Latilla Soft Floor MSc

8/2/2019 Latilla Soft Floor MSc

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I N F L U E N C E O F W E A K F L O O R O N

T H E S T A B I L I T Y O F P I L L A R S I N

S O U T H E R N A F R I C A N

C O L L I E R I E S

Jonathan William Latilla


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DECLARATION

I declare that this project report is my own unaided work with the exception of assistancereceived from D. Neal of Ingwe Rock Engineering i n building and running the MAP3DNumerical modelling (6.2). Determination of input parameters and interpretation of resultswere my own unaided work.

It is being submitted for partial fulfilment of the degree of Master of Science in theUniversity of the Witwatersrand, Johannesburg. It has not been submitted before for anydegree or examination in any other University.

Information used in this project report was obtained while the author was employed by theRock Engineering department of Ingwe Collieries (the South African division of BHPBilliton Energy Coal).

-----------------------------------J W Latilla

------------------day of ---------------------- --------- ----------------


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ABSTRACT

A number of pillar collapse cases have been recorded in southern African collieries whereweak floor rock was considered to be influential. This study attempts to analyse a few ofthese cases to determine if there are any relationships between them.

Initially a literature survey was carried out which concentrated on local as well asAustralian and United States reports and papers. This was followed by collating data fromfour failed and four stable, southern African case studies. Three and two -dimensionalnumerical modelling was done for each of the case studies as well as analysing each from a

civil engineering foundation stability point of view.

It became apparent from t he literature that pillar settlement and surface subsidence aremore prevalent overseas while locally pillar failure with its corresponding safetyimplications is more likely to occur.

A few relationships were established during analysis of the case studies. However, itappears that only a small number of the currently worked seams in southern Africa havefloors which are prone to failure, and that in these cases, it would be prudent to designpillars to higher factors of safety as well as to limit the width of panels. A provisional setof design guidelines have been proposed for the identified problem seams.

Two-dimensional numerical modelling was found to be a useful tool in studying thisproblem but considerably more geotechnical investigations are needed to evaluate the civilengineering design methods.

The problem is a rather complex one and more work, considered to be outside the scope ofthis project, will need to be carried out. T he additional work required is likely to be costly,in that a large volume of field data will have to be collected.


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ACKNOWLEDGEMENTS

Prof T.R. Stacey of the Mining Engineering Department at the University of t heWitwatersrand for his invaluable input as Mentor for this project.

Dr P.S. Buddery of Strata Engineering Australia and J.J. van Wijk of Ingwe RockEngineering for reviewing this project report.

D. Neal of Ingwe Rock Engineering for assistance with MAP3D modelling.

Ingwe Collieries (Messrs M Oppenheimer and E Scholtz) for permission to publish theEmaswati, Welgedacht and Zululand Anthracite Colliery (ZAC) case studies .

Eyesizwe Coal (Mr J Nel) for permission to publish the Matla case studies.

Ingwe Rock Engineering Department for assistance with access to computer facilities andprogrammes.

I. Canbulat of CSIR Miningtek for allowing access to the PHASE 2 program.

Prof. J.N. van der Merwe of Pretoria University for information on the Sigma case studyand papers on foundation failure from the Illinois coalfield in the USA.

B. Jack of CSIR Miningtek for locating two r eports and Dr G. Gurtunca for permission toquote from them.

Dr B. Madden for copies of reports on two Australian cases.

Dr C. Mark of NIOSH for numerous American papers on foundation failure.

Dr E. Sellers of CSIR Miningtek for various rock properties data.


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TABLE OF CONTENTS

DECLARATION ................................................................ ..................................... iiABSTRACT...................................................... ...................................................... iiiACKNOWLEDGEMENTS ..................................................................................... ivTABLE OF CONTENTS ............................................................ ............................. vLIST OF FIGURES ................................................... ............................................. ixLIST OF TABLES ..................................................... ............................................ xiiLIST OF SYMBOLS ..................... ...................................................... ................. xiii

LIST OF ABBREVIATIONS ...................................................... .......................... xiv

1 INTRODUCTION ................................................................................................... 1

2 THE GOAL OF THIS PROJECT ............................................... ............................. 1

3 INVESTIGATION INTO PREVIOUS WORK ON FOUNDATION STABILITY .... 1

3.1 Geological factors ................................................ ...................................... 23.1.1 Seams prone to floor problems ................................................. .................... 23.1.2 Clay minerals ................................................ .............................................. 33.1.3 Facies characterisation ................................................... ............................. 33.1.4 Conclusions .................................................. .............................................. 3

3.2 Heave Mechanisms ............................................... ...................................... 43.2.1 Buckling of floor beam and plastic flow modes ............................................ 43.2.2 Squeezing of weak floor rock ................................................... .................... 73.2.3 Floor failure due to horizontal stress .................................................. .......... 8

3.2.4 Coal pillar shearing through weak floor beam ............................................... 83.2.5 Swelling floor strata .............................................. .................................... 113.2.6 Bearing capacity failure around pillar edges ............................................... 113.2.7 Hard rock mines .................................................... .................................... 123 2 8 S f fl f il h i 13


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3.5.5 Colorado ....................................................... ............................................ 253.5.6 Cooranbong Colliery ...................................................... ........................... 263.5.7 Conclusions .................................................. ............................................ 273.5.8 General ................................................ .................................................... . 27

3.6 Design Procedures ................................................................................... 28 3.6.1 Numerical simulations .................................................... ........................... 283.6.2 Civil engineering approach ...................................................... .................. 303.6.3 General ................................................ .................................................... . 303.6.4 Conclusions .................................................. ............................................ 31

3.7 Main Findings of prev ious work on foundation stability ......................... 31

4 FAILED CASES ........................................................ ............................................ 32

4.1 Emaswati Colliery, main haulage south ................................................... 33 4.1.1 Introduction .................................................. ............................................ 334.1.2 Geology ........................................................ ............................................ 354.1.3 Geotechnical tests ................................................. .................................... 354.1.4 Background ................................................... ............................................ 364.1.5 Post collapse observations .............................................. ........................... 40

4.1.6 Discussion .................................................... ............................................ 414.1.7 Conclusions .................................................. ............................................ 41

4.2 Matla 1, 5 seam, panel R14 South .................................................... ........ 424.2.1 Introduction .................................................. ............................................ 424.2.2 Geology ........................................................ ............................................ 424.2.3 Geotechnical testing .............................................. .................................... 454.2.4 Post collapse observations .............................................. ........................... 464.2.5 SIMRAC investigation .................................................... ........................... 464.2.6 4 seam pillar collapse ..................................................... ........................... 48

4.2.7 Conclusions .................................................. ............................................ 48

4.3 Welgedacht, Alfred seam stooping ................................................... ........ 494.3.1 Introduction .................................................. ............................................ 494 3 2 Geology and geotechnical data 49


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5.1 Matla 1, 5 seam, Panel R14 West ..................................................... ........ 68

5.2 Matla 2, 5 seam, Panel M12 North ................................................... ........ 68

5.3 ZAC, Mngeni, panel MN ............................................... ........................... 70

5.4 ZAC, Maye, poor roof case ...................................................................... 71

5.5 Conclusions of study of stable cases ........................................................ 72

6 NUMERICAL MODELLING ..................................................... ........................... 72

6.1 Rock mass properties .................................................... ........................... 73

6.2 Three dimensional stress analysis .................................................... ........ 736.2.1 Emaswati, main haulage south ................................................. .................. 766.2.2 Matla 1, 5 seam, panel R14South ....................................................... ........ 786.2.3 Welgedacht, Alfred seam stooping ..................................................... ........ 806.2.4 ZAC, Mngeni, panel MEN1 ..................................................... .................. 816.2.5 Matla 1, 5 seam, R14West .............................................. ........................... 846.2.6 Matla 2, 5 seam, panel M12North ...................................................... ........ 846.2.7 ZAC, Mngeni, panel MN ................................................ ........................... 866.2.8 Summary of MAP3D results .................................................... .................. 886.2.9 Three-dimensional modelling conclusions .................................................. 88

6.3 Two dimensional stress analysis .............................................................. 89 6.3.1 Emaswati Colliery, Main haulage south ...................................................... 916.3.2 Matla 1, 5 seam, panel R14South ....................................................... ........ 956.3.3 Welgedacht Colliery, Alfred seam stooping ................................................ 966.3.4 ZAC, Mngeni, panel MEN1 ..................................................... ................ 1006.3.5 Matla 1, 5 seam, panel R14West .............................................. ................ 1066.3.6 Matla 2, 5 seam, panel M12North ...................................................... ...... 106

6.3.7 ZAC, Mngeni, panel MN ................................................ ......................... 1086.3.8 ZAC, Maye, soft roof problem ................................................. ................ 1116.3.9 Summary of two-dimensional modelling results ........................................ 112

