total porosity was 9.2% for a ρb = 3.28 t/m3; and for FDI from the Gongo Soco Mine, the porosity

was 5.8% for a ρb = 3.11 t/m3. Based on these studies, for fresh itabirites, it is expected that there

will be a minor porosity influence on the results.

Considering the dispersion induced by the discontinuity presence and geological features, the

samples (pre-test) and the failure surface (post-test) were evaluated and photographed in order to

identify and characterise the main geological feature responsible for the failure, and that could have

affected ρb and/or UCS results.

This evaluation resulted in a sample divided into two groups:

• Typical, presenting proper lithotype characteristics such as,

FDI – typical red colour from dolomites bands and rhythmic millimetre to centimetre

banding.

FQI – typical white colour from quartz bands and rhythmic centimetre to millimetre

banding.

FAI – typical brown-yellow colour from goethite and quartz bands and rhythmic

centimetre banding.

• Not typical, and which do not fit into the required lithotype standards for intact mass strength

evaluation are geological features such as: intense folding, pre-existing close fracture,

brecciated, line of pores, veins (quartz or calcite), and specularite levels. It is important to note

407

that some of these geological features are commonly observed on these lithotypes and are

generally subordinate to the βangle. The tested samples, photographs and identification are

shown in Table 4.

Table 4 Sam

ples geological features and characterisation

Sample

number

Id Photo pre-test

Photo post-test Sam

ple

number

Id Photo

pre-test

Photo

post-test

1 N

ot typical

(quartz vein) 6

Typical

2 N

ot Typical

(fracture) 7

Not typical

(porous lines)

3 Typical

8 Typical

4 Typical

9 Typical

5 N

ot typical

(fracture)* 10

Typical

408

11 N

ot typical

(quartz vein) 17

Not typical

(folded)

12 Typical

Not avalable

18 Typical

13 N

ot typical

(brecciaed) 19

Not typical

(folded)

14 N

ot typical

(brecciaed) 20

Typical

15 Typical

21 Typical

16 Typical

22 Typical

409

410

The first approach carried out by research, aimed to evaluate all samples in order to identify

possible drawbacks that may influence the results. Due to the preliminary nature of this first

testing campaign and the small number of tests, no statistical evaluation was performed.

4 PRELIMINARY FINDINGS

The first evaluation considered all samples in order to determine the overall UCS value

distribution in terms of ρb variation is shown in Figure 5.

Figure 5 Correlation between bulk density and UCS for different compositional itabirites and

anisotropy direction

For this distribution, the UCS values were divided into five categories: low (UCS ≤ 50 MPa),

moderate (50 MPa < UCS ≤ 150 MPa), high (150 MPa < UCS ≤ 250 MPa), very high

(250 MPa < UCS ≤ 350 MPa) and extremely high (UCS > 350 MPa). The bulk density was divided

into low (ρb < 3.3 t/m3) and high (ρb > 3.3 t/m3).

At this stage, is not possible to present quantitative results due to the reduced number of

samples tested and the large number of non-typical samples. However, some qualitative results,

due to the influence of the geological features and index proprieties relationships, are

recognised.

411

4.1 GEOLOGICAL FEATURES INFLUENCE

The presence of non-typical geological features presented in Table 4 showed some possible

drawbacks that may have interfered with the UCS and/or ρb results:

• Samples 2 and 5 present a previously closed fracture.

• Samples 13 and 14 present a brecciated material.

• Samples 17 and 19 presented folded banding.

• Samples 1 and 11 present quartz and calcite veins.

• Sample 7 presents large pore alignment.

By evaluating these sample results and correlating them with the observed geological features,

it is possible to identify some relationships:

• For samples 2, 7 and 5, due to the presence of fracture and alignment of large pores,

the UCS results were lower than expected, and for samples 7 and 5, a ρb reduction was

noted due to the presence of pores.

• For samples 1 and 11, due to the presence of quartz and calcite veins, a lower ρb was

evident. However, no considerable UCS reduction was observed.

• For samples 13 and 14, due to the presence of brecciated material, a higher ρb was

evident. However, it was not possible to discuss the UCS results.

• For samples 17 and 19, the presence of folded banding seems to not affect UCS or ρb

results.

Based on these evaluations, it was assumed that the presence of veins, closed fractures and

brecciaed material could produce biased data. For folding considerations, more evaluations are

necessary.

4.2 UCS AND BULK DENSITY FRESH ITABIRITE CORRELATION

Overall, the distribution of results presents a trend that is possible to see in Figure 5. However,

an important dispersion is noted and some considerations about this variation are:

• The FQI presented the higher range in UCS values, from extremely high to moderate.

However, the ρb was consistently high.

412

• The FAI ranged from very high to moderate UCS values with ρb varying from low to

high values. Excluding the non-typical samples 2, 5 and 7, the range reduced to very

high UCS and high- ρb values.

• The FAI and FQI average presented closer UCS and ρb values. However, considering the

anisotropy (β), FQI (90°) presented higher values.

• The low UCS results for FAI (90°) samples 7 and 5 could be partially associated with the

low ρb results.

• For the FDI, UCS values vary from high to moderate, presenting the lowest UCS average

compared to other lithotypes. However, ρb presents a high dispersion if the non-typical

samples 13 and 14 were included. Considering the anisotropy (β) influence in UCS and

ρb, the results for 90° samples presented higher values and were respectively lower for

45° and 0° for the two indices.

Evaluating the results per lithotype, it is possible to recognise some relationships about the index

parameters:

• For the FDI, the presence of brecciaed material increased the ρb and the UCS effects

were not well determined. However, these features must be considered separately

due to the directed result influence.

• For the FQI, the presence of folded banding tends to not influence the UCS values.

• For the FAI, the presence of large pores affects the UCS and ρb results.

• For all materials, closed fracture, and alignment of pores influence ρb and UCS results

especially when the β is different to 90°.

• For all materials, the presence of quartz and calcite veins directly influence the bulk

density values. However, the influence of these features for the UCS results is

associated with the anisotropy (β) direction.

5 CONCLUSION AND NEXT STEPS

Despite only 22 samples being tested from three lithotypes, the preliminary findings present the

same trend demonstrating similar results as obtained for other authors researching different

iron ore deposits. As Hack & Huisman (2002) previously proposed, this represents an important

interrelationship that could be used for the physical index characterisation of fresh itabirites.

413

These evaluations have emphasised the importance of identifying even the simplest geological

characteristics and/or structural features in order to have a proper correlation able to represent

the different types of fresh itabirites. Also, they suggest that the correlation between the UCS

and ρb could be an important pathway to infer the UCS values. The results outline the need for

more laboratory tests to support a statistically valid correlation from the Vale ρb database and

UCS test.

To achieve this primary aim, the number of UCS and ρb tests for fresh rock must be increased to

populate the graph and achieve the appropriate minimum sample size.

Further studies must be undertaken to determine the total porosity and iron content for the

fresh materials relationship and to investigate the influence of the heterogeneity associated

with the typical banding. After establishing these relationships for fresh materials, the study is

then likely to move forward to partially and completely weathered itabirites.

ACKNOWLEDGEMENTS

The authors thank Vale S.A. for permission to present this paper and their sponsorship of the

research undertaken by the Australian Centre for Geomechanics, The University of Western

Australia. The author’s appreciation extends to Vale colleagues Pedro Apolonio dos Santos, Diniz

Ribeiro, Fernando Machado, Eduardo Motta; and Ariel Hsieh and Eduardo Ansaloni for their

contributions.

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419

APPENDIX II

This paper was presented and published in 2015 at Slope Stability 2015 International Symposium

proceedings- SS03, Cape Town, South Africa.

WEATHERED BANDED IRON FORMATIONS IN VALE IRON ORE MINES ON

THE WESTERN SIDE OF THE IRON QUADRANGLE, BRAZIL: WEAK

HEMATITITE AND WEATHERED ARGILLACEOUS ITABIRITE GEOTECHNICAL

CHARACTERISTICS AND IMPLICATIONS OF MATRIC SUCTION EFFECTS ON

SLOPE STABILITY

COSTA, T.A.V.; DIGHT, P.M.; MERCER, K.; AND MARQUES, E.A.G.

ABSTRACT

Brazilian banded iron formations (BIF), locally known as itabirite, is the main host rock for iron

ore and hematitite is the high-grade ore in the Iron Quadrangle mines, Brazil. Their geneses are

controversial, but it is understood that weathering processes are principally responsible for

reducing the original high intact rock strength, and for generating deep weathered profiles with

low strength rocks commonly observed in Vale’s iron ore mines.

This paper presents a review of the geological and weathering characteristics and related

geomechanical and strength assessments of two important lithotypes commonly associated with

slope failure mechanisms in Vale mines: weathered argillaceous itabirite (WAI) and weak

hematitite (WHE). These studies are based on early research results from the PhD thesis being

undertaken by the lead author at the Australian Centre for Geomechanics, School of Civil,

Environmental and Mining Engineering, The University of Western Australia, sponsored by Vale

S.A.

The results show that minor changes in bulk density, texture, and the percentage of clay minerals

lead to distinct geomechanical characteristics in terms of plasticity, intact mass strength and

mainly matric suction effects. These changes influence both the shear strength and intact rock

behaviour and consequently the overall slope stability as shown in a preliminary insight into the

open pit slope design for short and long-term considerations.

420

1 INTRODUCTION

Vale S.A. Iron Quadrangle mines in Brazil generally exhibit weathering profiles that can reach

over 400 m in depth, and for these mines, the weathered rocks exhibit soil type behaviour. The

historical slope design approach and identification of possible failure mechanisms used by Vale

is largely based on applying classical soil mechanics principles as well as historical trial and error

experience. While this approach has been largely satisfactory, there are nevertheless several

key geotechnical issues that are still not well understood, and there have been continued

instances of large slope failures which have resulted in significant disruptions to the mines.

A PhD research project is currently being undertaken by the lead author of this paper at the

Australian Centre for Geomechanics, School of Civil, Environmental and Mining Engineering, The

University of Western Australia, sponsored by Vale S.A. The main objective of the thesis is to

investigate how the weathering process affects the geological and geotechnical characteristics

ranging from hard and fresh to weak and completely weathered rocks. This will ultimately

facilitate improved optimisation of final pit slope design and promote a better understanding of

potential failure mechanisms and, in turn, lead to a reduced risk of slope failure and hence, an

improvement of operational productivity and safety of Vale’s mines.

This paper shows the first results of this research and begins by presenting the geological setting

of the Iron Quadrangle mines in Brazil and goes on to summarise the key geological and

geotechnical characteristics of the weathered argillaceous itabirites and weak hematitites. The

general weathering process found at the mines is also described, followed by a review of the

implications of the weathering process on current slope stability. Material characterisation

studies which included laboratory tests for determination of bulk density, particle size

distributions (PSD), Atterberg limits, soil-water characteristic curves (SWCC) as well as saturated

and unsaturated direct shear tests are also presented. These test results are summarised and

evaluated to determine a correlation between key parameters and matric suction effects.

The results were applied in slope stability analysis using different pore pressure and matric

suction approaches. The paper ends with a discussion and conclusions on the possible

implications of these results on Vale’s mine slope stability.

421

2 GEOLOGICAL SETTING

2.1 LOCALISATION

The mines under study are part of Vale’s south ferrous division (DIFL) and comprise of fifteen

mines located in the centre of Minas Gerais, Brazil: Águas Claras (MAC), Mutuca (MUT), Mar

Azul (MAZ), Capão Xavier (CPX), Tamanduá (TAM), Capitão do Mato (CMT), Abóboras (ABO),

Galinheiro (GAL), Sapecado (SAP), Pico (PIC), Córrego do Feijão (CFJ), Jangada (JGD), João Pereira

(JPE), Alto Bandeira (BAN) and Fábrica (FAB). The location of these fifteen mines is shown in

Figure 1.

2.2 REGIONAL GEOLOGICAL SETTING

The focus area is in the western part of the Iron Quadrangle located in the southern border of

São Francisco Craton. The mines are sited along the Moeda and Don Bosco Synclines and Curral

Homocline ranges.

The Iron Quadrangle is delineated by a roughly quadrangular arrangement with

Paleoproterozoic banded iron formations (BIF) of the Minas Supergroup, as proposed by Dorr

(1969), composed of hundreds of metres of iron ore rich metamorphic rocks belonging to the

Itabira Group/Cauê Formation. The Minas Supergroup comprises, from bottom to top: the

Caraça, Itabira, Piracicaba and Sabará groups; superimposed by the Itacolomi Group.


domes of Archean and Proterozoic crystalline rocks as studied by (Machado et al. 1989;

Machado & Carneiro 1992 and Noce 1995).

The regional structure is the result of two main deformational super-positional events as

described in Chemale Jr. et al. (1994). The first produced the nucleation of regional synclines in

the uplift of the gneissic domes during the Trans Amazonian Orogenesis (2.1–2 Gyr); and the

second is related to an east–west verging thrust fault belt of Pan African/Brazilian age (0.8–

0.6 Gyr) described by Marshak & Alkmim (1989). This event deformed the earlier structures and

was mainly responsible for the deformational gradient.

Locally the main iron ore deposits of DIFL/Vale are in the western low strain and green schist

metamorphic grade region as described by Hertz (1978). The geological settings are shown in

Figure 1.

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Figure 1 Mine locations and Iron Quadrangle geological settings (modified from Morgan et

al. 2013)

2.3 ITABIRITES GEOLOGICAL SETTINGS

The Iron Quadrangle BIF are mainly metamorphic rocks presenting a millimetre to centimetre

rhythmic alternation banding of iron and non-iron minerals recrystallised from original chert or

jasper bands.

Compositional metamorphic banding is the most typical characteristic that defines a strong

heterogeneity and anisotropy. This variation could be controlled by the original sedimentary

bedding, tectonic setting, metamorphic grade, hydrothermal or supergene processes. However,

the superposition of these processes causes partial or total mineralogical and textural changes,

making it difficult to identify the main banding control.

Dorr (1969) defined, for this type of iron deposit, two main lithologies: hematite or hematitite

the high-grade ore (Fe ≥ 64%); and itabirite – the low-grade ore (Fe < 64%), which are divided

into three main compositional lithotypes: quartzitic, dolomitic and amphibolitic. From these

proto-ores, the tectonic, metamorphic, and weathering process changed in different ways and

magnitude resulting in multiple settings of iron ore lithotypes.

The origin of itabirites and associated high-grade hematitite orebodies remains controversial


For the friable orebody geneses, some authors agree on a supergene process and residual

itabirite enrichment, leaching the gangue mineral by surface waters.

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3 THE WEATHERING PROCESS IN IRON QUADRANGLE MINES

In this paper, the weathering process and supergene concentration sequence have been

considered as being the main phenomena responsible for the chemical and physical changes, as

described by Ribeiro (2003), Ribeiro & Carvalho (2002) and Spier (2005) and are directly

responsible for the reduction of the original itabirite strength. It should be noted that there are

other geological processes, such as tectonic events and metamorphism that could reduce the

original rock strength.

In terms of highly to completely weathered rocks, these low strength rocks could also be defined

as soft, friable or weak, and are invariably the result of alteration of the original fresh rock

imposed by weathering processes, particularly to completely leaching as proposed by Dorr

(1969, and 1973), Boulangé et al. (1997), Ribeiro & Carvalho (2002), Spier et al. (2006),

Ramanaidou (2009) and Ramanaidou and Morris (2010). However, metamorphic, and tectonic

events cannot be ignored as suggested by Pires (1979 and 1995).

Due to the effectivity of the weathering process in tropical climates induced by a high

temperature variation range and a high pluviometric index, in addition to the favourable

topography featured, the weathering profile can often present more than 200 m and reach over

400 m in depth. Also, the high permeability of BIF rocks, in association with structural feature

control such as syncline and anticline configurations and the presence of high banding angles

and extensional fractures, can facilitate the superficial and groundwater penetration and

circulation, increasing chemical and physical weathering and, thus, the weathering profile.

As presented by Ramanaidou & Morris (2010) the supergene iron enrichment and subsequent

strength reduction can be divided into two main processes. The supergene mimetic mechanism,

that occurs below the watertable as described by Morris et al. (1980) and Morris (2003) that is

associated with the structures, topography, and climate, and is responsible for producing deep

and large iron rich deposits. The second process was denominated as supergene lateritic

weathering and is a result of ganga minerals dissolution and iron reconcentration above the

watertable. This process produces several weathered profiles characterised by different relative

iron enrichment and ganga minerals composition.

Mineral changes are not the only effect of the weathering process. The total porosity increase

is also recognised as an important change observed in those rocks. Studies by Morris (2003) and

Taylor et al. (2001) argue that the supergene process can reduce the thickness of the BIF by 32 %

to 40%, and increase the total porosity from 6% to 30%, as described by Mourão (2007).

424

These studies suggest that the total porosity increase, and the mineral changes are the main

changes induced by the weathering processes that produce an effective softening in the original

fresh rocks. The leaching of the non-iron minerals provides the most important void increase on

non-iron bands while mineral changes are responsible for the total porosity increase on iron

bands. The remaining iron and non-iron bands present a high total porosity that could be

cemented by secondary iron oxides or hydroxides depending on various factors.

These materials present weak strength caused mainly by high total porosity; however, the

relative iron enrichment promotes an increase in bulk density due to iron minerals

concentration. As both parameters are important in determining the rock strength, it is crucial

to understand the balance between the iron ore concentration (which affects the original bulk

density) and the total porosity increase, in order to determine the most influential propriety

affecting rock strength.

As the weathering processes develop (close to the surface and in high water flow rate areas)

another important effect noted is the increase in percentage of hydroxides (ochreous goethite)

and clay minerals (kaolinite and gibbsite). The presence of these minerals in typical weathered

itabirites can change the geomechanical behaviour due to the increase in the rock plasticity or

filling the secondary porosity.

The weathering as a continuum and multiple conditioning processes generates a range of

different materials (lithotypes) for each compositional itabirites and the weathering profiles

present specific characteristics and boundaries between the different weathered levels. In

general, weathering processes are more effective over dolomitic itabirite due to the high

solubility of the iron dolomitic bands. In mines with this proto-ore (e.g. MAC, CPX and MUT), the

weathered profiles are generally deeper and the weathered lithotypes are more homogeneous

in terms of iron content and gangue minerals. In these mines, the transition between weathered

to fresh rocks is abrupt and it is difficult to identify the partially weathered material (Figure 2B).

As the weathering process is not highly effective over the quartzitic and amphibolitic itabirites

the presence of partially weathered material is more frequent and can reach tens of metres (e.g.

TAM, PIC and JPE). Also, the broad boundary of these partially weathered lithotypes could have

different characteristics for each type of compositional itabirite due to the gangue minerals

content, structures associated and topography (Figure 2, cross-section A).

Figure 2 shows a typical iron ore mine geological cross-section that presents on the surface

several metres of covering itabirites and hematitites. In mines where the geology and

topography are favourable to the supergene mimetic mechanism as proposed by Ramanaidou

425

& Morris (2010), large bodies of weak hematitite (WHE) are occurring, which can reach more

than 400 m depth. In areas where this process was not effective, the first hundred metres below

the surface are mainly composed of weathered rich itabirites (WRI), weathered goethite

itabirites (WGI) and weathered quartz itabirites (WQI). The decrease in iron content increasing

depth could be associated with the supergene lateritic weathered enrichment, as proposed by

Ramanaidou & Morris (2010).

For a typical weathering profile as presented in Figure 2 (cross-section B), the first passage of

partially weathered or fresh material is reached at depths exceeding 400 m although profiles as

presented in Figure 2 (cross-section A) where it can be observed partially or fresh material even

on surface and below 400 m is composed mainly of fresh and partially weathered rocks.

Figure 2 Typical geological cross-section highlighting two different weathering profile

interference patterns

3.1 IMPLICATIONS OF THE WEATHERING PROCESS ON THE STABILITY OF WEAK

ROCK SLOPES

Currently in DIFL/Vale mines, more than 70% of exposed slope rocks are weak and shallow

rotational shear failures at the batter or multi batter scale are the most common failure

mechanisms associated with these weathered rock slopes. Large-scale slope failures are rare

and more complex, composed of rotational and planar surfaces, the last ones controlled by

anisotropic planes with reduced shear strength. Further, secondary influences such as high

426

stress concentrations at the toe of slopes and discontinuities may also act as contributing

factors.

The current approach used to undertake slope stability analyses in these types of weak rocks is

based on limit equilibrium analyses using the Mohr–Coulomb shear strength criterion and, in

some special cases, supported by numerical models. The shear strength parameters for these

designs are normally based on a limited number of intact rock laboratory tests which are mainly

saturated triaxial and direct shear and supported by adapted rock mass classifications. While

this approach has been mostly successful, it would appear from experience that they are

conservative. Despite that, there have been examples of large slope failures not captured by

these design approaches that have had significant negative impacts for mine production.


tectonic settings of the weak rocks that are very often located at the toe of high slopes, where

the stress is concentrated. To address this issue, Vale geotechnical engineers have historically

designed a buttress of ore to be left at the toe of the wall, resulting in a loss of ore reserves, as

suggested in Costa et al. (2009).

In addition to the classical soil mechanics approaches, several weak material characteristics such

as total porosity, bulk density, grain size and shape, mineralogy, and anisotropy have been

evaluated and considered in relation to the observed slope stability behaviour. Describing and

correlating these parameters remains a key challenge to the author due to the difficulties

associated with sample acquisition, testing, scale effect and sample representativeness.

An important characteristic generally neglected for iron formations due to the high permeability

and low plasticity observed on those materials is the matric suction effect on slope stability.

However, a minor presence of clay minerals can influence the slope stability as presented by Lu

& Likos (2004) which argue that the matric suction contributes to the cohesion and increases

the shear strength of unsaturated soils. Studies by Grgic et al. (2005) on iron formations

suggested that an improvement on the strength and cohesion of partially saturated oolithic iron

ore can occur due to the negative pore water pressure effects.

It has been noted from work outside the mining industry that the shear strength of a soil with

negative porewater pressure can increase the stability of slopes, especially those having shallow

but steep surfaces as supported by Fredlund et al. (1978). So far, matric suction effects have not

been considered in Vale open pit slope stability evaluations where effective stress parameters

are obtained by conventional saturated laboratory testing. This approach seems to be adequate

for high pluviometric index areas where long-term slope stability is required. However, it tends

427

to be too conservative when applied to the design of short-term slopes above the water level.

The use of unsaturated or partially saturated soil mechanics is therefore necessary to correct

the balance between safety and economical slope design.

In Iron Quadrangle mines, studies by Soares (2008) and Ventura & Bacellar (2012) for waste rock

showed the importance of suction effects on iron ore mines’ slope stability analyses, especially

for shallow surfaces. However, no studies focusing on BIF have been published yet.

4 WEATHERED ARGILLACEOUS ITABIRITE AND WEAK HEMATITITE

GEOLOGICAL AND GEOTECHNICAL CHARACTERISTICS

In Vale mines, BIF receive a local denomination defined by the iron ore and non-iron mineral

(gangue) content, and by the rock strength. Hematitite has the richest iron content (more than

62% iron) and itabirite has the poorest iron content varying from 54 to 62%. Itabirites are

reclassified by the main gangue mineral found in three main lithotypes: quartz or quartzitic;

dolomitic or carbonatic; amphibolitic or goethitic. Others can be described as manganese or

specularite, but these are not so prolific in Vale mines.

Hematitite and itabirites are also subdivided using the rock hardness rated at Vale by

technological crusher laboratory tests used to simulate the industrial process. This test consists

of crushing a known sample to less than 31.5 mm, then sieving it through a 6.35 mm sieve,

resulting in three main typologies: hard (more than 50% above 6.35 mm); medium (50 to 25%

above 6.35 mm); and weak (less than 25% above 6.35 mm).

In general terms, the actual Vale crusher classification can be used as a guide to identify the

weathering level and the strength index (hardness), where typology is defined as ‘hard’

representing the fresh to slightly weathered material; ‘medium’ representing moderately to

highly weathered material; and ‘weak’ representing completely weathered to residual soil. It

could also be associated with the field intact rock strength as ‘hard’ representing extremely hard

to medium hard material; ‘medium’ as medium hard to medium soft; and ‘weak’ as medium soft

to extremely soft material, as presented in the ISRM (1981) tables.

The weak hematitite (WHE) and the weathered argillaceous itabirite (WAI) represent some of

the lower-strength resistance BIF present in Vale mines and the main geotechnical characteristic

of these weak materials is low rock matrix strength (UCS < 5 MPa) ranging from extremely weak

(R0) to weak (R2) overlapping with very stiff soil (S5) to hard soil (S6) as suggested by Martin &

Stacey (2013) that include these materials in a boundary zone with rock and soil behaviour.

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The importance of characterisation and understanding the slope stability behaviour of these two

weak materials is due to different reasons. For WHE as the main orebody (economic and areal

distribution), it is especially important for Vale mines in order to reduce the buttress left in the

toe of final slope for a proper balance between safety and productivity. For WAI, the lower

strength band composed by clay mineral works as a slippery surface which controls several

failure mechanisms. Also, typical soil characteristics such as matric suction and plasticity can

influence the increase or decrease of slope stability.

The main visual characteristics of the WHE, WAI and the mix of these materials denominated

rich weathered argillaceous itabirites (RWAI) are presented in Figure 3.

(A) (B) (C)

Figure 3 A (left), WAI: note the presence of kaolinite and gibbsite level (white). B (centre),

RWAI: levels of hematite interplayed by clay levels. C (right), WHE: layers of less

porous hematite with more porous hematite

The main geological and geotechnical characteristics of these lithotypes can be described as

follows:

4.1 WEAK HEMATITITE

WHE presents the larger and richest supergene ore bodies in iron ore mines with iron content

higher than 62% and SiO2 + Al2O3 lower than 2.5 %, and loss of ignition lower than 3.5%. The

hydraulic conductivity for the intact rock presents an average of 6.6×10-3cm/s and are

considered good aquifers; natural moisture content of 5% and a bulk density of 3.5 t/m3 is the

average.

Typically, present are weathering grades varying from W5 to W6 and field intact rock strength

grade from R0 to R2 according to ISRM (1981). The associated UCS values are lower than 5 MPa,

generally from 1 MPa to 2 MPa and classified as IV or V according to Bieniawski (1989).

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This low resistance (friable) material consists of thin (millimetre to centimetre) opaque/dark

grey colour bands of hematite microplates and granular hematite/martite, with higher total

porosity, representing the lower relative strength band, alternated with more consistent layers

of dark metallic bands of bigger crystals of granuloblastic hematite and martite with lower total

porosity and higher relative strength. The global porosity varies significantly from 25% to 30%,

according to Costa et al. (2009), or even higher, from 29% to 37% according to Ribeiro (2003).

A study by Costa (2009) argues that this lithotype presents small band heterogeneity, but some

varieties present sensitive anisotropy. In most cases, it represents the tectonic foliation, but can

also preserve the original banding. The total porosity, grain size and shape of the minerals vary

also with the tectonical settings. The larger hematite crystals are granuloblastic and the smaller

are microplates, and the specularite presents lepidoblastic texture. In size, these iron minerals

vary from 0.005 mm to 0.5 mm, with a maximum 0.1 mm as presented in Rosière (2005).

In depth, these lithotypes are associated with synclines, or found in the upper surfaces above

the WQI and can also occur in the shear zones and brittle failures presenting specularite levels.

Authigenic breccia is very common and can represent a load deformation structure associated

with collapse imposed by the weathering process.

The mineral texture and related physical properties are important characteristics of WHE due to

the potential for collapse observed in past failure mechanisms; for example, the Patrimônio wall

failure at Águas Claras mine, as discussed by Franca (1997) and Costa (2009). As argued by

Martin & Stacey (2013), when the cohesion is superimposed, this can lead to a rapid failure

mechanism, mainly caused by the overload transferred to granular particle contacts thus

mobilising the frictional strength, which could not support the total load resulting in rapid

failures.

Geomechanical behaviour for WHE lower confinement levels can be observed in a typical

consolidated undrained triaxial test (CIU) from Costa (2009). Figure 4A shows the stress-strain

curve and in the centre (4B) shows the WHE sample after test and 4C the porewater pressure

strain curve.

Figure 4A presents low reduction for post-peak values and the pore pressures in Figure 4C show

rapid dissipation during the shearing. This behaviour resembles coarse stiff (dense) material

associated with the WHE.

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(A) (B) (C)

Figure 4 A (left), WHE stress-strain curve. B (centre), WHE sample after test. C (right),

porewater pressure/axial deformation. After Costa (2009)

4.2 WEATHERED ARGILACEOUS ITABIRITE

WAI presents iron content ranging from 30% to 62% and Al2O3 ≥ 2.5%, and loss of ignition (3.5%

≤ LOI < 5%). When the iron content is greater than 62%, they are considered rich and are

denominated RWAI.

They are defined as a fine laminated lithotype composed by layers of hematite microplates, fine

granular hematite and goethite alternated by layers of microplates of hematite, goethite, quartz,

gibbsite, kaolinite, and ochreous goethite. Some manganese minerals, such as pyrolusite and

cryptomelanite, are commonly found cementing the rock pores.

Typically, present is a weathering grade of W6, field intact rock strength grade varying from R0

to R1 according to ISRM (1981), and associated UCS values lower than 1 MPa, and classified as

V according to Bieniawski (1989). Macroscopically, they are characterised by the dark brown

colour determined by the high goethite and clay mineral content (gibbsite and kaolinite) that

attributes a plastic rock characteristic.

The average bulk density used is 2.7 t/m3 and the natural moisture content presenting an

average of 10%. The total porosity is mainly associated with the iron bands and the authigenic

breccia texture. However, the hydraulic conductivity is considered low with an average value

equal to 4.6×10-4 cm/s, generally representing an aquitard or, depending on the argillaceous

band thickness, an aquiclude for the aquifer.

Important information about the mineral shape and texture were presented by Zapparoli et al.

(2007) who defined brecciaed as the most common micro texture and secondary mylonitised or

foliated, the first resulting from collapses mainly associated with supergene process (mimetic

and laterisation) and the second due to discrete shear zones.

431

Spier (2005) argues that the WAI are formed by the effective leaching process over the dolomitic

itabirites. However, recent studies by Zapparoli et al. (2007) and Suckau et al. (2005) suggested

two other different facies for the WAI: a basal unit, associated with the Batatal formation,

representing a gradational pelitics to chemical deposition, and an intraformational unit,

associated with exhalative volcanic deposits.