6.4 Numerical modelling conclusions........................................................... 112


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9.3 Numerical modelling ..................................................... ......................... 127

9.4 Civil engineering based stability analyses ............................................. 127

10 STABILTY RATING ....................................................................................... 128

10.1 Discussion of findings ............................................................................ 128

10.2 Stability rating ...................................................................................... 132

10.3 Design procedure ................................................................................... 134

11 CONCLUSIONS .............................................................................................. 135

12 RECOMMENDED FURTHER WORK ............................................................ 136

13 REFERENCES ....................................................... .......................................... 136

14 BIBLIOGRAPHY ............................................................................................ 142

APPENDIX A. GEOTECHNICAL TESTS .................................................... .......... aImpact Splitting ...................................................................................................... aFloor rock testing ...................................................... .............................................. bAPPENDIX B. FAILED CASES ................................................. ............................. eAPPENDIX C. STABLE CASES ............................................................................ .. i


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LIST OF FIGURES

Figure 3.1. Type I floor heave. ................................................... ............................................. 5

Figure 3.2. Type II floor heave. ............................................................................. ................. 6

Figure 3.3. Position of horizontal stress induced roof and floor damage ................................... 9

Figure 3.4. Newcastle coalfield floor heave model .................................................. ................ 10

Figure 3.5. Shear failure of a foundation. ..................................................... .......................... 12

Figure 3.6. Stress distribution in a pillar with a weak band at the top. ........... .......................... 13

Figure 3.7. Stress change in the floor of a longwall tailgate. ................................................... 19

Figure 3.8. Zones of horizontal and vertical floor movement. .................................................. 21

Figure 4.1. Emaswati, main haulage south, showing pillar condition ratings. ........................... 34

Figure 4.2. Section (E-W) through collapsed Area at Emaswati (after Petzer, 1991). ................ 36

Figure 4.3. E maswati, convergence monitoring. ...................................................... ................ 40

Figure 4.4. Open topped and closed ridge types of floor heave. ............................................... 41

Figure 4.5. Matla 1, 5 seam pillar collapse area. .............................................................. ....... 43

Figure 4.6. Matla 1 - showing position of dolerite sill. ..................................................... ....... 44

Figure 4.7. Two pillar sidewall failure mechanisms at Matla. .................................................. 47

Figure 4.8. Welgedacht, Alfred seam, section entrapment area. ............................................... 50

Figure 4.9. Welgedacht. Effect of weathering on bord width. .................................................. 51

Figure 4.10. Near vertical floor buckling mechanism. ............................................................. 53

Figure 4.11. Load distribution through a failing pillar (after Wagner, 1974). ........................... 54

Figure 4.12. Mngeni, pillar deterioration area. .............................................. .......................... 56

Figure 4.13. ZAC, typical chequerboard layouts. .................................................... ................ 58


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Figure 6.8. MAP3D. Welgedacht, Alfred seam, vertical window, zz . ...................................... 81

Figure 6.9. MAP3D. ZAC, Mngeni, panel MEN1, horizontal window, step 1, zz . .................... 82

Figure 6.10. MAP3D. ZAC, Mngeni, panel MEN1, horizontal window, step2, zz . ................... 83

Figure 6.11. MAP3D. ZAC, Mngeni, panel MEN1, vertical window, step 2, zz . ...................... 83

Figure 6.12. MAP3D. ZAC, Mngeni, panel MEN1, vertical window, step 2, U t. ....................... 84

Figure 6.13. MAP3D. Matla 1, 5 seam, panel R14West, horizontal window, zz . ...................... 85

Figure 6.14. MAP3D. Matla 2, 5 seam, panel M12North, horizontal window, zz . .................... 86

Figure 6.15. MAP3D. ZAC, Mngeni, panel MN, before chequerboarding, zz . ......................... 87Figure 6.16. MAP3D. ZAC, Mngeni, panel MN, after chequerboarding, zz . ............................ 87

Figure 6.17. PHASE2. Emaswati, Main haulage south, 1(MPa), sensitivity analysis,

adjusted for percentage extraction. .............................................. ................................... 92

Figure 6.18. PHASE2. Emaswati, Main haulage south, 1(MPa), using actual pillar width. ....... 93

Figure 6.19. PHASE2, Emaswati, Main haulage south, 3(MPa). .............................................. 93

Figure 6.20. PHASE2, Emaswati, Main haulage south, U t (m). ................................................. 94

Figure 6.21. PHASE2. Emaswati, Main haulage south, U t(m), sensitivity analysis. ................... 94

Figure 6.22. PHASE2, Matla 1, 5 seam, panel R14South, (MPa). ............................................. 95

Figure 6.23. PHASE2, Matla 1, 5 seam, panel R14South, 3 (MPa). ......................................... 96

Figure 6.24. PHASE2, Welgedacht, Alfred seam, 1(MPa), overview. ...................................... 97

Figure 6.25. PHASE2, Welgedacht, Alfred seam, 1(MPa), road nearest goaf. .......................... 98

Figure 6.26. PHASE2, Welgedacht, Alfred seam, 1(MPa), first row of superimposed bords. ... 98

Figure 6.27. PHASE2, Welgedacht, Alfred seam, 3(MPa), road nearest goaf. ......................... 99

Figure 6.28. PHASE2, Welgedacht, Alfred seam, 3(MPa), first row of superimposed bords. ... 99

Figure 6.29. PHASE2. ZAC, Mngeni, panel MEN1, 1(MPa), before chequerboarding............. 101


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Figure 6.38. PHASE2. ZAC, Mngeni, panel MEN1, U t(m), 3 pillars away from

chequerboarding. ....................................................... ................................................... 105

Figure 6.39. PHASE2. Matla 1, 5 seam, panel R14West, 1 (MPa). ......................................... 106

Figure 6.40. PHASE2. Matla 1, 5 seam, panel R14West, 3 (MPa). ......................................... 107

Figure 6.41. PHASE2. Matla 2, 5 seam, panel M12North, 1 (MPa). ....................................... 107

Figure 6.42. PHASE2. Matla 2, 5 seam, panel M12North, 3 (MPa). ....................................... 108

Figure 6.43. PHASE2. ZAC, Mngeni, panel MN, 1(MPa), before chequerboarding. ................ 109

Figure 6.44. PHASE

2

. ZAC, Mngeni, panel MN, 1(MPa), after chequerboarding. .................. 109Figure 6.45. PHASE2. ZAC, Mngeni, panel MN, 3 (MPa), before chequerboarding. ............... 110

Figure 6.46. PHASE2. ZAC, Mngeni, panel MN, 3(MPa), after chequerboarding. .................. 110

Figure 6.47. PHASE2. ZAC, Maye, soft roof, 1(MPa). .................................................... ...... 111

Figure 6.48. PHASE2. ZAC, Maye, soft roof, 3(MPa). .................................................... ...... 112

Figure 7.1. Foundation bearing capacity factors. ..................................................... ............... 115

Figure 7.2. Determination of cohesive strength. ...................................................... ............... 116

Figure 8.1. Relationship between slake durability and class of floor heave. ............................ 125

Figure 10.1. Relationships plotted for stable and failed cases. ................................................ 129

Figure 10.2. Safety factor vs. mining height. ................................................................. ........ 129

Figure 10.3. safety factor vs. panel width. ............................................................................. 130

Figure 10.4. Safety factor vs. seam dip. ............................................... .................................. 130

Figure 10.5. Safety factor vs. roof impact splitting rating. .......... ........................................... 131

Figure 10.6. Safety factor vs. average weighted slake durability rating. .................................. 131

Figure 10.7. Safety factor and panel width plotted against FSR. ............................................. 133

Figure 10.8. Floor Stability Rating design chart. ..................................................... ............... 134


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LIST OF TABLES

Table 3.1. Sedimentary facies, strength properties and underground behaviour. ........................ 4

Table 3.2. Newcastle coal field; mining geometries and floor properties. ................................ 10

Table 3.3. Comparison between Klipriver and other failed cases. ............................................ 22

Table 3.4. Dimensions of some pillar collapse cases used by Salamon and Munro (1967). ........ 24

Table 3.5. Sigma Colliery failure compared to case studies. .................................................... 25

Table 3.6. Colorado failure compared to case studies ....................................................... ....... 25

Table 3.7. Cooranbong panel North B compared to case studies. ............................................. 27

Table 4.1. Emaswati Colliery, Roof and floor ratings. ...................................................... ....... 37

Table 4.2. Pillar condition rating. ............................................... ............................................ 39

Table 4.3. Matla 1, 5 seam, roof and floor ratings. .................................................. ................ 45

Table 4.4. Pillar damage rating. ................................................. ............................................ 47