This material typically behaves as soil, and its mineral texture and composition are crucial to the

geomechanical behaviour specially associated with the plasticity index and suction effects that

can drastically change the failure mechanism and the slope stability features.

Geomechanical behaviour for WAI (low confinement levels) can be observed on typical CIU tests

from Barbosa & Silva (2008). Figure 5A shows the stress-strain curve, while Figure 5B shows the

WAI sample after test, and Figure 5C presents the porewater pressure strain curve.

Figure 5A presents low and slower value reduction for post-peak values. The porewater pressure

curves observed in Figure 5C present low and minor dissipation values and this behaviour is

typical from fine-medium to stiff plastic soil.

(A) (B) (C)

Figure 5 A (left), WAI stress-strain curve. B (centre) WAI sample after test. C (right)

porewater pressure/axial deformation. After Barbosa & Silva (2008)

Figures 4A and 5A presented low stress reduction for the post-peak characterising a moderate

strain softening curve. Some of them present more pronounced peak (e.g. 50 kPa for the WAI

and 800 kPa for the WHE). These characteristics could be associated with the higher bulk density

and lower total porosity observed in those samples presenting minor ductile behaviour.

Recent studies by Martin & Stacey (2013) argue that for these two lithotypes the failure

mechanisms normally are resulting from cohesion loss (WAI) and collapse (WHE) of the

intergranular texture. For both materials, the key physical characteristics that control the

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geomechanical behaviour were porosity and fabric (micro texture). These features are

intimately related to the mineral shape, PSD, void content, and density.

5 PRELIMINARY FINDINGS

5.1 LABORATORY TESTS

Laboratory tests were performed on eight samples and included bulk density, saturated and

unsaturated, single and multistage direct shear tests (DST); soil-water characteristic curve

(SWCC); particle size curve distribution (PSD) and Atterberg limits (ATT).These are performed on

original 77.8 mm cores casted with plaster diamond drill cores trimmed to 61.8 mm in order

obtain the necessary anisotropy angle (β) and ensure minimal sample disturbance.

The tests were undertaken in accordance with the following Australian standards: Direct Shear,

AS 1289.6.2.2 (AS 1998); PSD/Hydrometer, AS 1289.3. 6.3 (AS 2003) and AS 1289.3. 6.1 (AS

2009); Atterberg limits, AS 1289.0 (AS 2000); SPD/SG, AS 1289.3. 6.1 (AS 2009); Bulk Density,

AS 1289.3. 6.1 (AS 2009); Moisture Content, AS 1289.6.4.1 (AS 2016) and UCSCS, ASTM D2487-

00 (ASTM 2000).

The presence of anisotropy (βangle) due to banding was considered in all test work. These

materials were tested at β = 90° (perpendicular) and β = 0° (parallel) to the anisotropy plan. Also,

due to the heterogeneity promoted by the compositional metamorphic banding thickness and

related sample scale effects, represents a challenge to obtain samples with similar physical

characteristics (banding, total porosity, bulk density, etc). Therefore, some samples were

denominated as RWAI, defining a sample with ambiguous characteristics from WAI and WHE.

5.2 MATERIAL CHARACTERISATION

The section summarises the material characterisation from the test results. Table 1 lists the

Atterberg limits for the <425 μm fraction for each material and the associated USCS classification

of the samples according to ASTM D2487-00 (ASTM 2000). Natural moisture content is not

reported due to disturbance from long-term storage and transport. Table 1 also presents the

PSD and correlated parameters, and the dry density values.

Ta ble 1 Physical index results

Id Atterberg

PSD

Classification Physical index

Sample

Lithotype LL

PL PI

SL LS

A gravel

%

sand

%

silt

%

clay

%

Cc Cu

USCS

Bulk density

(t/m3)

1 W

AI 37

21 17

––

2 0

17 70

13 2

7 CL

1.95

2 RW

AI 35

21 14

17 7

2 20

52 26

2 1

34 SC

2.84

3 RW

AI 24

17 8

15 5

2 3

62 34

2 2

16 SC

3.09

4 RW

AI 28

24 4

22 3

1 13

63 22

2 17

68 SM

2.38

5 RW

AI/WAI

25 19

6 17

2 1

4 55

38 2

1 18

SC-SM

2.60

6 W

AI/RWAI

34 26

8 23

5 2

1 47

49 2

1 7

ML

2.71

7 W

HE 11

8 3

––

25

55 38

2 1

12 SM

2.96

8 W

HE 22

19 3

19 2

–17

82 1

4 SP

2.67

LL: Liquid index; PL: Plastic limit; PI: Plastic index; SL: Shrinkage lim

it; LS: Linear shrinkage, A: Activity; Cu: Coefficient of uniformity; Cc: Coefficient of curvature; CL: Clay; SC: Clayey sand; SM

: Silt sand; ML: Very fine

sands; SP: gravelly sand .

433

434

The particle size curve distribution is shown in Figure 6. Most materials contained very little clay size

fraction (2% < 2 μm) but can reach 12% in argillaceous bands and relatively low percentages of gravels

(2% to 20% > 2 mm) and were therefore characterised as fine-medium sand to silt material. The coarse

materials present a PSD of well graded materials with the exception of sample 4, which is gap graded

and sample 8 which is poorly graded.

Figure 6 Particle size distribution

As anticipated, samples cover a range of different material types from fine-medium sands, to silts and

clays According to the USCS, WHE is classified as SP (sand poorly graded) and SM (sand silt), presenting

very low or no plasticity. RWAI materials are classified as SC, SC-SM or SM, with low plasticity index,

while WAI is classified between a LC (lean clay), SC (sand clay) and ML (silt), with plasticity varying

from low to medium plasticity.

Figure 7 shows the normalised Fredlund & Xing (1994) SWCC curve fit with a correction factor C (ψ)

for all samples without considering the influence of anisotropy. Fitting curve parameters are listed in

Table 2. Parameters a, n and m are the fitting parameters while Cr is the parameter related to residual

suction that has been selected as Cr = 1,500 kPa. Өs is the saturated volumetric water content, in the

air entry value (AEV) and Ψr is the residual suction values. These SWCC curves are typical of sandy

materials.

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Figure 7 SWCC curves for normalised water content

Table 2 SWCC fitting parameters

Id Fitting Parameters

Sample Lithotype a (kPa) n m Cr (kPa) өs AEV

(kPa) Ψr (kPa)

8 WHE 70 3 1.4 1,500 0.721 40 213

6 WAI/RWAI 150 2.4 2.6 1,500 0.838 66 324

5 RWAI/WAI 230 1.8 5 1,500 0.622 46 393

4 RWAI 95 1.95 2.7 1,500 0.916 34 237

3 RWAI 70 1.1 2 1,500 0.438 14 565

2 RWAI 360 1.5 2 1,500 0.443 107 600

θs: calculated volumetric water content; a: fitting parameter related to the air entry value of the soil (kPa); n: fitting parameter related to the maximum slope of the curve; m: fitting parameter related to the curvature of the slope; ψr = fitting parameter related to the residual suction of the soil (kPa); AEV: air entry values; Cr: residual suction parameter.

Due to the reduced number of tests, the typical suction characteristics, and limits to considerer these

effects of each material are still unclear. However, the range presented in Table 2 from WAI to WHE

SWCC Curves

436

shows higher relative air entry value, lower relative residual porewater pressure and higher suctions

values associated with WAI, which contains a higher percentage of clay mineral compared to WHE.

5.3 DIRECT SHEAR TESTS

Intact saturated single-stage direct shear tests (DST - SS) as well as single-stage unsaturated direct

shear test (UDST) are summarised in Table 3.

Table 3 Direct shear test result

Id Lithotype Test

Test

direction

βangle (°)

Natural

density

(t/m3)

cpk

(kPa)

Φpk

(°)

Cres

(kPa)

φres

(°)

φb

(°)

6 WAI/RWAI DST (SS) 0 2.7 0 32 0 32 –

5 RWAI/WAI DST (SS) 0 2.62 33 29 21 30 –

4 RWAI DST (SS) 0 2.28 52 32 18 30 –

5 RWAI/WAI DST (SS) 90 2.52 47 33 0 35 –

4 RWAI DST (SS) 90 2.27 52 24 55 19 –

3 RWAI UDST 0 3.09 – – – – 22

2 RWAI UDST 0 2.84 – – – – 15

Cpk: peak cohesion; ɸpk: peak friction angle; Cres: residual cohesion; ɸres: residual friction angle ɸb: unsaturated friction angle.

The peak (pk) and residual (res) values were obtained using adjusted fit curves. Due to the different

nature of the multistage and single-stage DST test, anisotropy (β) and moisture content test results

presented a high range mainly for the cohesion values. However, they are consistently low strength

values for all terms.

Figure 8 shows the peak and residual shear stress versus matric suction curves from unsaturated DST

(sample 3) for a constant normal stress of 75 kPa in all suction stages. The unsaturated friction angle

φb values, which is the angle indicating the rate of increase in shear relative to matric suction, was

estimated using a linear relationship between stage (S2) and stage (S3) matric suction values of 65 kPa

and 260 kPa. Stage 2 corresponded to the AEV for sample 6.

437

Figure 8 Sample 3, nonlinear failure envelope on the τ - matric suction plane for peak and residual

values at σN, = 75 kPa, showing the AEV, Ψr, Φb. and the stages (S1, 2 and 3)

6 SLOPE STABILITY ANALYSES

The final aspect of this work was to evaluate the potential change in the Factor of Safety (FoS) of the

open pit slopes developed in these WAI and WHE materials when applying unsaturated material

parameters using a bi-linear extended Mohr–Coulomb failure envelope. The stability analysis was

undertaken using 2D limit equilibrium methods (LEM). The matric suction within the slope was

modelled both explicitly and using a finite element groundwater simulation (FEGS).

A representative slope profile selected for the analysis has both WAI and WHE material present in the

lower slope. This is characterised by a stress-strain state marked by 10 m to 20 m long tension cracks

with an aperture of 100 mm, and 2 m depth along the intrusive rock and itabirites contact at an

elevation of 1,240 m to 1,260 m. The phreatic surfaced was defined by a hydrogeological model and

calibrated by water level indicators

Slide 5.0 (Rocscience Inc. 2014) was used as the LEM software and the GLE/Morgenstern Price method

was used to calculate the FoS using non-circular trial surfaces. Pore pressure profiles within the slope

were established using two different techniques. In the first one, porewater pressures values were

input explicitly in grids and interpolated using the Modified Chugh grid interpolation method. The

second technique used the built-in Slide FEGS groundwater seepage analysis, the results from which

were calibrated. Saturated permeabilities were used from Vale’s database and unsaturated

permeabilities of the two materials were established using a best fit method of Leong & Rahardjo’s

(1997) equation to the SWCC statistical model. The second technique is considered more accurate for

establishing both positive and negative porewater pressure distributions.

438

The stability analyses implemented in Slide 6.0 uses a bi-linear failure envelope on the τ - matric

suction plane with φb = φ’ for Ψ < AEV and φb< φ’ for Ψ > AEV. This approach was more conservative

that using the full unsaturated shear strength capacity that was determined from lab testing.

The WHE saturated shear strength values were obtained from CIU triaxial tests from Barbosa & Silva

(2008); for WAI, the average value from Table 3 was used; and for other materials, standard values

used in DIFL/Vale mines were applied. The φb value of WAI was selected from sample 2 and for WHE,

sample 3 test results were used.

Three stability analyses were undertaken. The first did not consider any contribution from unsaturated

shear strength; the second used porewater pressures established using the grid method; and the third

analysis used the porewater pressure derived using the FEGS.

Figure 9 presents material strength and matric suction values used on slope analyses. Figure 10

presents the results for the first analysis considering the watertable with no matric suction effects,

and a FoS = 1.2 was obtained. Figure 11 presents the results for the second analysis showing a FoS of

1.4 using the gridding pore pressure method to establish the suction values. In this scenario, the grid

points were assigned as follows: zero porewater pressure at water level, WAI material suction values

equal to 180 kPa, and WHE material suction equal to 110 kPa.

Figure 12 shows results of the third analysis with FEGS seepage analysis porewater pressures and

considering suction effects on slope stability, giving a FoS of 1.7.

Comparing the three analyses, the FoS presents an increase of 20% when using the LEM gridding

technique (Figure 11) and 38% when using the FEGS (Figure 12).

Figure 9 Material strength and matric suction parameter

439

Figure 10 Slope stability analysis using water level and no suction effects, resulting in an FoS equal

to 1.20

Figure 11 Slope stability analysis using gridding of pore pressure conditions and suction effects,

resulting in an FoS equal to 1.42

440

Figure 12 Slope stability analysis using FEGS seepage pore pressures and suction effects (Leong &

Rahardjo 1997) resulting in a FoS of 1.7

7 DISCUSSION

The weathering process affected the itabirites in different ways and intensity producing a range of

partially to completely weathered material. However, in some way, these products kept part of the

original structures resulting in stiff soils that can reach hundreds of metres deep.

The WAI and WHE can be widely characterised as fine sand silt materials with low plasticity and low

to medium suction values. However, it is not at all uncommon to find specimens of those ores that

present small amounts of clay content with soil minerals such as kaolinite and gibbsite. In this case,

plasticity and suction values are relatively higher, which can have an influence on slope stability due

to the low intact rock strength and suction effects of these materials. When the increase in clay

mineral contents is more relevant to those materials, they are called WAI and the suction values

normally observed can reach up to double compared to the WHE.

Due to the reduced number of tests for the WAI, it is not possible to associate different strength values

related to βangle. However, based on previous experiences, it is agreed to consider these materials as

isotropic. Additionally, comparing the WHE and WAI results, the strength values related to WHE are

slightly higher than WAI, especially for the friction angle. This consideration is supported by WHE

values presented by Costa (2009).

The critical failure surfaces normally associated with these ores are relatively shallow, which suggests

that WAI and WHE are subjected to low confining stress. Thus, a linear failure envelope

(Mohr–Coulomb) is a reasonable assumption. Further, there is no evidence of brittle behaviour for

441

WAI. These two aspects suggest that the friction angle must be the most important shear strength

parameter to be determined for these weak rock materials as also recognised by Martin & Stacey

(2013). However, for the WHE, due to the occurrence of some past events with deep failure surfaces,

more evaluations are necessary in order to understand the collapse potential of the mineral fabric.

This also contributes to the importance of determining the τ - matric suction plane of the extended

Mohr–Coulomb envelope for unsaturated conditions that can considerably increase the overall shear

strength resistance of weathered materials when evaluating the slope stability of these pit slopes.

When adopting the unsaturated shear strength, the variation of the unsaturated state of the materials

along critical surfaces from the wet to dry season needs to be carefully evaluated.

8 CONCLUSION AND NEXT STEPS

The preliminary results presented in this paper demonstrate a potential increase in the material shear

strength when considering the matric suction effects for WAI, which shows the potential benefit of

considering the negative porewater pressure distribution when evaluating slope stability.

The historical approach used in Vale mines which is based on saturated tests, effective strength

parameters and without considering matric suction effects, is suitable for long-term and post-closure

slope stability analyses. However, for short-term slope stability without water implications, this

approach is conservative.

To better evaluate the matric suction effects, more representative laboratory tests and more accurate

and calibrated groundwater simulations must be undertaken. The non-linearity of the failure envelope

on the τ - matric suction plane should be considered in the analysis. In addition, consideration of the

climate (seasonal) induced fluctuation of the near surface matric suction values (transient flow) as

well as the influence of the clay mineralogy must be evaluated to establish robust parameters for

undertaking sensitivity studies and achieving a high degree of reliability in the designs.

In terms of the next steps, further studies will be addressed to determine the porewater pressure

effects, physical parameters, and geomechanical relationship for a large number of WAI and WHE

samples. It will also be necessary to evaluate the matric suction effects for other weathered materials

(e.g. WQI and WGI) in order to establish a comprehensive understanding of the unsaturated shear

strength behaviour across the full range of the weak weathered iron formations in Vale S.A. iron

mines.

442

ACKNOWLEDGEMENTS

The author’s appreciation extends to Vale S.A. for supporting the research, the Australian Centre for

Geomechanics and Vale colleagues for their contribution, and special thanks to Victor Suckau and

Professor Waldyr Oliveira (EM/UFOP) for discussions about the nature of the WAI and suction effects.

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446

APPENDIX III

Rock test – Uniaxial Com

pressive Strength test

Sequence

number

Block ID M

ine Lithotype

Diameter

(mm

)

Height

(mm

)

Anisotropy

(β)

Bulk density

(t/m3)

E stat

(GPa)

σ1 (kPa)

UCS

(MPa)

VP (m

/s) Lab

Failure Mode

1 10333

TAM

HHE 3.60

8.20 0

4.72 55

31 5798

Geocontrole Shearing along sb

2 10333

TAM

HHE 3.60

8.40 0

4.92 28.60

82 45

6789 Geocontrole

Axial splitting

3 10333

TAM

HHE 3.60

8.60 45

4.78 33.30

89 49

7155 Geocontrole

Shearing along

porous level

4 10333

TAM

HHE 3.60

7.20 90

5.05 97

54 6343


5 10331

TAM

HHE 3.60

7.90 0

4.97 30.70

101 56

6639 Geocontrole

Shearing along

single plane

6 10333

TAM

HHE 3.60

7.40 45

4.98 133

74 7568

Geocontrole Shearing along

single plane

7 10331

TAM

HHE 3.60

7.30 45

5.15 11.00

136 75

6952 Geocontrole

Axial splitting

8 10345

TAM

HHE 3.60

7.40 0

5.20 12.20

146 81

7872 Geocontrole

Axial splitting

9 10344

TAM

HHE 3.70

8.20 0

4.96 56.70

163 91

7193 Geocontrole

Axial splitting

10 10331

TAM

HHE 3.60

7.60 45

5.08 90.00

166 92

6909 Geocontrole

Shearing along Sb

11 10344

TAM

HHE 3.70

8.30 0

5.02 169

94 7130

Geocontrole Axial splitting

12 10345

TAM

HHE 3.70

7.60 0

5.06 33.30

194 108

7041 Geocontrole

Axial splitting

13 10343

TAM

HHE 3.60

8.30 0

5.09 206

113 7281


14 10344

TAM

HHE 3.60

8.20 0

5.02 45.00

209 115

7545 Geocontrole

Axial splitting

15 10335

TAM

HHE 3.60

8.40 45

5.00 26.70

223 123

6308 Geocontrole

Axial splitting

16 10331

TAM

HHE 3.70

8.20 90

4.89 100.00

248 138

6357 Geocontrole

Axial splitting

17 10343

TAM

HHE 3.60

8.70 0

5.19 286

158 7311


18 10345

TAM

HHE 3.70

7.60 0

5.04 90.00

391 217

7755 Geocontrole

Axial splitting

19 10343

TAM

HHE 3.60

7.90 0

5.19 6.40

435 240

7383 Geocontrole

Axial splitting

20 10344

TAM

HHE 3.70

7.60 0

5.06 68.60

458 255

7308 Geocontrole

Axial splitting

447

21 10335

TAM

HHE 3.60

7.00 0

5.10 35.00

520 287

7447 Geocontrole

Axial splitting

22 69

SEG HHE

20.60 49.70

0 5.16

384.0 311

7766 U

WA

Axial splitting

23 69

SEG HHE

20.60 49.70

0 5.19

558.0 7000

UW

A Splitting

24 69

SEG HHE

20.60 49.70

0 5.16

521.0 7646

UW

A Pre-existing

Weathered

25 69

SEG HHE

20.60 49.70

45 5.16

7766 U

WA

26 17_2

SEG HHE

65.50 331.00

5.24

27 10405

TAM

FQI

3.60 6.50

45 3.19

6.00 29

16 2211


28 10403

TAM

FQI

3.60 6.80

90 3.09

6.20 32

18 3402


29 10404

TAM

FQI

3.60 6.80

0 3.17

9.30 36

20 4024


30 10405

TAM

FQI

3.60 6.80

45 3.27

8.40 38

21 3254


31 10346

TAM

FQI

3.60 7.50

0 3.14

29.00 42

23 4095


32 10405

TAM

FQI

3.60 8.80

45 3.40

16.40 52

29 2902

Geocontrole Shearing along Sb

33 10403

TAM

FQI

3.50 6.80

0 3.24

16.20 66

36 4564


34 10403

TAM

FQI

3.60 7.60

45 3.20

19.60 71

39 3551


35 10404

TAM

FQI

3.60 7.20

0 3.09

59.20 71

39 4832


36 10403

TAM

FQI

3.60 6.60

90 3.05

14.30 77

43 3492


37 10346

TAM

FQI

3.60 6.90

0 3.42

3.70 77

43 5520


38 10403

TAM

FQI

3.60 7.90

45 3.25

18.70 88

48 3450


39 10404

TAM

FQI

3.60 7.40

0 3.07

98 54

4252 Geocontrole

Shearing along Sb

40 10404

TAM

FQI

3.60 6.90

90 2.95

76.70 100

55 3557


41 10404

TAM

FQI

3.60 6.70

90 2.94

16.40 108

60 4214


42 10403

TAM

FQI

3.60 6.90

45 2.92

22.50 130

72 3966


43 10403

TAM

FQI

3.60 6.90

90 3.18

132 73

3080 Geocontrole

Axial splitting

44 10404

TAM

FQI

3.60 7.30

90 3.02

133 74

3842 Geocontrole

Shearing along

double plane

45 10403

TAM

FQI

3.50 7.70

0 2.88

21.40 143

78 4302


46 2

GAL FQ

I 20.50

49.40 0

3.78 251.0

203 7265

UW

A Pre-existing Sb

47 7

GAL FQ

I 20.60

49.40 90

3.34 116.00

255.0 207

6099 U

WA

Splitting

48 1

GAL FQ

I 20.60

49.40 90

3.61 122.00

410.0 332

6099 U

WA

Axial splitting

448

49 3_1

ABO

FQI

20.40 39.40

90 3.82

6043 U

WA1

50 10337

TAM

FAI 3.70

7.40 45

3.09 40.00

53 29

5324 Geocontrole

Shearing along sb

51 10337

TAM

FAI 3.70

6.60 45

3.15 81.00

70 39

5546 Geocontrole

Axial splitting

52 10337

TAM

FAI 3.60

7.40 0

3.28 100.00

79 44

5522 Geocontrole

Shearing along sb

53 10337

TAM

FAI 3.70

7.50 45

3.21 46.20

83 46

5597 Geocontrole

Shearing along Sb

54 10337

TAM

FAI 3.70

6.80 90

3.19 93

52 5271


55 10337

TAM

FAI 3.60

7.30 90

3.26 25.00

103 57

5271 Geocontrole

Axial splitting

56 10337

TAM

FAI 3.70

6.80 90

3.18 133

74 5252


57 10337

TAM

FAI 3.60

7.00 0

3.20 78.80

148 82

5224 Geocontrole

Axial splitting

58 10337

TAM

FAI 3.60

7.50 0

3.43 184

101 5814


59 43

FAI 20.60

41.40 45

3.73 115.00

206.0 167

5377 U

WA

Shearing

60 20

JGD FAI

20.60 47.50

45 3.33

97.60 256.0

208 6013

UW

A Splitting

61 14_1

CMT

FAI 19.30

50.00 0

3.24 108.06

350.5 280

6024 U

WA1

Pre-existing Sb

62 43

FAI 20.60

39.40 0

3.96 147.00

570.0 462

5549 U

WA

Axial splitting

63 15_1

CMT

FAI 19.20

50.00 0

3.39 6329

UW

A1

64 21

SEG FAI

20.50 50.00

45 3.78

4902 U

WA

65 42

FDI 20.60

38.30 45

3.24 48.00

67.0 54

3648 U

WA

Shearing

66 42

FDI 20.60

34.50 45

3.21 32.00

80.0 65

2782 U

WA

Splitting

67 36

JGD FDI

20.40 46.00

0 3.87

115.00 83.0

67 6053

UW

A Pre-existing Sb

68 31

JGD FDI

20.60 44.80

0 138.0

112 5818

UW

A Pre-existing

fracture

69 42

FDI 20.60

38.30 45

3.53 285.0

231 4910

UW

A Pre-existing

fracture

449

450

APPENDIX IV

Rock test – Physical and elastodynamic results

Sequence

number

Block ID M

ine Lithotype

Diameter

(mm

)

Height

(mm

)

Anisotropy

(β)

Total

Porosity

(%)

Bulk

density

(t/m3)

VP (m

/s) V

S (m/s)

Vp /V

s E

dyn (GPa)