Table 4.5. Welgedacht, Alfred floor, geotechnical results. ...................................................... 51

Table 4.6. Mngeni, geotechnical testing. ................... .................................................... ......... 57

Table 5.1. Matla 1, panel R14West, floor lithology. ................................................ ................ 68

Table 5.2. Matla 2, 5 seam, geotechnical results. .................................................................... 69

Table 6.1. Summary of rock mass properties ...................... .................................................... 74

Table 6.2. MAP3D Material properties. ............................................... ................................... 74

Table 6.3. MAP3D horizontal to vertical stress ratios. ............................................ ................ 74

Table 6.4. Summary of MAP3D results. ........................................................................ ......... 89Table 6.5. UCS from point load (MPa). ........................................................ .......................... 90

Table 6.6. PHASE2 transversely isotropic material. ................................................. ................ 90


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LIST OF SYMBOLS

A Dry mass before slaking (g)b Bord width (m)C Dry mass after two slaking cycles (g) c Pillar centre distance (m)Co Unconfinedcompressive strength (MPa)d30 Swelling displacement after 30 minutes (mm)

fs Fracture spacing (impact splitting) (m)hr Mean unit height into roof (impact splitting) (m) Id2 Slake durability indexL Initial length of sample (mm)Nr & Nq Bearing capacity factors

Angle of internal friction ()Pu Ultimate load applied over width B (MN/m)ru Unit rating (impact splitting)rw Weighted rating (impact splitting) S

30Swelling strain after 30 minutes

So Unconfined shear strength (MPa)To Indirect tensile strength (MPa)tu Thickness of unit (impact splitting) (m) Ut Total displacementw Pillar width (m)W Weight density (units)w/h Pillar width to height ratio1 Major Principal stress3 Minor Principal stress

zz Vertical stressx

/ Principal stress vector in the horizontal or near horizontal plane


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LIST OF ABBREVIATIONS

ACIRL Australian Coal Industry Research LaboratoryAPL Axial point load indexBDS Brazilian disk strengthCM Continuous minerCMRR Coal Mine Roof RatingCOMRO Chamber of Mines Research OrganisationCSIR Council for Scientific and Industrial ResearchDNC Durban Navigation Colliery

DPL Diametral point load indexDRMS Design Rock Mass strengthDS Duncan free swellDTS Direct tensile strengthFEM Finite element modellingFR Floor ratingFSR Floor stability ratingGENROC Gencor Rock Engineering DepartmentHF Heave factorIRED Ingwe Rock Engineering DepartmentIS Impact splittingMBC Mine-floor bearing capacityMC Natural moisture contentMSHA United States Mine Safety and Health AdministrationNIOSH National Institute of Safety and Health in the USAPLI Point Load IndexPS Pillar stressRQD Rock Quality DesignationSAIMM South African Institute of Mining and Metallurgy

SANGORM South African National Group on Rock MechanicsSD Slake durabilitySIMRAC Safety in Mines Research Advisory CommitteeSSCR Steady state creep rate


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1 INTRODUCTIONThe widely applied coal pillar design formula derived by Salamon and Munro (1967) hasbeen used by and large successfully over the past 30 years in the design of bord and pillarworkings in southern African collieries. However, over the years there have been severalcases in which workings with relatively high factor of safety pillars have failed and thepresence of soft floor strata has been judged to be a contributory or controlling factor. Themost newsworthy of these was the failure at Emaswati Colliery, Swaziland, in 1991 whichresulted in the temporary entrapment of 26 workers.

In order to avoid the problem in future there is a need to enable rock engineers to identifypotential floor stability problems and adjust t heir coal pillar design accordingly. To assistin this, cases in which soft floor strata were associated with pillar failure are analysed inthis project report together with cases in which the pillars remained stable even though softfloor was present. One case in which it was considered that the pillars were punching in toweak roof strata is also studied.

2 THE GOAL OF THIS PROJECTIt is intended that this project will i mprove understanding of pillar failures associated withsoft floor rocks in southern African collieries and enable the practical experience gainedfrom the case studies to be used to improve the design of pillars standing on weakfoundations.

The approach taken in this proj ect report, to achieve its goal of producing design guidelinesthat will reduce the likelihood of pillar foundation failures, is ou tlined below:

o Review of both local and overseas papers and reports.o Review of at least two accepted civil engineering foundation design

methods to assess their relevance to the solution of the problem.o Investigation of mining geometries, geology, convergence data and any


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approached for any anecdotal evidence or copies of any r elevant publications in theirpossession. The information gathered during this review is from South Africa, Australiaand the United States. No comparative data were found from other major coal miningcountries such as the United Kingdom, Germany, I ndia, China or Russia.

An Internet search was also conducted but produced few results.

The following aspects were considered:o Geological factorso Floor heave mechanismso Geotechnical testingo Site investigationso Pillar failureo Design procedures

These are dealt with in turn in t he sections to follow.

3.1 Geological factorsIn the following section certain geological factors are considered. These include seamswhich have been found to be prone to floor stability problems, the influence of clayminerals on floor stability and the characterisation of seam floor strata.

3.1.1 Seams prone to floor problemsProblems caused by weak floor during secondary extraction in South African collieries,mentioned by Fauconnier and Kersten (1982), are:

o The 5 seam of the Springs / W itbank coalfield has, locally, a laminated micaceousd t fl hi h t h i bl I th l ft hi hl


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3.1.2 Clay mineralsThe definition of clay minerals by Whitten and Brooks (1972) is worthy of mention. Itstates, inter alia,that: An important characteristic is their ability to lose or take up wateraccording to the temperature and amount of water present in a system. And notes further

that: The montmorillonite group is especially notable for the way in which it takes u p andloses water. Montmorillonites are formed by the alteration of basic rocks, or other silicateslow in K, under alkaline conditions.

These clay minerals are considered to be influential in the swelling floor of the 5 seam at

Matla 1 (Section 4.2).

3.1.3 Facies characterisationJermy and Ward (1988) consider that the UCS, while being easy to determine, providesvery little indication of potential roof conditions. It can however be used to understandfloor behaviour and in designing support pillars. Sedimentary facies likely to be

problematical, when encountered in the floor, are shown in Table 3.1, which is an extractfrom Jermy and Wards table of 24 di fferent facies.

There is a wide scatter of strengths for all the facies with the shaly rock being as strong andin some cases stronger than the sandy facies. The medium and coarse -grained sandstoneshave some of the lower UCS results. Jermy and Ward (1988) found that UCS and tensilestrengths are significantly reduced by increasing moisture content. No relationship wasfound to be immediately apparent between the UCS of rock and its underground behaviour.

Facies 1 has reportedly the worst geodurability, a similar facies type in northern KwaZulu-Natal (coal field and seam not specified) decomposes rapidly on exposure, resulting in roofand floor instability. Jermy and Ward (1988) conclude that: The UCS of the facies will be

of importance when considering heave associated with punching of pi llars into the floor,calculating the size of mining pillars and in the design of underground development in


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Table 3.1. Sedimentary facies, strength properties and underground behaviour.

Facies Description Strength(MPa)*

Properties of rock strataunderground

1 Massive dark grey to blackcarbonaceous siltstone.

BDS 5 8DTS 0.2 0.4

DPL 0.3APL 2 2.4UCS 45 - 75

Very poor roof and floor strata due tolow tensile strength and poor

durability. Deteriorates rapidly onexposure. Roof falls common andfloor heave occurs when depth of

mining exceeds 150m

2 Lenticular-bedded siltstone withdiscontinuous ripple cross

lamination.

BDS 7 - 10DTS 0.3 0.4

DPL 0.5

APL 2 3.5UCS 60 - 75

3 Alternation of 1cm thick layers offlat laminated siltstone and fine-

grained sandstone.

BDS 6 - 9DTS 0.2 0.4

DPL 0 - 1APL 1 4.5

UCS 80 - 105

11 Massive coarse grained whitefeldspathic sandstone.

BDS 3 - 5DTS 0.8 1.1DPL 1.5 2.5APL 1.8 2.5

UCS 45 - 55

Good roof and floor strata.Decomposes under prolonged

saturation giving rise to stabilityproblems.

12 Coarse grained and whitefeldspathic sandstone with planar /

trough crossbeds.

13 Intensely bioturbated carbonaceoussiltstone or fine-grained sandstone.

BDS 7 - 9DTS 0.2 2.3

DPL 3.5UCS 55 - 70

Deteriorates rapidly upon exposureand saturation to give roof and floor

stability problems.

* BDS = Brazilian disk strength / DTS = Direct tensile strength / DPL = Diametral point loadAPL = Axial point load / UCS = Uniaxial compressive stre ngth. Strength quoted is the central 50 % of

the data values.