vdyn

1 HC13B

TAM

HHE 37.55

48.24 90

2.14 5.07

7164 3019

2.4 129

0.392

2 HC13B

TAM

HHE 37.13

39.34 45

3.26 5.01

7091 3047

2.3 129

0.387

3 HC13B

TAM

HHE 37.06

32.75 45

0.20 5.16

6667 3404

2.0 158

0.324

4 HC13B

TAM

HHE 37.06

32.75 0

0.71 5.16

6538 3400

1.9 157

0.315

5 HC13B

TAM

HHE 37.55

48.24 90

2.14 5.07

7164 3019

2.4 129

0.392

6 HC13B

TAM

HHE 37.13

39.34 45

3.26 5.01

7091 3047

2.3 129

0.387

7 HC13B

TAM

HHE 37.06

32.75 45

0.20 5.16

6667 3404

2.0 158

0.324

8 HC13B

TAM

HHE 37.06

32.75 0

0.71 5.16

6538 3400

1.9 157

0.315

9 ICS3156

GAL FQ

I 37.57

30.83 90

3.20 3.31

4366 2026

2.2 37

0.363

10 ICS3156

GAL FQ

I 37.38

26.98 90

2.48 3.28

4262 1733

2.5 28

0.401

11 ICS3162

GAL FQ

I 36.99

36.77 90

1.42 3.41

4932 3363

1.5 82

0.066

451

12 ICS2430

GAL FQ

I 37.15

31.42 90

2.65 3.30

5439 3373

1.6 89

0.188

13 ICS2430

GAL FQ

I 37.43

27.87 90

3.66 3.28

5745 3214

1.8 86

0.272

14 ICS4308

GAL FQ

I 36.43

40.34 90

9.37 3.18

5333 2910

1.8 69

0.288

15 ICS4308

GAL FQ

I 36.83

40.49 90

8.38 3.34

4938 2797

1.8 66

0.264

16 ICS3156

GAL FQ

I 37.77

45.73 45

3.31 3.37

3309 2026

1.6 33

0.200

17 ICS3156

GAL FQ

I 37.39

45.36 45

3.48 3.12

3435 1923

1.8 29

0.272

18 ICS3162

GAL FQ

I 37.36

34.15 45

2.46 3.49

5152 3301

1.6 88

0.152

19 ICS3162

GAL FQ

I 37.02

47.48 45

1.24 3.30

4486 2437

1.8 51

0.291

20 ICS2430

GAL FQ

I 37.65

30.11 45

3.92 3.44

4762 2190

2.2 45

0.366

21 ICS2430

GAL FQ

I 37.86

29.26 45

2.85 3.51

5179 3021

1.7 80

0.242

22 ICS4308

GAL FQ

I 36.53

54.45 45

7.01 3.18

2471 1695

2.5 19

0.056

23 ICS3156

GAL FQ

I 37.43

36.18 0

4.07 3.06

3955 2057

1.9 34

0.315

24 ICS3161

GAL FQ

I 36.03

31.99 0

0.61 3.48

4156 2105

2.0 41

0.327

25 ICS3162

GAL FQ

I 36.78

29.39 0

3.31 3.47

4478 2479

1.8 55

0.279

26 ICS2430

GAL FQ

I 37.80

29.70 0

0.93 3.58

5273 1737

3.0 31

0.439

27 ICS4308

GAL FQ

I 37.59

61.26 0

7.84 2.08

4783 3140

1.5 46

0.121

452

28 ICS3156

GAL FQ

I 37.57

30.83 90

3.20 3.31

4366 2026

2.2 37

0.363

29 ICS3156

GAL FQ

I 37.38

26.98 90

2.48 3.28

4262 1733

2.5 28

0.401

30 ICS3162

GAL FQ

I 36.99

36.77 90

1.42 3.41

4932 3363

1.5 82

0.066

31 ICS2430

GAL FQ

I 37.15

31.42 90

2.65 3.30

5439 3373

1.6 89

0.188

32 ICS2430

GAL FQ

I 37.43

27.87 90

3.66 3.28

5745 3214

1.8 86

0.272

33 ICS4308

GAL FQ

I 36.43

40.34 90

9.37 3.18

5333 2910

1.8 69

0.288

34 ICS4308

GAL FQ

I 36.83

40.49 90

8.38 3.34

4938 2797

1.8 66

0.264

35 ICS3156

GAL FQ

I 37.77

45.73 45

3.31 3.37

3309 2026

1.6 33

0.200

36 ICS3156

GAL FQ

I 37.39

45.36 45

3.48 3.12

3435 1923

1.8 29

0.272

37 ICS3162

GAL FQ

I 37.36

34.15 45

2.46 3.49

5152 3301

1.6 88

0.152

38 ICS3162

GAL FQ

I 37.02

47.48 45

1.24 3.30

4486 2437

1.8 51

0.291

39 ICS2430

GAL FQ

I 37.65

30.11 45

3.92 3.44

4762 2190

2.2 45

0.366

40 ICS2430

GAL FQ

I 37.86

29.26 45

2.85 3.51

5179 3021

1.7 80

0.242

41 ICS4308

GAL FQ

I 36.53

54.45 45

7.01 3.18

2471 1695

2.5 19

0.056

42 ICS3156

GAL FQ

I 37.57

30.83 0

3.12 3.16

3619 1929

1.9 31

0.302

43 ICS3156

GAL FQ

I 37.43

36.18 0

4.07 3.06

3955 2057

1.9 34

0.315

453

44 ICS3161

GAL FQ

I 36.03

31.99 0

0.61 3.48

4156 2105

2.0 41

0.327

45 ICS3162

GAL FQ

I 36.78

29.39 0

3.31 3.47

4478 2479

1.8 55

0.279

46 ICS2430

GAL FQ

I 37.80

29.70 0

0.93 3.58

5273 1737

3.0 31

0.439

47 ICS3156

GAL FQ

I 37.38

26.98 90

2.48 3.28

4262 1733

2.5 28

0.401

48 ICS3162

GAL FQ

I 36.99

36.77 90

1.42 3.41

4932 3363

1.5 82

0.066

49 ICS2430

GAL FQ

I 37.15

31.42 90

2.65 3.30

5439 3373

1.6 89

0.188

50 ICS2430

GAL FQ

I 37.43

27.87 90

3.66 3.28

5745 3214

1.8 86

0.272

51 ICS4308

GAL FQ

I 36.43

40.34 90

9.37 3.18

5333 2910

1.8 69

0.288

52 ICS4308

GAL FQ

I 36.83

40.49 90

8.38 3.34

4938 2797

1.8 66

0.264

53 ICS3156

GAL FQ

I 37.77

45.73 45

3.31 3.37

3309 2026

1.6 33

0.200

54 ICS3156

GAL FQ

I 37.39

45.36 45

3.48 3.12

3435 1923

1.8 29

0.272

55 ICS3162

GAL FQ

I 37.36

34.15 45

2.46 3.49

5152 3301

1.6 88

0.152

56 ICS3162

GAL FQ

I 37.02

47.48 45

1.24 3.30

4486 2437

1.8 51

0.291

57 ICS2430

GAL FQ

I 37.65

30.11 45

3.92 3.44

4762 2190

2.2 45

0.366

58 ICS2430

GAL FQ

I 37.86

29.26 45

2.85 3.51

5179 3021

1.7 80

0.242

59 ICS4308

GAL FQ

I 36.53

54.45 45

7.01 3.18

2471 1695

2.5 19

0.056

454

60 ICS3156

GAL FQ

I 37.43

36.18 0

4.07 3.06

3955 2057

1.9 34

0.315

61 ICS3161

GAL FQ

I 36.03

31.99 0

0.61 3.48

4156 2105

2.0 41

0.327

62 ICS3162

GAL FQ

I 36.78

29.39 0

3.31 3.47

4478 2479

1.8 55

0.279

63 ICS2430

GAL FQ

I 37.80

29.70 0

0.93 3.58

5273 1737

3.0 31

0.439

64 ICS4308

GAL FQ

I 37.59

61.26 0

7.84 2.08

4783 3140

1.5 46

0.121

65 ICS3156

GAL FQ

I 37.38

26.98 90

2.48 3.28

4262 1733

2.5 28

0.401

66 ICS3162

GAL FQ

I 36.99

36.77 90

1.42 3.41

4932 3363

1.5 82

0.066

67 ICS2430

GAL FQ

I 37.15

31.42 90

2.65 3.30

5439 3373

1.6 89

0.188

68 ICS2430

GAL FQ

I 37.43

27.87 90

3.66 3.28

5745 3214

1.8 86

0.272

69 ICS4308

GAL FQ

I 36.43

40.34 90

9.37 3.18

5333 2910

1.8 69

0.288

70 ICS4308

GAL FQ

I 36.83

40.49 90

8.38 3.34

4938 2797

1.8 66

0.264

71 ICS3156

GAL FQ

I 37.77

45.73 45

3.31 3.37

3309 2026

1.6 33

0.200

72 ICS3156

GAL FQ

I 37.39

45.36 45

3.48 3.12

3435 1923

1.8 29

0.272

73 ICS3162

GAL FQ

I 37.36

34.15 45

2.46 3.49

5152 3301

1.6 88

0.152

74 ICS3162

GAL FQ

I 37.02

47.48 45

1.24 3.30

4486 2437

1.8 51

0.291

75 ICS2430

GAL FQ

I 37.65

30.11 45

3.92 3.44

4762 2190

2.2 45

0.366

455

76 ICS2430

GAL FQ

I 37.86

29.26 45

2.85 3.51

5179 3021

1.7 80

0.242

77 ICS4308

GAL FQ

I 36.53

54.45 45

7.01 3.18

2471 1695

2.5 19

0.056

78 ICS3156

GAL FQ

I 37.43

36.18 0

4.07 3.06

3955 2057

1.9 34

0.315

79 ICS3161

GAL FQ

I 36.03

31.99 0

0.61 3.48

4156 2105

2.0 41

0.327

80 ICS3162

GAL FQ

I 36.78

29.39 0

3.31 3.47

4478 2479

1.8 55

0.279

81 ICS2430

GAL FQ

I 37.80

29.70 0

0.93 3.58

5273 1737

3.0 31

0.439

82 ICS4308

GAL FQ

I 37.59

61.26 0

7.84 2.08

4783 3140

1.5 46

0.121

83 ICA3244

SAP FAI

37.58 35.93

90 3.51

3.24 5537

3130 1.8

80 0.265

84 ICA3620

SAP FAI

37.57 33.75

90 3.81

3.25 5593

2724 2.1

65 0.345

85 ICA3620

SAP FAI

37.57 33.75

90 2.23

3.25 4457

2781 1.6

59 0.181

86 ICA3244

SAP FAI

37.42 45.73

45 3.42

3.35 5998

3563 1.7

104 0.227

87 ICA3244

SAP FAI

36.95 28.14

45 7.07

3.25 4516

1482 3.0

21 0.440

88 ICA3620

SAP FAI

35.75 37.50

45 1.90

3.49 5069

3076 1.6

80 0.209

89 ICA3620

SAP FAI

37.10 39.82

45 9.52

3.24 4255

2273 1.9

44 0.300

90 ICA3244

SAP FAI

37.87 24.14

0 1.94

3.54 4898

1579 3.1

25 0.442

91 ICA3243

SAP FAI

37.52 36.18

0 5.22

3.43 5713

3714 1.5

107 0.134

456

92 ICA3620

SAP FAI

36.56 59.53

0 1.19

3.39 5172

3452 1.3

89 0.098

93 ICA3620

SAP FAI

37.36 40.58

0 5.33

3.38 5195

3276 1.6

85 0.170

94 ICA3244

SAP FAI

37.58 35.93

90 3.51

3.24 5537

3130 1.8

80 0.265

95 ICA3620

SAP FAI

37.57 33.75

90 3.81

3.25 5593

2724 2.1

65 0.345

96 ICA3620

SAP FAI

37.57 33.75

90 2.23

3.25 4457

2781 1.6

59 0.181

97 ICA3244

SAP FAI

37.42 45.73

45 3.42

3.35 5998

3563 1.7

104 0.227

98 ICA3244

SAP FAI

36.95 28.14

45 7.07

3.25 4516

1482 3.0

21 0.440

99 ICA3620

SAP FAI

35.75 37.50

45 1.90

3.49 5069

3076 1.6

80 0.209

100 ICA3620

SAP FAI

37.10 39.82

45 9.52

3.24 4255

2273 1.9

44 0.300

101 ICA3244

SAP FAI

37.87 24.14

0 1.94

3.54 4898

1579 3.1

25 0.442

102 ICA3243

SAP FAI

37.52 36.18

0 5.22

3.43 5713

3714 1.5

107 0.134

103 ICA3620

SAP FAI

36.56 59.53

0 1.19

3.39 5172

3452 1.3

89 0.098

104 ICA3620

SAP FAI

37.36 40.58

0 5.33

3.38 5195

3276 1.6

85 0.170

105 ICA3244

SAP FAI

37.58 35.93

90 3.51

3.24 5537

3130 1.8

80 0.265

106 ICA3620

SAP FAI

37.57 33.75

90 3.81

3.25 5593

2724 2.1

65 0.345

107 ICA3620

SAP FAI

37.57 33.75

90 2.23

3.25 4457

2781 1.6

59 0.181

457

108 ICA3244

SAP FAI

37.42 45.73

45 3.42

3.35 5998

3563 1.7

104 0.227

109 ICA3244

SAP FAI

36.95 28.14

45 7.07

3.25 4516

1482 3.0

21 0.440

110 ICA3620

SAP FAI

35.75 37.50

45 1.90

3.49 5069

3076 1.6

80 0.209

111 ICA3620

SAP FAI

37.10 39.82

45 9.52

3.24 4255

2273 1.9

44 0.300

112 ICA3244

SAP FAI

37.87 24.14

0 1.94

3.54 4898

1579 3.1

25 0.442

113 ICA3243

SAP FAI

37.52 36.18

0 5.22

3.43 5713

3714 1.5

107 0.134

114 ICA3620

SAP FAI

36.56 59.53

0 1.19

3.39 5172

3452 1.3

89 0.098

115 ICA3620

SAP FAI

37.36 40.58

0 5.33

3.38 5195

3276 1.6

85 0.170

116 ICA3244

SAP FAI

37.58 35.93

90 3.51

3.24 5537

3130 1.8

80 0.265

117 ICA3620

SAP FAI

37.57 33.75

90 3.81

3.25 5593

2724 2.1

65 0.345

118 ICA3620

SAP FAI

37.57 33.75

90 2.23

3.25 4457

2781 1.6

59 0.181

119 ICA3244

SAP FAI

37.42 45.73

45 3.42

3.35 5998

3563 1.7

104 0.227

120 ICA3244

SAP FAI

36.95 28.14

45 7.07

3.25 4516

1482 3.0

21 0.440

121 ICA3620

SAP FAI

35.75 37.50

45 1.90

3.49 5069

3076 1.6

80 0.209

122 ICA3620

SAP FAI

37.10 39.82

45 9.52

3.24 4255

2273 1.9

44 0.300

123 ICA3244

SAP FAI

37.87 24.14

0 1.94

3.54 4898

1579 3.1

25 0.442

458

124 ICA3243

SAP FAI

37.52 36.18

0 5.22

3.43 5713

3714 1.5

107 0.134

125 ICA3620

SAP FAI

36.56 59.53

0 1.19

3.39 5172

3452 1.3

89 0.098

126 ICA3620

SAP FAI

37.36 40.58

0 5.33

3.38 5195

3276 1.6

85 0.170

127 ID2402

CPX FDI

37.97 31.07

90 2.89

3.38 5082

1856 2.7

33 0.423

128 ID2402

CPX FDI

37.87 25.54

90 4.68

3.01 5108

2453 2.1

49 0.350

129 ID2402

CPX FDI

38.01 28.67

90 2.91

3.48 5088

2871 1.8

73 0.266

130 ID1780

CPX FDI

37.70 42.30

90 0.49

2.56 6364

3621 1.8

85 0.261

131 ID1780

CPX FDI

37.70 40.60

90 0.77

3.27 5857

2611 2.2

61 0.376

132 ID3294

CPX FDI

38.03 36.93

90 1.04

2.85 4867

2569 1.9

49 0.307

133 ID3294

CPX FDI

38.21 41.30

90 3.33

2.78 6212

2993 2.1

67 0.349

134 ID3202

CPX FDI

37.59 51.34

90 2.37

3.43 5667

2833 2.0

73 0.333

135 ID3284

CPX FDI

37.21 28.43

90 1.38

3.71 4375

2074 2.1

43 0.355

136 ID2212

CPX FDI