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Applying civil engineering concepts to coal mine floors (Wuest, 1992) is possible but themore complex loading system and far greater area of floor exposed need to be borne inmind.

Two types of floor heave, generally similar to those identified by Wuest (1992), wereanalysed by Peng, Wang and Tsang (1995) at two mines in West Vir ginia. The mechanismsof the two types are as follo ws:

Type I

The floor consists of several thin layers of relatively strong rock. The sequence of floorfailure is:

o On development, where vertical stress is not high enough to break the floor, thehorizontal stress will not be able to cause heave.

o During retreat pillaring (stooping) the vertical stress increases. This means a buildup of shear stresses on t he bedding planes. Shear failure occurs first on beddingplanes as they have the lowest shear strength.

o The shear failure of the bedding planes changes the floor from a composite beaminto several individual beams (A in Figure 3.1). Tensile failure may then occur as aresult of redistributed tensile stresses in t he individual beams (B).

o Vertical stress is relieved by the tensile and shear failures mentioned above. If highhorizontal stress exists this will cause buckling of the broken and separatedindividual beams (C).

A B


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o Most floor heaves of t his type occur during secondary extraction (retreat pillaringor stooping) when the abutment stresses on the out-bye pillars cause the floor tofail. More importantly, the horizontal stress causes the broken floor to heave up.

Type II

The immediate floor consists of massive soft rock such as mudstone. The vertical stressexceeds the floor bearing capacity. The sequence of failure i s:

o The floor fails once the vertical stress exceeds the bearing capacity.o After failure, the bearing capacity is reduced to the residual value which remains

nearly constant while the pillar keeps on punching into the floor.o Due to pillar punching, the roof above the pillars deflects. The magnitude of heave

is dependant on the roof deflection above the pillar.

A

Residualbearingcapacity


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For control of floor heave Peng et al, 1995,recommend that:o Type I floor heave is best handled by designing the pillar such that tensile stresses

in the floor and shear stresses on the bedding planes are less than the tensile andshear strengths of the floor and bedding planes respectively.

o The decision must be made as to whether any degree of type II heave is acceptable.If none is allowed then the floor load must be kept below the floor bearingcapacity, or alternatively, some controlled heave is allowed for in the design stage.

Two control techniques were put forward:o Cut a vertical slot in the floor to relieve the stored ho rizontal stress, the main cause

of type I heave.

o Floor bolting to prevent type I heave by forming a composite floor beam and toprevent failed floor material from intruding into the entry (type II heave).Mechanical bolting was tried and found to be not effective. Floor trusses, with orwithout a centre floor cut, are effective for floor heave control.

3.2.2 Squeezing of weak floor rockIn a case study of floor heave in the Zeigler mines near Murdock, Illinois, Speck (1981)attempted to develop procedures for the detection of potential floor heave problems duringthe exploration phase of mine design. These mines lie between 45.7 and 76.2m (150 to250ft) deep and the seam is 1.8 to 2.1m ( 6 to 7ft) thick.Speck (1981) found that underclay being squeezed from under the coal pillar formed bulgesof displaced material along the pillar edges (type II heave). Another result of this squeezingwas that the outward moving underclay often caused the pillar edges to be dragged outwardand break in tension, or rash.

While a large variation in strengths was measured for floor lithologies, the UCS bestcorrelated with natural moisture content.

The closer the stronger floor units are to the coal pill ar the more resistant the floor will be


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This hypothesis and the modified equation have been tested for the South African casestudy sites and found to predict floor failure or stability in seven of the eight case studies(this is dealt with in more detail in Section 7.2).

Covering much of the same ground as Speck (1981), Stephenson and Rockaway (1981),also studied floor heave at the Zeigler mines. They state that as the floor material beneaththe pillar fails the pillar moves down, transferring its load to the adjacent pillars. If theadjacent pillar foundations are only marginally stable they may then also be overloaded.This could lead to the progressive spread of floor heave (not pillar failure) through a largearea. Stephenson and Rockaway (1981) conclude that:

o Underclay thickness up to 1.8m (6ft) causes significant changes in bearingcapacity. For underclay thicker than 1.8m this e ffect is reduced.

o Floor heave occurred at sites where the natural moisture content of the underclaywas highest.

o The ratio of claystone (the stronger unit underlying the underclay) to underclaystrength, is also an i mportant factor contributing to floor bearing capacity.

o No single set of data at any site will always predict floor behaviour.

3.2.3 Floor failure due to horizontal stressGale (1991) maintains that high horizontal stresses cause the floor to fail in tensionmanifested by a vertical crack starting at the floor surface. The cracks may start at eitherthe centre of the roadway (symmetrical loading) or closer to the pillar edge (possibledifferential loading). The magnitude of the horizontal stress may be much higher in oneparticular direction.

The effect of mining direction directly influences the effect of high horizontal stress, with

roads parallel to it being least affected while those perpendicular to it may experience roofgutter and floor failure. The locations of such failures are largely influenced by theirposition in relation to the direction of the principal horizontal stress. Figure 3.3 (StrataControl Technology, 1993) illustrates the position of roof and floor damage with respect to


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o Foundation engineering principles can be applied to the study of weak floors incoal mines and pillar settlement can be analysed using simple elastic theory.

Modulus values for claystone are an order of magnitude lower than for typical co alfloor rocks.

o In agreement with findings in the Illinois coal fields, referred to i n section 3.2.2,moisture content may be used to predict the UCS and elastic modulus.

o Due to the high safety factors of the pil lars at Cooranbong and Chain Valley,Seedsman and Gordon (1991) considers that it was unlikely that the spalling wasrelated to the onset of pillar failure. In the case of Wyee it would appear that thepillars were under designed when the overburden of the overlying Great Northernseam was active. Longwalling in the Fassifern seam caused the remnant pillars to

fail in the Great Northern seam.o For the generally low aspect ratios of the thin claystones seen in the Newcastle coal

field it is unlikely that late ral flow of claystone would tear the pillars apart.

GOOD CONDITIONS

Major horizontal stress (MHS) MHS

MHSMHS

Directionofdriveage

Direction of driveage

Stress concentration


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Table 3.2. Newcastle coal field; mining geometries and floor properties.

Chain Valley Cooranbong Wyee

Cover depth (m) 140 110 - 120 180

Mining height (m) 2.3 2.6 2.9 3.1

Pillar centres (m) 24 x 30 27 x 95 30 x 95

Bord width (m) 5.5 5.5 5.5

Pillar width- of narrowestside (m)

18.5 21.5 24.5

Extraction (%) 37 25 23Pillar safety factor

(Salamon)

3.03 4.89 3.13

Pillar width to height ratio(narrowest side)

8.04 8.27 7.90 8.44

Soft floor thickness (m) 1.08m with 0.3mcoal capping

2.8m with 0.3m coalcapping

0.5m with 1.2mcoal capping

Soft floor strength (MPa) 1.3 23.4 8 - 53 32.7

Maximum floor heave (mm) 280 >300 550Horizontal floor movement

(mm)

85 34 Not measured

Abutment zone

Peak stress


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3.2.5 Swelling floor strataA floor heave database was compiled by Hill, Gordon and Madden (1994) in which,contrary to findings detailed elsewhere (Section 2.3), the claystone mineralogy, inparticular the presence of smectite (swelling clay), is found to be of great significance indetermining floor behaviour in that it determines the propensity of the floor rock to swell.

Hill et al (1994)postulate a new model for floor heave, at least in the low stressenvironment of the first workings at Cooranbong. In this model floor heave is pri marily dueto unloading and subsequent expansion of the claystone. Pillar punching is thought not tobe a significant mechanism.

The model offers explanations for the following phenomena:o Floor heave greatly exceeds the vertical displacement of subsidence to the extent

that heave and subsidence cannot be rationally explained in terms of lateralextrusion of claystone from beneath pillars.

o Bearing capacity concepts borrowed from civil engineering fail to accuratelypredict the magnitude of heave.

o Floor heave tends to develop very slowly, whereas stress changes due to mining areusually quick.

o Dinting of roadways is often not effective in controlling heave.This model is not applicable within the context of this study as the pillars are notconsidered to have any great influence on the floor heave.

3.2.6 Bearing capacity failure around pillar edgesMadden et al (1995) state that foundation failure can take a number of forms depending o nthe strength, thickness and location of the weak stratum within the roof or f loor horizons. Acase from Galvin (1995) is included in which if the floor material is only marginally


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o Increase in pillar load and reduction in strength, being sufficient to induce pillarfailure.

o Whether the effect of increasing bord width, leading to reduced roof stability and inturn taller, less stable pillars is sufficient to cause pillar failure.

The last two factors mentioned above indicate accelerated deterioration and are not thoughtto lead to the re-establishment of stability.