37.80 49.50

45 0.99

2.96 6494

3030 2.1

74 0.361

137 ID2402

CPX FDI

37.47 57.20

45 2.58

3.02 4524

3115 1.5

61 0.049

138 ID1780

CPX FDI

3.77 48.80

45 0.66

2.30 5568

2367 2.4

36 0.390

139 ID3294

CPX FDI

37.22 39.57

45 1.40

2.84 5909

3611 1.6

89 0.202

459

140 ID3294

CPX FDI

38.16 37.04

45 2.98

2.77 5966

2215 2.7

39 0.420

141 ID3202

CPX FDI

37.39 57.14

45 2.94

3.82 5429

2676 2.0

73 0.340

142 ID3202

CPX FDI

37.98 50.56

45 1.07

3.67 5952

3311 1.8

103 0.276

143 ID3284

CPX FDI

37.15 36.66

45 3.79

3.55 4457

3007 1.5

69 0.082

144 ID2212

CPX FDI

37.73 39.78

0 2.99

2.53 4444

2920 1.5

48 0.120

145 ID2401

CPX FDI

37.63 32.60

0 3.94

3.55 5156

2870 1.8

75 0.276

146 ID2402

CPX FDI

37.12 22.55

0 2.32

3.41 5227

1825 2.9

32 0.431

147 ID17780

CPX FDI

37.70 51.40

0 0.09

2.41 5102

2795 1.8

48 0.286

148 ID3294

CPX FDI

37.88 24.83

0 3.11

2.80 6944

3425 2.0

88 0.339

149 ID3294

CPX FDI

38.11 29.56

0 3.71

2.84 5472

2417 2.3

46 0.379

150 ID3202

CPX FDI

37.90 42.88

0 2.20

3.51 5316

2500 2.1

60 0.358

151 ID3202

CPX FDI

37.42 29.70

0 2.10

3.68 4545

2086 2.2

44 0.367

152 ID3284

CPX FDI

37.50 35.80

0 1.63

3.52 4545

2397 1.9

53 0.307

153 ID2402

CPX FDI

37.97 31.07

90 2.89

3.38 5082

1856 2.7

33 0.423

154 ID2402

CPX FDI

37.87 25.54

90 4.68

3.01 5108

2453 2.1

49 0.350

155 ID2402

CPX FDI

38.01 28.67

90 2.91

3.48 5088

2871 1.8

73 0.266

460

156 ID1780

CPX FDI

37.70 42.30

90 0.49

2.56 6364

3621 1.8

85 0.261

157 ID1780

CPX FDI

37.70 40.60

90 0.77

3.27 5857

2611 2.2

61 0.376

158 ID3294

CPX FDI

38.03 36.93

90 1.04

2.85 4867

2569 1.9

49 0.307

159 ID3294

CPX FDI

38.21 41.30

90 3.33

2.78 6212

2993 2.1

67 0.349

160 ID3202

CPX FDI

37.59 51.34

90 2.37

3.43 5667

2833 2.0

73 0.333

161 ID3284

CPX FDI

37.21 28.43

90 1.38

3.71 4375

2074 2.1

43 0.355

162 ID2212

CPX FDI

37.80 49.50

45 0.99

2.96 6494

3030 2.1

74 0.361

163 ID2402

CPX FDI

37.47 57.20

45 2.58

3.02 4524

3115 1.5

61 0.049

164 ID1780

CPX FDI

3.77 48.80

45 0.66

2.30 5568

2367 2.4

36 0.390

165 ID3294

CPX FDI

37.22 39.57

45 1.40

2.84 5909

3611 1.6

89 0.202

166 ID3294

CPX FDI

38.16 37.04

45 2.98

2.77 5966

2215 2.7

39 0.420

167 ID3202

CPX FDI

37.39 57.14

45 2.94

3.82 5429

2676 2.0

73 0.340

168 ID3202

CPX FDI

37.98 50.56

45 1.07

3.67 5952

3311 1.8

103 0.276

169 ID3284

CPX FDI

37.15 36.66

45 3.79

3.55 4457

3007 1.5

69 0.082

170 ID2212

CPX FDI

37.73 39.78

0 2.99

2.53 4444

2920 1.5

48 0.120

171 ID2401

CPX FDI

37.63 32.60

0 3.94

3.55 5156

2870 1.8

75 0.276

461

172 ID2402

CPX FDI

37.12 22.55

0 2.32

3.41 5227

1825 2.9

32 0.431

173 ID17780

CPX FDI

37.70 51.40

0 0.09

2.41 5102

2795 1.8

48 0.286

174 ID3294

CPX FDI

37.88 24.83

0 3.11

2.80 6944

3425 2.0

88 0.339

175 ID3294

CPX FDI

38.11 29.56

0 3.71

2.84 5472

2417 2.3

46 0.379

176 ID3202

CPX FDI

37.90 42.88

0 2.20

3.51 5316

2500 2.1

60 0.358

177 ID3202

CPX FDI

37.42 29.70

0 2.10

3.68 4545

2086 2.2

44 0.367

178 ID3284

CPX FDI

37.50 35.80

0 1.63

3.52 4545

2397 1.9

53 0.307

179 ID2402

CPX FDI

37.97 31.07

90 2.89

3.38 5082

1856 2.7

33 0.423

180 ID2402

CPX FDI

37.87 25.54

90 4.68

3.01 5108

2453 2.1

49 0.350

181 ID2402

CPX FDI

38.01 28.67

90 2.91

3.48 5088

2871 1.8

73 0.266

182 ID1780

CPX FDI

37.70 42.30

90 0.49

2.56 6364

3621 1.8

85 0.261

183 ID1780

CPX FDI

37.70 40.60

90 0.77

3.27 5857

2611 2.2

61 0.376

184 ID3294

CPX FDI

38.03 36.93

90 1.04

2.85 4867

2569 1.9

49 0.307

185 ID3294

CPX FDI

38.21 41.30

90 3.33

2.78 6212

2993 2.1

67 0.349

186 ID3202

CPX FDI

37.59 51.34

90 2.37

3.43 5667

2833 2.0

73 0.333

187 ID3284

CPX FDI

37.21 28.43

90 1.38

3.71 4375

2074 2.1

43 0.355

462

188 ID2212

CPX FDI

37.80 49.50

45 0.99

2.96 6494

3030 2.1

74 0.361

189 ID2402

CPX FDI

37.47 57.20

45 2.58

3.02 4524

3115 1.5

61 0.049

190 ID1780

CPX FDI

3.77 48.80

45 0.66

2.30 5568

2367 2.4

36 0.390

191 ID3294

CPX FDI

37.22 39.57

45 1.40

2.84 5909

3611 1.6

89 0.202

192 ID3294

CPX FDI

38.16 37.04

45 2.98

2.77 5966

2215 2.7

39 0.420

193 ID3202

CPX FDI

37.39 57.14

45 2.94

3.82 5429

2676 2.0

73 0.340

194 ID3202

CPX FDI

37.98 50.56

45 1.07

3.67 5952

3311 1.8

103 0.276

195 ID3284

CPX FDI

37.15 36.66

45 3.79

3.55 4457

3007 1.5

69 0.082

196 ID2212

CPX FDI

37.73 39.78

0 2.99

2.53 4444

2920 1.5

48 0.120

197 ID2401

CPX FDI

37.63 32.60

0 3.94

3.55 5156

2870 1.8

75 0.276

198 ID2402

CPX FDI

37.12 22.55

0 2.32

3.41 5227

1825 2.9

32 0.431

199 ID17780

CPX FDI

37.70 51.40

0 0.09

2.41 5102

2795 1.8

48 0.286

200 ID3294

CPX FDI

37.88 24.83

0 3.11

2.80 6944

3425 2.0

88 0.339

201 ID3294

CPX FDI

38.11 29.56

0 3.71

2.84 5472

2417 2.3

46 0.379

202 ID3202

CPX FDI

37.90 42.88

0 2.20

3.51 5316

2500 2.1

60 0.358

203 ID3202

CPX FDI

37.42 29.70

0 2.10

3.68 4545

2086 2.2

44 0.367

463

204 ID3284

CPX FDI

37.50 35.80

0 1.63

3.52 4545

2397 1.9

53 0.307

205 IM

S2348 GAL

PWQ

I 37.70

53.70 90

26.42 2.92

2978 1398

2.1 16

0.359

206 IM

S2348 GAL

PWQ

I 37.60

49.30 90

26.30 2.92

2818 1056

2.7 9

0.418

207 IM

S2348 GAL

PWQ

I 37.70

53.40 90

27.30 2.81

2733 1274

2.1 12

0.361

208 IM

S2348 GAL

PWQ

I 37.60

29.20 90

22.32 2.47

2377 1213

2.0 10

0.324

209 IM

S2348 GAL

PWQ

I 37.50

26.90 90

27.38 2.86

2766 1238

2.2 12

0.375

210 IM

S2348 GAL

PWQ

I 37.70

45.20 0

29.99 2.79

2242 1327

1.7 12

0.230

211 IM

S2348 GAL

PWQ

I 37.70

43.20 0

25.97 2.94

2099 1269

1.7 11

0.212

212 IM

S2348 GAL

PWQ

I 37.70

53.70 90

26.42 2.92

2978 1398

2.1 16

0.359

213 IM

S2348 GAL

PWQ

I 37.60

49.30 90

26.30 2.92

2818 1056

2.7 9

0.418

214 IM

S2348 GAL

PWQ

I 37.70

53.40 90

27.30 2.81

2733 1274

2.1 12

0.361

215 IM

S2348 GAL

PWQ

I 37.60

29.20 90

22.32 2.47

2377 1213

2.0 10

0.324

216 IM

S2348 GAL

PWQ

I 37.50

26.90 90

27.38 2.86

2766 1238

2.2 12

0.375

217 IM

S2348 GAL

PWQ

I 37.70

45.20 0

29.99 2.79

2242 1327

1.7 12

0.230

218 IM

S2348 GAL

PWQ

I 37.70

43.20 0

25.97 2.94

2099 1269

1.7 11

0.212

219 IM

S2348 GAL

PWQ

I 37.70

53.70 90

26.42 2.92

2978 1398

2.1 16

0.359

464

220 IM

S2348 GAL

PWQ

I 37.60

49.30 90

26.30 2.92

2818 1056

2.7 9

0.418

221 IM

S2348 GAL

PWQ

I 37.70

53.40 90

27.30 2.81

2733 1274

2.1 12

0.361

222 IM

S2348 GAL

PWQ

I 37.60

29.20 90

22.32 2.47

2377 1213

2.0 10

0.324

223 IM

S2348 GAL

PWQ

I 37.50

26.90 90

27.38 2.86

2766 1238

2.2 12

0.375

224 IM

S2348 GAL

PWQ

I 37.70

45.20 0

29.99 2.79

2242 1327

1.7 12

0.230

225 IM

S2348 GAL

PWQ

I 37.70

43.20 0

25.97 2.94

2099 1269

1.7 11

0.212

226 ICA2766

SAP PW

GI 37.80

34.20 90

22.01 3.07

3208 1965

1.6 28

0.200

227 ICA2766

SAP PW

GI 37.60

51.90 90

23.79 2.94

3239 2072

1.6 29

0.154

228 ICA2766

SAP PW

GI 37.70

48.20 90

25.02 2.82

3221 2072

1.6 28

0.147

229 IM

G1928 SAP

PWGI

37.70 48.50

90 27.56

2.57 3609

2051 1.8

27 0.261

230 IM

G1928 SAP

PWGI

37.60 48.40

90 27.47

2.58 3609

2008 1.8

27 0.276

231 ICA2766

SAP PW

GI 37.70

55.40 45

22.06 3.04

2321 1193

1.9 11

0.320

232 ICA2766

SAP PW

GI 37.70

54.20 45

23.39 2.98

1818 1074

1.7 8

0.232

233 IM

G2536 SAP

PWGI

37.70 56.70

45 32.08

2.44 2394

1435 1.7

12 0.220

234 IM

G2535 SAP

PWGI

37.70 53.20

45 27.30

2.38 2543

1522 1.7

13 0.221

235 IM

G1928 SAP

PWGI

37.80 27.10

45 28.33

2.57 2464

1667 1.5

15 0.078

465

236 ICA2766

SAP PW

GI 37.70

39.00 0

22.61 2.93

2484 1343

1.3 14

0.293

237 ICA2766

SAP PW

GI 37.70

46.80 0

24.93 2.84

1373 893

1.5 5

0.133

238 IM

G2536 SAP

PWGI

37.80 32.90

0 27.28

2.59 2270

1311 1.7

11 0.250

239 IM

G2536 SAP

PWGI

37.70 31.10

0 28.91

2.54 2109

1449 1.5

11 0.053

240 ICA2766

SAP PW

GI 37.80

34.20 90

22.01 3.07

3208 1965

1.6 28

0.200

241 ICA2766

SAP PW

GI 37.60

51.90 90

23.79 2.94

3239 2072

1.6 29

0.154

242 ICA2766

SAP PW

GI 37.70

48.20 90

25.02 2.82

3221 2072

1.6 28

0.147

243 IM

G1928 SAP

PWGI

37.70 48.50

90 27.56

2.57 3609

2051 1.8

27 0.261

244 IM

G1928 SAP

PWGI

37.60 48.40

90 27.47

2.58 3609

2008 1.8

27 0.276

245 ICA2766

SAP PW

GI 37.70

55.40 45

22.06 3.04

2321 1193

1.9 11

0.320

246 ICA2766

SAP PW

GI 37.70

54.20 45

23.39 2.98

1818 1074

1.7 8

0.232

247 IM

G2536 SAP

PWGI

37.70 56.70

45 32.08

2.44 2394

1435 1.7

12 0.220

248 IM

G2535 SAP

PWGI

37.70 53.20

45 27.30

2.38 2543

1522 1.7

13 0.221

249 IM

G1928 SAP

PWGI

37.80 27.10

45 28.33

2.57 2464

1667 1.5

15 0.078

250 ICA2766

SAP PW

GI 37.70

39.00 0

22.61 2.93

2484 1343

1.3 14

0.293

251 ICA2766

SAP PW

GI 37.70

46.80 0

24.93 2.84

1373 893

1.5 5

0.133

466

252 IM

G2536 SAP

PWGI

37.80 32.90

0 27.28

2.59 2270

1311 1.7

11 0.250

253 IM

G2536 SAP

PWGI

37.70 31.10

0 28.91

2.54 2109

1449 1.5

11 0.053

254 ICA2766

SAP PW

GI 37.80

34.20 90

22.01 3.07

3208 1965

1.6 28

0.200

255 ICA2766

SAP PW

GI 37.60

51.90 90

23.79 2.94

3239 2072

1.6 29

0.154

256 ICA2766

SAP PW

GI 37.70

48.20 90

25.02 2.82

3221 2072

1.6 28

0.147

257 IM

G1928 SAP

PWGI

37.70 48.50

90 27.56

2.57 3609

2051 1.8

27 0.261

258 IM

G1928 SAP

PWGI

37.60 48.40

90 27.47

2.58 3609

2008 1.8

27 0.276

259 ICA2766

SAP PW

GI 37.70

55.40 45

22.06 3.04

2321 1193

1.9 11

0.320

260 ICA2766

SAP PW

GI 37.70

54.20 45

23.39 2.98

1818 1074

1.7 8

0.232

261 IM

G2536 SAP

PWGI

37.70 56.70

45 32.08

2.44 2394

1435 1.7

12 0.220

262 IM

G2535 SAP

PWGI

37.70 53.20

45 27.30

2.38 2543

1522 1.7

13 0.221

263 IM

G1928 SAP

PWGI

37.80 27.10

45 28.33

2.57 2464

1667 1.5

15 0.078

264 ICA2766

SAP PW

GI 37.70

39.00 0

22.61 2.93

2484 1343

1.3 14

0.293

265 ICA2766

SAP PW

GI 37.70

46.80 0

24.93 2.84

1373 893

1.5 5

0.133

266 IM

G2536 SAP

PWGI

37.80 32.90

0 27.28

2.59 2270

1311 1.7

11 0.250

467

468

APPENDIX V

Rock test: Brazilian tensile strength test

Sequence number

Block ID

Mine

Lithotype Length

(mm

)

Diameter

(mm

)

Bulk density

(t/m3)

Anisotropy

(β)

UTS

(MPa)