In strata of moderate or higher shear strength, bearing capacity failure tends to progressgradually, with resistance to the process building up as it progresses. Unless the situation isone of deadweight loading, the load required to drive the process is not always avai lable. Inthe mining situation the load input is generally governed by displacement of the roof strata,

with the roof stiffness controlling the rate of loading of t he pillar system. With increasedpillar width, greater confinement is provided to the foundation with an increasedprobability of arresting failure.

Loaddistribution

in a pillar

Shear failureof foundation


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They quote a set of equations to calculate the bearing capacity of a cohesive, frictionalmaterial such as soft rock where the angle of friction and unit weight of the floor rock is

known.

Coates (1970) shows the stress distribution in a pillar with a weak band at the top ( Figure3.6). This geology results in transverse tension at the top of the pillar and in addition, thecompressive stresses increase towards the pillar edge near the base of the pillar. At mid -height the axial stress is slightly higher towards the pillar centre while near the floor thehighest axial stress is towards the pillar edge. If this model is inverted a weak floorscenario is r epresented.

Axial stress

at pillarheight

Axial stress

at pillarheight

Transverse stressnear centreline

Weak layer


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through the weak floor beam. This failure mode appears to contain certain aspects of thetwo types mentioned above.

It is often difficult to differentiate between the various mechanisms as they often work inconjunction.

High horizontal stresses have also been reported as causing the floor to fail in tension.While basic analysis using civil engineering concepts is possible, it must be recognised thatthe underground loading system is more complicated.

A new floor heave model put forward for low stress environments in Australia suggests that

it is pri marily due to unloading and subsequent expansion of claystone and not p illarpunching.Where the natural water content of t he immediate floor (underclay) is highest, heave ismost likely to occur.

3.2.9 ConclusionsAustralian and United States experience indicates that while pillar settlement is common inplaces, pillar failure is more likely to occur in southern Africa. This is probably due to thegenerally more conservative pillar designs practiced in these two overseas countries.

Pillar settlement and floor heave as experienced elsewhere are more of economicconsideration than a safety hazard as they are responsible for surface subsidence andclosure of underground roadways. In southern Africa, pillar failures as a result offoundation instability can have significant economic consequences but t hey also potentiallypose a greater threat to the safety of underground employees.

3 3 Geotechnical Testing


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o Borehole shear tests to obtain unconfined shear strength ( So) and angle of internalfriction ()

o Plate loading tests of immediate floor strata, under as-mined and soaked-wetconditions, to obtain UBC and deformation modulus at 50% and 90% of UB C(DM50 and DM90 ). These tests were done in freshly mined areas with 152, 203 and305mm (6, 8 and 12in) square plates.

o Engineering index properties, which included: natural moisture content (MC),density, particle size analysis, Atterberg limits, clay mineralogy, axial swellingstrain, and slake durability.

o Laboratory strength and deformation properties which included unconfinedcompressive strength (Co) and modulus of elasticity at 50% of Co (E50), Point-LoadIndex across bedding planes (PLI) and indirect tensile strength (To).

Chugh et al come to the following conclusions and recommendations:o UBC under test plates cannot be estimated with significant confidence from

unconfined compressive strength data. This approach generally overestimates theUBC.

o UBC underneath test plates can be esti mated with significant confidence from Toand values obtained from borehole shear tests. UBC is slightly underestimated

with this approach. In both this and the above conclusion, no mention was made ofadjusting the result either up or down to better reflect the UBC under test plates.o Angle of internal friction, , is a very i mportant parameter to enable accurate

estimation of UBC under plates or full sized pillars.o Axial Point Load Index (PLI) does not correlate as well as To with engineering

index properties or strength-deformation properties of floor strata.o Rock Quality Designation (RQD) does not correlate well with in situ strength-

deformation properties.o The determination of compressive strength of underclay as propo sed by Speck

(1981) may not be valid when the moisture content is less than 6%.o The following simple engineering tests may be used to characterise floor strata:

natural moisture content, indirect tensile strength, Atterberg Limits a nd axialswelling strain. The first two should be conducted at 150mm intervals while the lasttwo should be at 300mm intervals.


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3.3.2 Swelling potentialShakoor and Sarman (1992) carried out engineering tests on mudrock samples collected inmany different states of the United States. The properties determined are as follows: grainsize distribution, clay content, clay type, texture, percent absorption, Atterberg limits,specific gravity of solids, dry density, void ratio, second cycle slake durability index, U CS,volumetric increase and swelling pressure. The percentage of 0.005mm clay was used toclassify mudrocks into: claystone (>66% clay), mudstone (33-66% clay), mudshales (33-66% clay, laminated) and siltstones (


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determined quickly and simply from any roof exposure higher than the bolted interval, suchas in roof falls or air-crossings. No time consuming and expensive laboratory tests are

required nor is it necessary to drill boreholes. UCS for example, is estimated from the typeof damage sustained by the rock after a blow with a ball -peen hammer.

The modifications to the CMRR to produce the floor r ating (FR) include:o The CMRR has an adjustment factor to take into consideration the strengthening

effect of a strong band within the bolted horizon. This has been eliminated for thefloor strata rating.

o Where the CMRR rates discontinuities and unit strength as 70% and 30% of totalrating respectively, the FR assumes that the rock strength is probably as important

a factor in floor stability analysis as discontinuities. For this reason the t wo aspectshave equal weighting for the FR.

Both CMRR and FR have been determined (where possible) as part of the analysis of thecase studies for this research (Sections 4 and 5). While the method is quick and simple touse, the lack of access to most of the failed and stable sites resulted in less than half beingrated. From the limited data available no correlation is apparent and, therefore, no furtherwork on either the CMRR or FR was undertaken.

3.3.4 Slake durability and Duncan swell testsBuddery and Oldroyd (1992) published a methodology for r ating coal mine floors asdetermined from slake durability and Duncan free swell tests. This work was re-evaluatedby Latilla et al (2002). The system is relatively quick and simple to use and allows testingof the typically weak floor rock encountered beneath some seams. These tests have beenused on borehole cores drilled at or near some of the case study sites and are more fully

described in Appendix A.


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3.3.6 ConclusionsCost constraints preclude the drilling of boreholes close to existing failed and stable areasto carry out further geotechnical tests. As a result, the most useful tests are t hose which canbe carried out simply and quickly on relatively small i rregular samples taken undergroundas close to the areas of interest as possible. Of these, the most useful is the determinationof natural moisture content. As noted prev iously, this value, utilised in Specksmodification of Vesics formula, has been used successfully to determine foundation

stability in seven out of eight of the case study sites ( section 7.2).

Natural moisture content, slake durability and Duncan swell should be routinely collectedfor core samples of the floor, especially in seams with a history of floor failure, namely,the Highveld coalfield 5 seam, the 3 seam of the Free State coalfield, the Alfred seam inKwaZulu-Natal and the Main seam of the Zululand and Swaziland coalfields.

3.4 Floor Behaviour MonitoringThree cases where the behaviour of the floor was monitored were identified during theliterature survey, two in South Africa and one in t he United States.

3.4.1 Durban Navigation CollieryOzan (1991b) studied floor behaviour in the tailgate of a longwall at Durban NavigationColliery (DNC) as the face approached.

Panel parameters were as follow:Cover depth: 218mSeam thickness: 0.9 to 1.0mMining height in gateroads: 2.0m


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Figure 3.7. Stress change in the floor of a longwall tailgate.

3.4.2 Natal Anthracite CollieryA study was carried out into the behaviour of the surrounding strata during pillar extractionby Ozan and Prohaska (1992).

Panel parameters were as follow:Cover depth: 210mSeam thickness: 1.8mPillar centres: 23x23m and 23x46m

TailgateChain pillar

Longwall face

Horizontal stressincrease in floor:4.76MPaVertical stress

increase in floor:2.9MPa


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3.4.3 Central IllinoisChugh, Chandrashekhar and Caudle (1988) studied laboratory and field results and appliedfinite element modelling (FEM) to try to understand the interaction of floor, pillar and roofstrata. This study was carried out in two central Illinois mines in order to:

o Analyse stress changes in coal pillars and floor strata.o Determine the character of horizontal and vertical floor movements.o Correlate floor heave, roof sag, pillar settlement and opening convergence.o Study the variation of pillar settlement and floor heave rates as a function of ti me,

mining geometry and location within the panel.o Establish the functional relationships between pillar settlement and floor heave.

Available panel parameters were as follow:Seam thickness: 1.8m (assumed) (6ft)Pillar centres: 27.7 and 32.3mBord width: 5m (assumed)Pillar width: 22.9 and 27.4m (75 to 90ft)Pillar width/height ratio: 12.5 and 15.0m

It was found (inter alia) that:

o Pillar settlement of 25 to 50mm is relatively uniform across the panel except for thepillars adjacent to the barrier.

o A stress relief zone extended 0.9 to 1.2m (3 to 4ft) into the pillars. Sloughing ofcoal from the lower half of the pillars was common at all sites. The zone ofhorizontal floor movement extended about 3 to 4.3m (10 to 14ft) into t he floor(Figure 3.8).

o Horizontal floor movements were generally directed across entr ies towards thecentre with a magnitude of between 7 and 22mm (0.3 to 0.9in). The lateralmovements were associated with significant shearing stresses.

o

Horizontal floor movement is consistent with type II (closed ridge) floor heave.o Negligible horizontal movement was observed parallel to the entries.o Most sub-floor vertical movement was limited to the upper 2.4m (8ft) of underclay

(Figure 3.8).Th f d b d i l i i ifi b d


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As was the case in the work by Stephenson and Rockaway (1981), there was no mentionmade of pillars actually failing due to foundation failure.

3.4.4 ConclusionsIn the three documented floor monitoring cases, while there was floor heave and even somepillar damage, no mention of subsequent pillar failure is made. It is assumed that thelongwall chain pillars did not fail as no mention is made of such an occurrence.

In the DNC and Central Illinois cases the floor heave was type II (closed ridge) asmovement or increase of stress in the horizontal plane was detected. The other case (NatalAnthracite Colliery) was type I heave (open crack).

The magnitude of floor heave was larger for the type I heave at 120mm compared to 2mmand between 7 and 22mm in the two type II cases.

Pillar stress

relief zone

Coal sloughing from

lower half of pillar.

Z f


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Similarities between these cases and the failed case st udies (Section 4) are also highlightedwhere they are considered to be significant.

3.5.1 Swaziland and KwaZulu-NatalOldroyd and Buddery (1988) report on two cases of non -violent pillar failure at collieriesin Swaziland and KwaZulu-Natal. One of these was considered in the case studies (Section4.5.1). They conclude that the load is largely controlled by the panel geometry and the

elastic moduli of the roof strata and coal seam (barriers). The pillar response is determinedby the load/deformation characteristics of the pillars themselves and the physical propertiesof the roof and floor. Depending on the strength of the roof, pillars and floor, failure maybe accompanied by roof fracturing or floor heave.

3.5.2 Klipriver CoalfieldA number of cases of falls of ground in the Klipriver coalfield were investigated byBuddery and McGregor (1990), one of which is deemed to have been caused by weak floor.

The mining geometry is compared to those of the f ailed cases (Section 8) in Table 3.3:

Table 3.3. Comparison between Klipriver and other failed cases.

Klipriver Failed cases (Section 8)

Cover depth (m) 26.1 to 26.2 32.5 to 165

Mining height (m) 2.1 to 2.2 2.4 to 3.8Pillar centre distance (m) 12 11 to 24.4

Bord width (m) 5.9 to 6.06 5.9 to 8

Pillar width (m) 6.1 to 5.94 5.1 to 18.1S f f (S l ) 3 63 1 51 2 3


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o There was considerable evidence of floor heave and failure near the fall and thepillars appeared to have punched into the floor to a depth of 0.5 to 1m.

o Joints had opened up, generally sympathetic to the pillar sides, resulting inconsiderable spalling of t he coal pillars.

o The fall area was the lowest portion in that area of the mine. This result ed in watercollecting there which saturated the floor rock. From borehole information, thesaturated material would have been 3.5m thick.

o The average pillar stress after mining was 2.5MPa. Based on consolidation tests onsaturated floor rock, the resulting settlement beneath the pillars would be 80mmwith a corresponding 200mm of swell in the (unloaded) bords.

o Release of load at the pillar edges allowed the floor to swell, which assisted thespalling process. The increased load on the remaining pillar co re resulted in greatersettlement. About 900mm of spalling would effectively double t he pillar loadresulting in 110mm of settlement. The load/settlement/spalling sequence wouldhave continued until the floor and pillars failed.

o The tendency for parts of the floor to settle (beneath pillars) and others to swell(beneath bords) would have created internal shear stresses, which would contributeto floor failure.

o The presence of major joints would have contributed to the fall as they would resultin localised stress changes and provide planes of weakness along which pillars androof could fail.

It is clear from t he above that the floor failure was of the class II type and that the presenceof swelling materials combined with water was influential in causing f oundation failure.

3.5.3 Salamon and Munro collapsed casesMadden and Hardman (1992) studied the long-term stability of coal pillars. They quoteSalamon as giving natural causes as one of the reasons for the scatter of pillar strength andload values, that is, variations in coal strength, seam structure and roof and floor quality.


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3.5.4 Sigma CollieryTwo cases of pillar failure were reported by van der Mer we (1998), both on the No 3 seamat Sigma Colliery.

Case 1 occurred in bord and pillar workings at a depth of 120m with a 3m mining height.The pillar centres were 24m and the bord width 6.6m. The floor was reportedly a micaceoussandstone similar to that of the Witbank and Highveld No 5 seam. The section was very we tand the sandstone turned to a clay -like substance, more liquid than plastic inperformance. The floor sandstone flowed out from beneath the pillars and opened cracks

through the centre of pillars dividing them in half.

The sandstone is generally 0.2 to 0.5m thick in t his area, but in this case it was 1m thick.

Case 2 was in a longwall panel very close to completion, mining into the equipment take -off road. The face stopped for one shift due to belt problems. At this stage the pillar

between the face and take-off road was 4m wide and 100m long. This long slender pillarpunched into the soft floor, which heaved, trapping the shearer. The face supports wentiron bound resulting in a six month production delay. The mining geometry of the Sigma

Colliery case 1 is compared to those of the failed sites from Section 8 in Table 3.5.

Table 3.5 shows that Sigma Colliery case 1 falls within t he mining geometry ranges of thefailed cases for all but pillar width to height ratio.

Table 3.4. Dimensions of some pillar collapse cases used by Salamon and Munro

(1967).

Case No. 54 55 58

Colliery Welgedacht Blesbok South WitbankSeam Springs Witbank No. 5 Witbank No. 5

Depth (m) 62.5 65.0 39.6

Pillar width (m) 6.10 3.27 5.20

Bord width (m) 7 60 5 83 6 40


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Table 3.5. Sigma Colliery failure compared to case studies.

Sigma Colliery case 1 Failed cases (Section 8) Cover depth (m) 120 32.5 to 165

Mining height (m) 3 2.4 to 3.8Pillar centre distance (m) 24 11 to 24.4

Bord width (m) 6.6 5.9 to 8

Pillar width (m) 17.4 5.1 to 18.1

Safety factor (Salamon) 2.3 1.51 to 2.3

Pillar width to height ratio 5.8 2.1 to 5.6

van der Merwe (1995) states that foundation failure may occur where the pillar loa d is morethan twice the UCS of the floor rock. Lateral displacement of the floor material in thesecases resulted in the pillars being torn apart with wide vertical cracks being visible.

The lack of UCS results for the case study sites and the fact that obtaining samples frommost of these areas is unlikely in the foreseeable future makes it difficult to verify thisstatement for the case study sites.

3.5.5 ColoradoIn a mine fatal accident report published in 1999 by the Mine Safety and HealthAdministration (MSHA), it is stated that severe floor heave was present during pillarextraction at a mine in Colorado. In this area the floor heaved 0.9 to 1.2m (3 to 4ft) duringthe final stages of pillar extraction. At the ti me a bounce was experienced which is

described as, when the pillar actually yields, you can feel the floor give. Substantial floorheave was reported in the section prior to the accident. The D seam overlies the previously

worked B seam by 82.3m (270ft). The roof consisted of interbedded shal es, sandstones andcoal while the floor was described as primarily shale. The mining geometry at this site iscompared to that of the failed case study sites in Table 3.6.


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Only mining height and pillar width to height ratio of the Colorado case fall within the

range for the failed case study sites. The presence of dolerite intrusions is common to thefailed cases and the burn line was probably caused by a similar feature. It was not clear

from the report if the pillar sloughage was associated with floor movement or not.

3.5.6 Cooranbong CollieryIn a study at Cooranbong Colliery, Hill (1994) concludes that:

o Heave and subsidence were only loosely connected. This is due to swelling ofsmectite clay and buckling of the floor as a result of horizontal stress. Neither canbe explained in terms of extrusion of claystone beneath the pillars.

o Subsidence was largely attributed to consolidation of the claystone as water wasforced out resulting in consolidation and an increase in the shear strength of theclaystone.

o Subsidence tends to occur fairly rapidly after mining and then slows considerablywith very small movements still taking place several years after mining.

o A conservative pillar design approach was recommended, namely:i. Pillar safety factors should be in excess of 3.

ii . Percentage extraction not to exceed 50%.iii. Minimum pillar width to height ratio of 5.iv. Regular pillar geometries to be adopted.v. Substantial barriers to be left between panels, with w/h10.

vi. Leave floor coal to strengthen the base of pillars.vii. Restrict total extraction in proximity to areas where the surface subsidence

must be prevented.