Failure mode

Lab

1 G

AL-FG00010 G

AL FQ

I 38.8

62.8 3.61

90 22.3

cutting bedding U

WA

2 G

AL-FG00010 G

AL FQ

I 40.8

62.8 3.69

90 17.9

cutting bedding U

WA

3 G

AL-FG00010 G

AL FQ

I 40.1

62.8 3.68

0 13.4

Pre-existing Sb U

WA

4 G

AL-FG00010 G

AL FQ

I 41.5

62.8 3.62

45 17.3

cutting bedding U

WA

5 G

AL-FG00010 G

AL FQ

I 40.8

63.0 3.54

90 14.7

cutting bedding U

WA

6 G

AL-FG00010 G

AL FQ

I 41.6

63.0 3.45

90 13.3

cutting bedding U

WA

7 G

AL-FG00010 G

AL FQ

I 40.4

63.3 3.49

90 15.6

Pre-existing Sb U

WA

8 G

AL-FG00010 G

AL FQ

I 41.1

63.3 3.49

0 12.0

cutting bedding U

WA

9 G

AL-FG00010 G

AL FQ

I 38.6

62.2 3.67

0 18.1

Pre-existing Sb U

WA

10 G

AL-FG00010 G

AL FQ

I 39.5

62.2 3.53

90 16.1

cutting bedding U

WA

11 G

AL-FG00010 G

AL FQ

I 38.0

62.2 3.29

45 15.7

cutting bedding U

WA

12 G

AL-FG00010 G

AL FQ

I 40.0

63.0 3.65

45 16.1

cutting bedding U

WA

13 G

AL-FG00010 G

AL FQ

I 39.0

63.0 3.55

0 16.2

Pre-existing Sb U

WA

14 G

AL-FG00010 G

AL FQ

I 39.2

63.0 3.45

90 18.2

cutting bedding U

WA

15 JG

D-FD00007 JGD

FAI 37.5

63.0 3.17

90 11.3

cutting bedding U

WA

16 JG

D-FD00007 JGD

FAI 39.5

63.0 3.05

0 14.3

Pre-existing Sb U

WA

17 JG

D-FD00007 JGD

FAI 38.8

63.0 2.89

45 10.9

cutting bedding U

WA

469

18 SEG-FG0006

SEG FAI

39.8 66.6

3.10 90

8.0 cutting bedding

UW

A

19 JGD-FD00056-98

JGD FDI

41.5 50.4

3.57 45

12.0 cutting bedding

UW

A

20 JGD-FD00056-98

JGD FDI

41.4 50.4

3.47 0

10.2 Pre-existing Sb

UW

A

21 JGD-FD00056-99

JGD FDI

41.1 50.6

4.51 90


UW

A

22 JGD-FD00056-99

JGD FDI

41.3 50.6

4.58 0


UW

A

23 JGD-FD00056-100

JGD FDI

39.9 50.7

3.63 0

9.0 Pre-existing Sb

UW

A

24 JGD-FD00056-100

JGD FDI

37.8 50.7

3.69 90


UW

A

25 JGD-FD00056-101

JGD FDI

40.5 50.7

3.31 90

8.4 cutting bedding

UW

A

26 JGD-FD00056-101

JGD FDI

39.8 50.7

3.39 90


UW

A

27 JGD-FD00056-101

JGD FDI

41.0 50.3

3.66 0

6.5 cutting bedding

UW

A

28 JGD-FD00056-101

JGD FDI

40.6 50.3

3.61 45

7.5 cutting bedding

UW

A

29 JGD-FD00056-101

JGD FDI

40.0 50.0

3.72 45

4.8 cutting bedding

UW

A

30 JGD-FD00056-101

JGD FDI

40.2 50.0

3.65 45

9.1 cutting bedding

UW

A

31 JGD-FD00056-101

SEG HH

39.9 65.4

5.23 90


UW

A

32 JGD-FD00056-101

SEG HH

38.6 65.4

5.24 0


UW

A

33 JGD-FD00056-101

SEG HH

38.9 65.4

5.25 45


UW

A

34 1

TAM

HH 62.0

36.0 4.69

45 18.31

Not Available

GEOCO

NTRO

LE

35 2

TAM

HH 57.0

36.0 4.75

45 23.27

Not Available

GEOCO

NTRO

LE

36 3

TAM

HH 64.0

30.0 4.64

45 9.17

Not Available

GEOCO

NTRO

LE

37 1

TAM

FAI 60.0

37.0 3.06

45 8.95

Not Available

GEOCO

NTRO

LE

38 2

TAM

FAI 50.0

36.0 3.07

45 7.65

Not Available

GEOCO

NTRO

LE

39 3

TAM

FAI 40.0

36.0 3.27

45 9.86

Not Available

GEOCO

NTRO

LE

470

40 1

TAM

FAI 60.0

37.0 3.15

0 21.74

Not Available

GEOCO

NTRO

LE

41 2

TAM

FAI 68.0

35.0 3.15

0 29.42

Not Available

GEOCO

NTRO

LE

42 3

TAM

FAI 67.0

35.0 3.04

0 32.03

Not Available

GEOCO

NTRO

LE

43 1

TAM

HH 65.0

36.0 4.79

0 18.94

Not Available

GEOCO

NTRO

LE

44 2

TAM

HH 44.0

36.0 4.70

0 26.93

Not Available

GEOCO

NTRO

LE

45 3

TAM

HH 53.0

36.0 4.82

0 24.69

Not Available

GEOCO

NTRO

LE

46 1

TAM

HH 47.0

37.0 4.76

90 22.15

Not Available

GEOCO

NTRO

LE

47 2

TAM

HH 55.0

36.0 4.81

90 18.39

Not Available

GEOCO

NTRO

LE

48 3

TAM

HH 43.0

36.0 4.83

90 17.23

Not Available

GEOCO

NTRO

LE

49 1

TAM

FQI

53.0 35.0

3.16 45

14.96 N

ot Available GEO

CON

TROLE

50 2

TAM

FQI

43.0 35.0

3.07 45

16.62 N

ot Available GEO

CON

TROLE

51 3

TAM

FQI

35.0 34.0

3.29 45

7.81 N

ot Available GEO

CON

TROLE

52 1

TAM

FQI

53.0 36.0

3.21 90

8.94 N

ot Available GEO

CON

TROLE

53 2

TAM

FQI

50.0 36.0

3.12 90

15.74 N

ot Available GEO

CON

TROLE

54 3

TAM

FQI

38.0 36.0

3.16 90

17.23 N

ot Available GEO

CON

TROLE

55 1

TAM

FQI

61.0 36.0

3.00 0

20 N

ot Available GEO

CON

TROLE

56 2

TAM

FQI

44.0 36.0

3.00 0

9.4 N

ot Available GEO

CON

TROLE

57 3

TAM

FQI

40.0 36.0

3.03 0

9.15 N

ot Available GEO

CON

TROLE

58 1

TAM

FQI

59.0 36.0

2.96 90

16.69 N

ot Available GEO

CON

TROLE

59 2

TAM

FQI

56.0 36.0

3.10 90

12.41 N

ot Available GEO

CON

TROLE

60 3

TAM

FQI

39.0 35.0

3.21 90

12.51 N

ot Available GEO

CON

TROLE

61 M

AC FDI

22.77 54.05

3.31 90

9.5 Plane surface along Sb

Nogueira 2000

471

62 M

AC FDI

31.97 54.18

3.31 90


Nogueira 2000

63 M

AC FDI

27.63 54.1

3.31 90


Nogueira 2000

64 M

AC FDI

18.92 54.08

3.31 90

9.6 Diam

etral surface

Parallel to Sb N

ogueira 2000

65 M

AC FDI

24.33 54.12

3.31 90


Nogueira 2000

66 M

AC FDI

28.23 54.17

3.31 90

14 Diam

etral plane

Surface with no Sb

Nogueira 2000

67 M

AC FDI

20.42 54.1

3.31 90


Nogueira 2000

68 M

AC FDI

32.25 54.25

3.31 90


Nogueira 2000

69 M

AC FDI

35.43 54.18

3.31 90


Nogueira 2000

70 M

AC FDI

34.38 54.48

2.75 90


Nogueira 2000

71 M

AC FDI

19.13 54.53

2.75 90


Nogueira 2000

72 M

AC FDI

34.97 54.5

2.75 90


Nogueira 2000

73 M

AC FDI

27.4 54.43

2.94 90


Nogueira 2000

74 M

AC FDI

34.15 54.58

2.94 90

19.2 Diam

etral plane

Surface with no Sb

Nogueira 2000

75 M

AC FDI

29.65 54.53

2.94 90

11.1 Diam

etral plane

Surface with no Sb

Nogueira 2000

76 M

AC FDI

23.18 54.55

2.94 90

15.6 Diam

etral plane

Surface with no Sb

Nogueira 2000

77 M

AC FDI

22.1 54.42

2.75 90

14.9 Diam

etral plane

Surface with no Sb

Nogueira 2000

78 M

AC FDI

22.33 54.13

2.75 90

11.2 Diam

etral plane

Surface with no Sb

Nogueira 2000

472

79 M

AC FDI

29.07 53.92

2.64 90

13.7 Diam

etral plane

Surface with no Sb

Nogueira 2000

80 M

AC FDI

22.52 54.05

2.64 90

6.1 Along axial plane

Nogueira 2000

81 M

AC FDI

24.97 53.95

2.64 90

6 Along axial plane

Nogueira 2000

Nogueira, JA 2000. ‘Iron ore m

echanical intact rock properties’. Masted degree. U

niversity of Minas Gerais, Belo Horizonte, Brazil. pp.106

473

474

APPENDIX VI

Soil test: Drained direct shear test

Sequence Num

ber Block ID

Sam

ple ID Lithotype

Anisotropy

(β)

Bulk density

(t/m3)

Initial Moisture Content

(%)

σ peak

(kPa)

δpeak

(kPa)

σres

(kPa)

δres

(kPa)

1 TAM

-BL-08 10395

WHE

0 2.84

10.5 100

201.1

2 TAM

-BL-08 10395

WHE

0 2.69

13.1 400

428.3

3 TAM

-BL-08 10395

WHE

0 2.69

11.8 800

588.9

4 TAM

-BL-17 10425

WHE

0 2.16

8.4 100

130.0

5 TAM

-BL-17 10425

WHE

0 2.22

7.6 400

427.8

6 TAM

-BL-17 10425

WHE

0 2.25

5.8 800

693.9

7 TAM

-BL-18 10426

WHE

0 2.56

9.9 100

177.2

8 TAM

-BL-18 10426

WHE

0 2.44

9.8 400

352.2

9 TAM

-BL-18 10426

WHE

0 2.44

10.1 800

779.2

10 TAM

-BL-20 10428

WHE

0 1.42

14.6 100

111.4

11 TAM

-BL-20 10428

WHE

0 1.62

13.1 400

396.4

12 TAM

-BL-20 10428

WHE

0 1.74

16.0 800

720.0

13 3a

WHE

0 3.25

2.6 100

227.0 100

176.0

14 3b

WHE

0 3.25

2.6 400

557.0 400

400.0

15 3c

WHE

0 3.25

2.6 800

955.0 800

947.0

16 TAM

-BL-18 10426

WHE

45 2.33

9.3 100

130.8

17 TAM

-BL-18 10426

WHE

45 2.15

11.6 400

372.8

475

18 TAM

-BL-18 10426

WHE

45 2.25

10.8 800

631.4

19 5a

WHE

45 3.32

3.7 100

323.0 100

292.0

20 5b

WHE

45 3.32

3.7 400

658.0 400

571.0

21 5c

WHE

45 3.32

3.7 800

1243.0 800

1145.0

22 TAM

-BL-08 10395

WHE

90 3.12

9.0 100

208.6

23 TAM

-BL-08 10395

WHE

90 2.74

13.7 400

379.2

24 TAM

-BL-08 10395

WHE

90 2.82

9.5 800

803.3

25 TAM

-BL-17 10425

WHE

90 2.66

9.4 100

136.3

26 TAM

-BL-17 10425

WHE

90 2.67

10.3 400

445.6

27 TAM

-BL-17 10425

WHE

90 2.26

8.0 800

616.7

28 2.1194.04

WHE

90 502

528.0

29 2.1197.04

WHE

90 1002

930.0

30 2.1200.04

WHE

90 2002

1779.0

31 2.1199.04

WHE

90 4013

2759.0

32 2.1198.04

WHE

90 7025

4420.0

33 10a

WHE

90 3.27

3.6 100

243.0 100

164.0

34 10b

WHE

90 3.27

3.6 400

626.0 400

510.0

35 10c

WHE

90 3.27

3.6 800

1229.0 800

1210.0

36 JAN

PC -03 CIS 08-17

WHE

2.20 11.9

100 138.5

99.8

37 JAN

PC -03 CIS 08-17

WHE

2.47 11.9

200 221.2

220.0

38 JAN

PC -03 CIS 08-17

WHE

2.19 11.9

400 435.0

428.0

39 JAN

PC -03 CIS 08-17

WHE

2.60 11.9

800 628.0

622.0

476

40 JAN

PC-03 CIS 08 -18

WHE

2.23 11.7

100 158.0

117.8

41 JAN

PC-04 CIS 08 -19

WHE

2.32 11.7

200 203.5

196.3

42 JAN

PC-05 CIS 08 -20

WHE

2.14 11.7

400 387.0

374.0

43 JAN

PC-06 CIS 08 -21

WHE

2.66 11.7

800 654.0

640.0

44 TAM

-BL-03 10390

WHE

0 2.97

8.4 100

98.9

45 TAM

-BL-03 10390

WHE

0 2.85

8.2 400

569.7

46 TAM

-BL-03 10390

WHE

0 3.00

8.6 800

815.6

47 TAM

-BL-03 10390

WHE

45 2.76

7.6 100

173.9

48 TAM

-BL-03 10390

WHE

45 2.81

10.2 400

531.1

49 TAM

-BL-03 10390

WHE

45 2.79

8.2 800

807.6

50 TAM

-BL-05 10392

WQ

I 0

2.30 4.9

100 252.5

51 TAM

-BL-05 10392

WQ

I 0

2.03 10.5

400 374.2

52 TAM

-BL-05 10392

WQ

I 0

1.74 10.0

800 410.6

53 TAM

-BL-09 10396

WQ

I 0

2.95 5.8

100 234.7

54 TAM

-BL-09 10396

WQ

I 0

2.85 8.0

400 518.6

55 TAM

-BL-09 10396

WQ

I 0

2.75 10.1

800 811.9

56 TAM

-BL-11 10419

WQ

I 0

1.86 5.2

100 146.9

57 TAM

-BL-11 10419

WQ

I 0

1.68 9.8

400 365.6

58 TAM

-BL-11 10419

WQ

I 0

1.80 5.5

800 638.6

59 TAM

-BL-13 10421

WQ

I 0

1.98 7.9

100 97.8

60 TAM

-BL-13 10421

WQ

I 0

1.85 7.7

400 260.3

61 TAM

-BL-13 10421

WQ

I 0

2.21 7.2

800 522.2

477

62 TAM

-BL-15 10423

WQ

I 0

1.96 3.6

100 136.7

63 TAM

-BL-15 10423

WQ

I 0

2.09 2.6

400 425.6

64 TAM

-BL-15 10423

WQ

I 0

1.92 2.5

800 701.4

65 2.1206.04

WQ

I 0

505 479.0

66 2.1208.04

WQ

I 0

3006 1905.0

67 2.1210.04

WQ

I 0

6028 2987.0

68 TAM

-BL-13 10421

WQ

I 45

1.95 7.2

100 145.3

69 TAM

-BL-13 10421

WQ

I 45

1.86 8.2

400 346.4

70 TAM

-BL-13 10421

WQ

I 45

1.92 10.0

800 416.1

71 TAM

-BL-15 10423

WQ

I 45

1.97 3.0

100 111.9

72 TAM

-BL-15 10423

WQ

I 45

2.02 3.5

400 388.3

73 TAM

-BL-15 10423

WQ

I 45

1.98 2.7

800 754.4

74 TAM

-BL-05 10392

WQ

I 90

2.45 13.9

100 110.0

75 TAM

-BL-05 10392

WQ

I 90

1.96 24.7

400 372.2

76 TAM

-BL-05 10392

WQ

I 90

2.38 17.6

800 834.4

77 TAM

-BL-09 10396

WQ

I 90

2.88 6.9

100 186.1

78 TAM

-BL-09 10396

WQ

I 90

2.67 7.5

400 390.0

79 TAM

-BL-09 10396

WQ

I 90

2.80 8.1

800 742.2

80 TAM

-BL-11 10419

WQ

I 90

1.85 8.3

100 113.1

81 TAM

-BL-11 10419

WQ

I 90

1.90 5.1

400 310.6

82 TAM

-BL-11 10419

WQ

I 90

1.84 6.5

800 651.4

83 2.1204.04

WQ

I 90

508 355.0

478

84 2.1205.04

WQ

I 90

3012 1830.0

85 2.1207.04

WQ

I 90

6016 2619.0

86 34a

WAI

0 2.62

1.3 100

89.0 100

70.0

87 34b

WAI

0 2.62

1.3 200

148.0 200

145.0

88 34c

WAI

0 2.62

1.3 600

369.0 600

368.0

89 34d

WAI

0 2.62

1.3 800

487.0 800

477.0

90 33a

WAI

90 2.52

1.3 100

116.0 100

55.0

91 33b

WAI

90 2.52

1.3 200

178.0 200

144.0

92 33c

WAI

90 2.52

1.3 600

418.0 600

387.0

93 33d

WAI

90 2.52

1.3 800

577.0 800

574.0

94 10388

WAI

0 1.88

25.4 100

153.3

95 TAM

-BL-01 10388

WAI

0 1.46

31.8 400

138.1

96 TAM

-BL-01 10388

WAI

0 1.49

36.0 800

215.6

97 TAM

-BL-01 10389

WAI

0 3.43

9.8 100

257.5

98 TAM

-BL-02 10389

WAI

0 3.28

21.2 400

325.8

99 TAM

-BL-02 10389

WAI

0 3.30

10.5 800

946.9

100 TAM

-BL-02 23a

WAI

0 2.28

4.6 100

124.0 100

96.0

101 23b

WAI

0 2.28

4.6 200

168.0 200

106.0

102 23c

WAI

0 2.28

4.6 600

434.0 600

365.0

103 24a

WAI

0 2.70

0.3 100

54.0 100

43.0

104 24b

WAI

0 2.70

0.3 200

109.0 200

97.0

105 24c

WAI

0 2.70

0.3 600

374.0 600

375.0

479

106 24d

WAI

0 2.70

0.3 800

476.0 800

467.0

107 24e

WAI

0 2.70

3.4 100

177.0 100

143.0

108 24f

WAI

0 2.70

3.4 400

420.0 400

415.0

109 24g

WAI

0 2.70

3.4 800

810.0 800

763.0

110 10388

WAI

90 1.10

56.5 100

138.3

111 TAM

-BL-01 10388

WAI

90 1.23

51.8 400

268.9

112 TAM

-BL-01 10388

WAI

90 1.43

43.8 800

520.0

113 TAM

-BL-01 10389

WAI

90 3.43

8.8 100

4385.8

114 TAM

-BL-02 10389

WAI

90 3.35

10.8 400

391.7

115 TAM

-BL-02 10389

WAI

90 3.41

9.1 800

901.1

116 TAM

-BL-02 6a

WAI

90 2.27

4.6 100

97.0 100

89.0

117 6b

WAI

90 2.27

4.6 200

142.0 200

123.0

480

APPENDIX VII

Soil test: CIU consolidated undrained tests

Sequence number

Sample ID


(β)