Considering that the range for pillar safety factors of the failed case studies (Section 8) is1.51 to 2.3, that percentage extraction is between 48.8 and 78.5% and that the pillar widthto height ratio ranges between 2.1 and 5.6, items i, ii and iii in the above list could havebeen compiled with the southern African failed cases in mind.


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Table 3.7. Cooranbong panel North B compared to case studies.

Cooranbong case Failed cases (Section 8) Cover depth (m) 75 32.5 to 165

Mining height (m) 4 2.4 to 3.8Pillar centre distance (m) 19 11 to 24.4

Bord width (m) 7 5.9 to 8

Pillar width (m) 12 5.1 to 18.1

Safety factor (Salamon) 1.92 1.51 to 2.3

Pillar width to height ratio 3 2.1 to 5.6

3.5.7 Conclusionso Pillar safety factors vary greatly in the South African collapsed cases, from 0.76 to

3.36 while in the failed case studies (Section 4) the safety factors range from 1.51to 2.3. In the two overseas cases the safety factor (Salamon) is between 1.37 and1.92.

o Average pillar width to height ratios are 2.96 and 4.29 for the South African casesand US / Australian cases, respectively. If the Sigma Colliery case is excluded theSouth African average drops to 2.25. For the failed case study sites the width toheight ratio is 2.1 to 5.6.

o The presence of water (saturated floor rock) or dolerite intrusions (burning) isrecorded in a significant number of cases both here and overseas.

o Most of the cases experienced type II floor heave with associated accelerated pillarspalling.

o Other problems recorded as occurring in two or more cases are horizontal stressdriven failures and irregularly sized pillars.

o Considering that the range for pillar safety factors of the failed case studies is1.51 to 2.3, for percentage extraction is 48.8 to 78.5% and for pillar width to heightratio is 2.1 to 5.6, the re commendations made by Hill ( 1994) could have beencompiled with the southern African failed cases in mind. These are: that the safetyfactor should exceed 3, the percentage extraction should not be more than 50% and


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o One case of a massive pillar collapse was experienced in a trona mine that wasclearly attributable to floor failure. It occurred at a depth of 490m and 1.6km 2

collapsed at once causing one fatality and generati ng a Richter 5.3 earthquake.

3.6 Design ProceduresA number of different design approaches have been identified in the literature and aresummarised in the following section.

3.6.1 Numerical simulationsOzan (1991a) looks in detail at various civil engineerin g approaches to foundation designand found that they have a significant shortcoming in that the influence of adjacent pillarsis not taken into account. He concludes that the methods have promise if this shortcomingcould be overcome.

Ozan (1991a) reports a programme of numerical modelling using the FLAC code in whichapplied load, material properties and mining height were kept constant while pillar widthand centre distance were changed. The ratio of centre distance and pillar width ( c/w) variedbetween 3 and 1.25.

w

bwwc

/ Equation 1

Where:c = pillar centre distance (m)b = bord width (m)


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then reducing further to 2.7mm at 20. As stated earlier, Chugh, Atri and Dougherty (1988)

found that only 16% of cases had an angle of internal friction lower than 20 .

Ozan (1991a) states that, in general, the three main r easons for floor heave are:o Swelling rock in the floor strata.o High horizontal stresses.o Pillar punching of smaller pillars in high vertical stress environments.

In another numerical simulation Ozan (1991a) used the Flac code to model a soft floor(E=20Mpa) beneath a single 10 x 40m footing, loaded with net intensity of 50kPa. Afterapproximately 1500 time steps equilibrium was reached with displacement of 23.5mm. The

average foundation settlement was found to be about 20mm. The loads and strengths usedin this simulation are far removed from the reality of underground coal mining and theresults should be viewed in this light.

Looking at the above items in more detail, he goes on to state that problems can beexpected in roadways where the floor consists of clay material, particularlymontmorillonite, but notes that no design methods applicable to this aspect of floor heavecould be found. The following r emedies are proposed:

o Prevent water from entering floor layers.o Provided the layer is not too thick, re move the swelling rock. This could prove

difficult from an operational point of view as the swelling rock would need to beeither handled by the washing plant or stowed in the workings.

o Leave a floor coal cover, practical in thicker seams only. o Remove swelling floor layer and replace with non-swell material. This may only be

practical in the case of a main travelling road as the cost of this operation would berelatively high.

The correct value of the kratio (horizontal/vertical stress) for South African coal mineswas little known at the time that Ozans report was written, but he quotes it as varying from

0.5 to over 2 dependent on geological structure (Fauconnier and Kirsten, 1992) whileSantos and Bieniawski (1989) are reported to have stated that in shallower mines k is wellabove 1.0.


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While the use of numerical modelling has been evaluated in a number of studies nodefinitive set of guidelines has been established to assist with the design of pillars standing

on soft floors. This aspect has been studied in more detail in Section 6.3 where it is foundthat the two-dimensional package PHASE2 gives encouraging results.

3.6.2 Civil engineering approachTerzaghis (1964) method for calculating the bearing capacity of foundations (which is

widely used in civil engineering) is evaluated by Madden et al (1995). This will not bediscussed in detail here, as the civil engineering approach to the problem is investigated inSection 7.1.

Rockaway and Stephenson (1982) state that various studies have at least attempted toqualitatively describe the floor failure process and suggest techniques to overcome theproblem. The varying success of the methods at different sites points to the problem beingmulti-faceted. Using Vesics equation (Vesic, 1975) th ey determine factors of safety, whichcorrelate well with observed mine floor conditions. It should be noted that in both papersby Rockaway and Stephenson (Rockaway and Stephenson, 1982 and Stephenson and

Rockaway 1981), no mention is made of the foundation failures leading to pillar failure,although surface subsidence is a problem above areas of punching pillars.

Speck (1981) quoted a modification to Vesics foundation strength equation that has been

applied as part of this research (Section 7.2).

3.6.3 GeneralYork, Canbulat and Jack (2000) note that even though pillars are designed according tocurrent accepted practice there are still cases of failure due to weakness in t he surroundingstrata They identified roof and floor stability as one of nine factors affecting coal pillar


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Coal pillar foundation failure is covered by Peng (1978) although he also makes no mentionof pillar failures associated with weak floor. In the majority of United States references,

floor heave is considered more of a production nuisance than a safety is sue. He quotes thefollowing equation to determine the ultimate bearing capacity of a pillar foundation usingthe Rankine wedge concept.

qru

u WhNNWB

B

PP

2

Equation 2.

Where:

Pu = the ultimate load applied along a width BW = Weight density of the foundation soilh = Pillar height

Nr and Nq are bearing capacity factors dependent upon the frictional characteristics ofthe soil. For = 20, Nr = 5 and Nq= 7.

In this book Peng also describes the bearing capacity t est and associated equipment. Thecomplexity and size of the apparatus is such that these tests will only be carried out inSouth African collieries if floor failure becomes a far more prevalent problem.

Galvin (1995) states that operators should be aware of t he following:o On the basis of soil and foundation engineering principles, the ratio of pillar width

to weak floor t hickness has a major controlling influence on the development ofbearing capacity failure. The bearing capacity of a weak foundation decreasesmarkedly with an increase in weak floor thickness. Increasing pillar width increasesthe pillar strength, reduces load and hence reduces the probability of bearingcapacity failure.

o With time, soft strata may undergo significant consolidation under the effects ofpillar load. Differential floor displacement and subsidence due to consolidation

should not be taken as indicators of pillar or bearing capacity failure.o The long-term effect of water on the strength of weaker foundation material needs

to be determined.


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studies in sections 4 and 5 of this project report. Most of the repor ts and papers studiedwere from South Africa, Australia and the United States of America.

The literature indicates that certain seams have floors more likely to fail than others, the 5seam of the Witbank, and Highveld coalfields as well as the Alfred and Main seams inKwaZulu-Natal being especially prone. Sandier facies generally form weaker pillarfoundations than those consisting of shaly material because they are able to absorb moremoisture. The relationship between moisture content and floor stabilit y is explored in moredetail in Section 7.2.

Australian and United States experience indicates that while pillar settlement is not

atypical in certain areas, pillar failure is more likely to occur in southern Afr ica. This isprobably due to the generally more conservative pillar designs practised in these twooverseas countries.

Pillar settlement and floor heave as experienced elsewhere tend to be more of economicconsideration than a safety hazard as they lead to surface subsidence and undergroundroadway closures. In southern Africa, pillar failures resulting from foundation instabilitycan have significant economic consequences but they also potentially pose a greater threatto the safety of underground personnel.