Bulk density

(t/m3)

Initial moisture content

(%)

σ3

(kPa)

σ1

(kPa)

σ3'

(kPa)

σ1'

(kPa)

U

(kPa)

1 10422

PWGI

0 3.26

8.3 100

544 66

510 34

2 10422

PWGI

0 3.28

8 400

1368 324

1292 76

3 10422

PWGI

0 3.19

7.4 800

2714 637

2551 163

4 10422

PWGI

90 3.44

5.7 100

563 N

A N

A

5 10422

PWGI

90 3.57

7.2 400

1852 387

1839 13

6 10422

PWGI

90 3.23

9.7 800

2682 614

2496 186

7 W

HE 6_45 W

HE 45

4.00 8.8

50 500

46 497

3

8 W

HE 6_45 W

HE 45

4.00 8.8

200 1344

200 1344

0

9 W

HE 6_45 W

HE 45

4.00 8.8

400 2207

397 2204

3

10 W

HE 6_45 W

HE 45

4.00 8.8

800 3518

797 3515

3

11 W

HE7_45 W

HE 45

3.20 9.2

100 996

90 986

10

12 W

HE7_45 W

HE 45

3.41 9.4

400 2032

2003 371

1661

13 W

HE7_45 W

HE 45

3.28 10.9

800 3108

2779 471

2637

14 10390

WHE

0 3.21

12.9 100

1119 N

A N

A

15 10390

WHE

0 3.34

12.9 399

2050 N

A N

A

16 10390

WHE

0 3.09

13 799

2779 674

2654 125

17 10390

WHE

45 3.33

10 100

858 N

A N

A

481

18 10390

WHE

45 3.25

9.4 400

1595 375

1570 25

19 10390

WHE

45 3.15

8.5 801

2629 714

2542 87

20 10395

WHE

0 3.43

12.1 100

820 95

815 5

21 10395

WHE

0 3.33

13 400

1842 317

1759 83

22 10395

WHE

0 3.35

9.8 800

2880 680

2760 120

23 10395

WHE

90 3.43

9.1 100

554 99

553 1

24 10395

WHE

90 3.52

9.2 400

1894 365

1859 35

25 10395

WHE

90 3.48

8.1 800

3624 749

3573 51

26 10425

WHE

0 3.56

12.6 100

309 41

250 59

27 10425

WHE

0 3.16

9.7 401

832 281

712 120

28 10425

WHE

0 3.35

9.7 800

2284 400

1884 400

29 10425

WHE

90 2.94

12.3 100

524 28

452 72

30 10425

WHE

90 2.98

4.7 401

1205 197

1001 204

31 10425

WHE

90 3.23

10.7 800

1943 481

1624 319

32 10426

WHE

0 2.83

7.6 100

735 N

A N

A

33 10426

WHE

0 2.76

8.5 399

1660 304

1565 95

34 10426

WHE

0 2.73

9.4 801

3108 617

2924 184

35 10426

WHE

45 3.16

9.5 100

546 N

A N

A

36 10426

WHE

45 2.79

14 400

1876 349

1825 51

37 10426

WHE

45 2.75

15.4 800

2766 623

2589 177

38 10428

WHE

0 3.10

9.1 99

530 92

523 7

39 10428

WHE

0 3.04

10.6 400

1469 317

1386 83

482

40 10428

WHE

0 3.03

9.3 798

2103 418

1723 380

41 CIU

08-069 W

HE 90

3.23 6.8

100 898

103 901

-3

42 CIU

08-069 W

HE 90

3.17 6.8

200 1367

201 1369

-1

43 CIU

08-069 W

HE 90

3.31 6.8

400 2109

399 2108

1

44 CIU

08-069 W

HE 90

3.33 6.8

800 3605

797 3603

3

45 CIU

08-079 W

HE 0

3.11 6.8

100 850

101 851

-1

46 CIU

08-079 W

HE 0

3.18 6.8

200 1098

200 1098

0

47 CIU

08-079 W

HE 0

2.93 6.8

400 2234

403 2237

-3

48 CIU

08-079 W

HE 0

3.22 6.8

800 3828

797 3826

3

49 2.1195.04

WHE

90 2.78

5.2 490

2273 588

2371 -98

50 2.1195.04

WHE

90 3.12

5 981

3933 868

3820 113

51 2.1195.04

WHE

90 2.86

5.2 1373

3362 789

2779 584

52 10392

WQ

I 0

3.01 12

99 320

33 254

66

53 10392

WQ

I 0

2.83 10.8

399 1571

262 1434

137

54 10392

WQ

I 0

2.72 4.6

798 2118

561 1881

237

55 10392

WQ

I 90

3.00 12.4

101 318

43 260

58

56 10392

WQ

I 90

2.00 32.2

349 656

178 485

171

57 10392

WQ

I 90

3.02 12

801 1762

569 1530

232

58 10396

WQ

I 0

3.17 6.4

100 525

32 457

68

59 10396

WQ

I 0

3.44 7

400 1487

271 1358

129

60 10396

WQ

I 0

3.17 7.3

800 2520

585 2305

215

61 10396

WQ

I 90

3.53 8

100 753

98 751

2

483

62 10396

WQ

I 90

3.59 5.9

400 1896

384 1880

16

63 10396

WQ

I 90

3.59 7.3

800 2936

609 2745

191

64 10419

WQ

I 0

2.14 5.1

100 642

100 642

0

65 10419

WQ

I 0

2.09 4.5

400 2063

398 2061

2

66 10419

WQ

I 0

2.06 5

800 3745

795 3740

5

67 10419

WQ

I 90

2.04 3.8

100 613

NA

NA

68 10419

WQ

I 90

2.18 4.5

400 1717

399 1716

1

69 10419

WQ

I 90

2.04 4.2

800 2897

786 2883

14

70 10421

WQ

I 0

2.27 12

100 544

NA

NA

71 10421

WQ

I 0

2.37 12.8

400 1473

376 1449

24

72 10421

WQ

I 0

2.50 9.4

800 2846

753 2799

47

73 10421

WQ

I 45

2.37 6.8

100 419

86 405

14

74 10421

WQ

I 45

2.46 9.3

400 1109

367 1076

33

75 10421

WQ

I 45

2.42 5.6

800 2551

648 2399

152

76 10423

WQ

I 0

2.38 4.9

100 544

NA

NA

77 10423

WQ

I 0

2.41 3.9

400 2140

375 2115

25

78 10423

WQ

I 0

2.30 4.1

800 3585

767 3552

33

79 10423

WQ

I 45

2.56 3.8

100 603

92 595

8

80 10423

WQ

I 45

2.44 3.8

400 2047

375 2022

25

81 10423

WQ

I 45

2.27 2.9

800 4397

769 4366

31

82 CIU

08-099 W

GI 0

2.25 14.7

100 1031

99 1030

1

83 CIU

08-099 W

GI 0

2.34 14.7

200 1422

198 1420

2

484

84 CIU

08-099 W

GI 0

2.07 14.7

400 2104

396 2100

4

85 CIU

08-099 W

GI 0

2.32 14.7

800 3789

788 3778

12

86 CIU

08-100 W

GI 90

2.08 14.7

100 794

101 795

-1

87 CIU

08-100 W

GI 90

2.26 14.7

200 1447

199 1446

1

88 CIU

08-100 W

GI 90

2.27 14.7

400 2320

393 2313

7

89 CIU

08-100 W

GI 90

2.33 14.7

800 3680

784 3663

17

90 W

AI 3_45 W

AI 45

3.03 5.8

50 217

35 203

14

91 W

AI 3_45 W

AI 45

3.03 5.8

200 848

163 812

36

92 W

AI 3_45 W

AI 45

3.03 5.8

400 1573

346 1519

54

93 W

AI 3_45 W

AI 45

3.03 5.8

800 2543

577 2320

223

94 10388

WAI

0 2.46

25.6 100

378 79

357 21

95 10388

WAI

0 2.00

50.4 399

856 177

634 222

96 10388

WAI

0 2.27

34.7 799

1602 369

1172 430

97 10388

WAI

90 2.16

12.7 99

321 61

283 38

98 10388

WAI

90 2.41

28.9 399

1113 209

923 190

99 10388

WAI

90 2.04

42 799

1680 368

1249 431

100 10389

WAI

0 3.63

7.8 99

543 41

485 58

101 10389

WAI

0 3.67

9.7 399

2754 206

2561 193

102 10389

WAI

0 3.97

7 800

4780 482

4462 318

103 10389

WAI

90 3.79

8.2 99

1129 39

1069 60

104 10389

WAI

90 3.76

10.7 400

2665 232

1497 1168

105 10389

WAI

90 3.69

6.8 801

4146 476

3821 325

485

106 CIU

08-064 W

AI 0

2.26 13.7

50 516

46 511

4

107 CIU

08-064 W

AI 0

2.20 13.7

100 697

98 695

2

108 CIU

08-064 W

AI 0

2.31 13.7

200 1114

191 1105

9

109 CIU

08-064 W

AI 0

2.29 13.7

400 1522

375 1498

25

110 CIU

08-065 W

AI 90

2.31 22.1

50 696

48 694

2

111 CIU

08-065 W

AI 90

2.39 22.6

100 948

95 943

6

112 CIU

08-065 W

AI 90

2.39 22.3

200 1173

193 1166

7

113 CIU

08-065 W

AI 90

2.30 22.3

400 1629

379 1607

21

486

APPENDIX VIII

SOIL TEST – U

ndrained Direct shear test

Sequence

number

Sample


(°)

Dry density

(t/m3)

Moisture content

(%)

Matric suction (Peak)

- kPa

Shear Stress (Peak)

kPa

Shear Stress (Res)

kPa

Norm

al Stress

kPa

1 W

HE4 W

HE 45

3.37 2.85

0 205

185 75

2 W

HE4 W

HE 45

3.37 2.85

46 341

222 75

3 W

HE4 W

HE 45

3.37 2.85

81 536

464 75

4 W

HE4 W

HE 45

3.37 2.85

401 134

117 75

5 W

HE9 W

HE 45

2.91 2.43

0 194

182 150

6 W

HE9 W

HE 45

2.91 2.43

50 369

353 150

7 W

HE9 W

HE 45

2.91 2.43

150 375

360 150

8 W

HE9 W

HE 45

2.91 2.43

250 355

248 150

9 W

HE10 W

HE 45

2.86 3.28

0 115

86 75

10 W

HE10 W

HE 45

2.86 3.28

100 223

135 75

11 W

HE10 W

HE 45

2.86 3.28

200 299

250 75

12 W

HE10 W

HE 45

2.86 3.28

300 314

235 75

13 TAM

BL 3 W

HE 0

3.38 6.56

0 79

77 75

14 TAM

BL 3 W

HE 0

3.38 6.56

10 95

94 75

15 TAM

BL 3 W

HE 0

3.38 6.56

30 105

102 75

16 TAM

BL 3 W

HE 0

3.38 6.56

90 112

111 75

17 TAM

BL 3 W

HE 90

3.52 6.35

0 84

84 75

18 TAM

BL 3 W

HE 90

3.52 6.35

10 122

120 75

487

19 TAM

BL 3 W

HE 90

3.52 6.35

30 146

136 75

20 TAM

BL 3 W

HE 90

3.52 6.35

90 206

204 75

21 TAM

BL 18 W

HE 0

2.87 11.93

0 91

91 75

22 TAM

BL 18 W

HE 0

2.87 11.93

15 129

128 75

23 TAM

BL 18 W

HE 0

2.87 11.93

40 172

169 75

24 TAM

BL 18 W

HE 0

2.87 11.93

71 98

96 75

25 TAM

BL 18 W

HE 90

2.96 3.09

0 88

86 75

26 TAM

BL 18 W

HE 90

2.96 3.09

15 125

108 75

27 TAM

BL 18 W

HE 90

2.96 3.09

41 143

134 75

28 TAM

BL 18 W

HE 90

2.96 3.09

70 76

69 75

29 TAM

BL 12 W

QI

0 2.2

6.32 0

133 130

75

30 TAM

BL 12 W

QI

0 2.2

6.32 22

169 168

75

31 TAM

BL 12 W

QI

0 2.2

6.32 70

189 153

75

32 TAM

BL 12 W

QI

0 2.2

6.32 153

129 119

75

33 TAM

BL 12 W

QI

90 2.28

7.53 0

117 114

75

34 TAM

BL 12 W

QI

90 2.28

7.53 20

189 164

75

35 TAM

BL 12 W

QI

90 2.28

7.53 70

140 134

75

36 TAM

BL 12 W

QI

90 2.28

7.53 150

117 116

75

37 TAM

BL 16 W

QI

0 2.76

3.96 0

148 144

75

38 TAM

BL 16 W

QI

0 2.76

3.96 20

210 206

75

39 TAM

BL 16 W

QI

0 2.76

3.96 40

242 239

75

40 TAM

BL 16 W

QI

0 2.76

3.96 80

184 184

75

488

41 TAM

BL 16 W

QI

90 2.71

3.96 0

124 114

75

42 TAM

BL 16 W

QI

90 2.71

3.96 20

171 169

75

43 TAM

BL 16 W

QI

90 2.71

3.96 40

215 192

75

44 TAM

BL 16 W

QI

90 2.71

3.96 89

177 174

75

45 Tam

5_105.90_106.05

WAI

0 3.09

0 63

60 75

46 Tam

5_105.90_106.05

WAI

0 3.09

65 164

105 75

47 Tam

5_105.90_106.05

WAI

0 3.09

260 250

209 75

48 Tam

5_105.90_106.05

WAI

0 3.09

325 234

128 75

49 Tam

5_105.90_106.05

WAI

0 3.09

500 220

104 75

50 Tam

5_SP20 W

AI 0

2.84 0

57 38

75

51 Ta m

5_SP20 W

AI 0

2.84 65

122 87

75

52 Tam

5_SP20 W

AI 0

2.84 260

177 141

75

53 Tam

5_SP20 W

AI 0

2.84 325

188 147

75

54 Tam

5_SP20 W

AI 0

2.84 500

205 182

75

55 TAM

B L 1 W

AI 0

2.34 4.31

0 73

73 75

56 TAM

BL 1 W

AI 0

2.34 4.31

18 98

98 75

57 TAM

BL 1 W

AI 0

2.34 4.31

57 121

121 75

58 TAM

BL 1 W

AI 0

2.34 4.31

107 113

105 75

59 TAM

BL 1 W

AI 90

2.15 7.62

0 86

86 75

489

60 TAM

BL 1 W

AI 90

2.15 7.62

18 106

106 75

61 TAM

BL 1 W

AI 90

2.15 7.62

56 141

140 75

62 TAM

BL 1 W

AI 90

2.15 7.62

106 123

123 75

490

Documents

Brazilian banded iron formations