It is relatively simple and inexpensive to determine moisture content as well as slakedurability and Duncan free swell indices (Appendix A). As such, this data may be routinelygathered from core samples of the floor, especially in seams with a h istory of floor failure,such as the Highveld coalfield 5 seam, the 3 seam of the Free State coalfield, the Alfredseam in Kwa-Zulu Natal and the Main seam of the Zululand and Swaziland coalfields.

In the three documented floor monitoring cases, while there was floor heave and even somepillar damage, no mention of subsequent pillar failure is made.

Average pillar width to height ratios are 2.96 and 4.29 for t he South African cases and US /Australian cases, respectively. If the Sigma Colliery case is excluded the South Africanaverage drops to 2.25. For the failed case study sites the width to height ratio is 2.1 to 5.6.


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Failed cases are defined as where there has been total pillar collapse or else severe pillar

damage (indicating that the pillars have passed their peak load carrying capacity) andwhere, at the same time, the floor has shown signs of severe instability.

The following aspects, identified during the literature survey, have been investigated orcarried out at the case study sites (where practical):

o The case study floor failures have been classified as type I, II or Newcastle,according to Peng, Wang and Tsang (1995) or Seedsman and Gordon (1991).

o Coal mine roof rating (CMRR) and floor rating (FR) have been determined at mostaccessible sites.

o Floor strata have been classified according to the facies descriptions of Jermy andWard (1988).In the following sections the circumstances surrounding the failures are described andavailable pertinent geotechnical and mining geometry data given. In addition, the factorsconsidered most likely to have caused the failures are detailed for each of the case studysites.

4.1 Emaswati Colliery, main haulage south4.1.1 IntroductionOn Saturday 8 th June 1991, a major collapse of ground occurred at Emaswati Collierysituated near Mpaka in the Kingdom of Swaziland. Twenty-six underground employees ofthe mine were trapped by the collapse and were subsequently all r escued uninjured by the

Chamber of Mines Rescue Brigade using the Rescue Drill based at the Colliery TrainingCollege, Witbank.

An entire drill and blast sections equipment was lost in this incident but work r esumed


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4.1.2 GeologyAll coal produced at Emaswati was mined from the main seam which is on average 3.1mthick (range, 1.5 to 4.3m) and dips towards the east at 1 to 2 ( locally up to 6). Theimmediate seam roof is a shale band overlain by a thin coal streak. This unit is usually 0.5to 0.6m thick but can reach 1.1m in places. It forms a very poor roof and is usually minedwith the seam. Above t his lies a f alse roof band consisting of coarse grained, carbonaceousshaly sandstone. When thin, this band tends to part with the production blast. Overlyingthese two weaker zones is massive, coarse-grained sandstone up to 14m t hick. The floor iscomprised of interlaminated bands of shale, siltstone and fine-grained sandstone which isshaly or silty.

The structural geology of the mine is fairly complex, with numerous dolerite dykes from 1to 30m thick and sills up to 12m thick. Occasional faults with displacement up to 10moccur and there are numerous joints in the seam associated with the aforementioned majorstructural units.

The failed pillars lay within an area completely enclosed by two dolerite sills and a d yke.As a result of the i ntrusions, the seam within this area was devolatilised. The thicker of thetwo dolerite sills (7m thick) lies between 30m and 70m below the seam (Figure 4.2).

The sediments within the failed area were tilted towards the south and east due to doleriteactivity. It is postulated that an elevated horizontal str ess environment was created by theseintrusions. Numerous slickensided joints, parallel to the bedding planes, were caused bythese stresses and weakened the floor strata.

4.1.3 Geotechnical testsAs this mine is closed it i s not possible to obtain access to the collapsed area. However,some exploration borehole cores were tested at Emaswati prior to the collapse (Oldroyd andLatilla 1989) of which three EM18 19a and 39) were within 500m of the failed area


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Figure 4.2. Section (E-W) through collapsed Area at Emaswati (after Petzer, 1991).


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Table 4.1. Emaswati Colliery, Roof and floor ratings.

Borehole

number

Distance from

collapse area

Roof rating

(impact splitting) Floor rating

EM 18 355m SW Good: IS=98

0.32m carbonaceous

sandstone overlain by

2.31m medium to coarse-

grained sandstone.

0.31m moderate - SD 14-26,overlying good floor.Weighted SD for 0.5m of floor 14.9

(Moderate)

0.21m interlaminated and micaceous

sandstone underlain by 0.10m

medium-grained sandstone and 0.50m

sandy shale.

EM 19a 440m SSW Very good: IS=127

0.21m carbonaceous and

micaceous sandstone

overlain by 1.99m coarse

grained sandstone.

0.08m good DS


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Oldroyd stated that convergence would continue and even accelerate but that a pillar ru n

was not expected due to relatively high width to height ratios and the fact that no spalledcoal had been cleaned up. Roof to floor convergence monitoring stations were established.

Mining was continuing to the east and south of this area, with the southerly workingsseparated from the failure area by long pillars along a dyke. After considering developing asecond access way and wrapping the pillars with mesh and haulage rope it was decided thatthe best option would be to fill three roadways and the splits i n between with wastematerial. Soil or sand supplied through a borehole from surface was considered best, as itwould have a lower void space requiring less compaction. This was to be placed to as close

to the roof as possible. The effect of the waste filling would be to provide lateral restraintto the pillars as well as to limit convergence once the waste began to be compacted.

Convergence monitoring indicated that although convergence was continuing, it was at areduced rate, (Latilla, 1990). The amount of sand filling required was subsequently reducedto filling in between clusters of four pillars as shown in Figure 4.1. The outsides of eachcluster of four pillars were to have been wrapped with diamond or weld mesh, attached tothe pillars by roofbolts and lined on the inside with brattice. This would ensure that theretaining wall of brattice was more flexible than if mine poles were used. This remedial

work was started but not finished before the colla pse occurred. Unfortunately, no record

can be found showing which pillar clusters had been completed but indications are thatthree out of seven had been filled.

In March 1991 an increase in convergence rate was detected as can be seen in Figure 4.3.At this stage it was recommended that the wire mesh wrapped around the pillar clustersshould be pulled closer to the pillar sides (Latilla, 1991).

At 10am on Saturday 8 June 1991, a major fall of ground occurred in thi s haulage trapping26 men. With the assistance of the Chamber of Mines r escue drill they were all brought to

safety by 5pm the following day. A detailed site investigation was carried out (Buddery,1991), and it was observed that the failed area was comple tely enclosed by two doleritesills and a dolerite dyke as shown in Figure 4.1. The collapsed panel was in contact withthese features for a great proportion of its perimeter.


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Table 4.2. Pillar condition rating.

ZAC

rating

number

ZAC pillar condition description

Equivalent

condition rating for

Emaswati.

1 Intact. CM cutter drum marks visible over more than 90% ofpillar.

Intact

2 Light spalling,


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-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Jan-90

Mar-90

May-90

Jul-90

Sep-90

Nov-90

Jan-91

Mar-91

May-91

Date

Convergence(mm

stn 1

stn 2

stn 2a

stn 3b

stn 4a

stn 5a

stn 6

Figure 4.3. Emaswati, convergence monitoring.

4.1.5 Post collapse observationsThe northern edge of t he collapse was plotted during the investigation, (Buddery, 1991)while the position of the southern edge is as r eported by the trapped miners (Figure 4.1).

In places it was observed that the roof had fallen out to a height of 1 to 2m and intactpillars could be seen in the collapsed area, as could a ventilation wall and mine pole (itemsI and X in Figure 4.1). The roof above the fall appeared reasonably intact and seemed toh b id d it


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4.1.6 DiscussionBuddery (1991) suggests that: Coal devolatilisation and severe jointing gave rise to pillar yield. Floor failure may also promote pillar yield as lateral floor movement peels off the

pillar edges. At some point (beneath the surface depression), the floor underwent massive failure

resulting in an extension of this failure mode around the perimeter. At this stage, thepillars, then still largely intact, would have punched into the floor.

The failed floor at Emaswati is structurally weak rather than compris ing of very weakrock.

The remedial measures being implemented were designed to control pillar failure andtheir ability to arrest floor deterioration is not known. Cluster stick packs, being verystiff, may have contributed to floor damage.

Floor beams remain the samelength

OPEN TOPPED

CLOSED RIDGE


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below the pillars occurred in this case. The overall failure mode was more akin to thatexperienced in the USA and Australia in that the pillars largely remained intact and the

roof failed around them. However, in those two countries this type of failure i s normallyassociated with type II (closed ridge) floor heave whereas in this case the floor heave wasof the type I (open topped) variety. This new classification of floor heave types isconsidered useful and will be done for all the failed and stable case studies.

The pillars exhibiting the greatest

Documents

Latilla Soft Floor MSc