Input to Orepass Design —

A Numerical Modeling Study

Jonny Sjöberg

Axel Bolin

Abel Sánchez Juncal

Thomas Wettainen

Diego Mas Ivars

Fredrik Perman

Development

Production drilling

and blasting

Mucking

Dumping in ore

passes

Dumping to trains and

transport to crusher

Sublevel caving & orepasses

Ore pass stability

Fall-outs in orepasses

Orepass 216

Commissioned april 2011

Closed for renovation

Orepass 225

Commissioned Oct 2012

Permanent closure April 2013

Spalling failure in ventilation shaft

1044 m Level 1079 m Level

Problems & Opportunities

• Orepass design guidelines required for

potentially continued mining at depth

• Observations validation design:

– Stress-induced failure

– Validate strength and stress values

– Investigate influence of nearby large-scale

structures

– Design options (location, orientation, shape)

Objective & Scope

• Validate rock strength and stress state

through comparison with observed

fallouts in orepasses and shafts

• Determine the optimal orientation and

location of orepasses for future mining

• Effects of wear only accounted for

implicitly by simulating a change in

orepass geometry

• Iron ore producer

• Two underground mines in

operation

– Kiruna

• 1 orebody (Kiirunavaara)

• Annual production 29 Mton

– Malmberget

• 10 actively mined orebodies

• Annual production 16 Mton

• Mining only with sublevel

caving method

The LKAB Mining Company

The LKAB Malmberget Mine

• Many orebodies of

varying size and shape

(8 km2 area)

• Mining currently at

550–850 m depth

• Mineralization to 1300

m depth (?)

• Hard, strong rock

mixed with weak, soft

rock + some large-

scale structures

• Several non-daylighting

orebodiesNorth

Alliansen

PrintzsköldDennewitz

The LKAB Malmberget Mine

• Many orebodies of

varying size and shape

(8 km2 area)

• Mining currently at

550–850 m depth

• Mineralization to 1300

m depth (?)

• Hard, strong rock

mixed with weak, soft

rock + some large-

scale structures

• Several non-daylighting

orebodiesNorth

Alliansen

PrintzsköldDennewitz

Problem Description – Ore Pass Fall-Out

Orepass

BS216

Structure DZ035

(green)

Structure DZ034

(red)

Orepass

BS225

Alliansen

orebody

Dennewitz

orebody

FLAC3D

coordinate system

z : Level

y : North

x : East

xz

y North

Modeling Approach

• Local model:

– 2D-section perpendicular to orepass axis

– Boundary stresses from mine-scale model

• Mine-scale model

– 3D model, calibrated

against stress measurements

Modeling Approach

• Analysis of two levels in each orepass:

– Upper portion (no fall-outs)

– Lower portion (extensive fall-outs)

• Parametric studies:

– Material models

– Strength values

– Location of large-scale structures

Geometry and Modeling Approach

Orepass

BS216

Structure DZ035

(green)

Structure DZ034

(red)

Orepass

BS225

Alliansen

orebody

Dennewitz

orebody

FLAC3D

coordinate system

z : Level

y : North

x : East

xz

y

Level 1148

Level 1218

Damage

zone (level

curves)

Perpendicular

sections to orepass

axis

Orepass

BS216

Damage

zone (point

cloud)

Level 1000

Level 1059

Perpendicular sections

to orepass axis

Orepass

BS225

Brittle Material Model; CWFS

Cohesion-Weakening Friction-Strengthening

Plastic straineps_Coh

Initial cohesion

Residual cohesion

Co

he

sio

n

Strain

Plastic

strain

YieldStr

ess

Elastic

strain

Peak strength

Residual

strength

Plastic straineps_Fric

Initial

friction

Frictio

n

Residual friction

Strain-softening model Variation of friction with

plastic strain

Variation of cohesion

with plastic strain

Material model for brittle failure

In other words:

"c and then tan fi"

not "c plus tan fi"

60 m

60 m

≈ 0.06 m

0.10 m

Finest

zone size

Zoom

Zoom

Diameter

3 m

Shell

4.5 m

North

90 m

33 m

15 m

2 m

Orepass

BS225

Orepass

BS216

North

90 m

90 m

FLAC (2D) model

• Model section

perpendicular to

orepass axis

• High resolution

(10 cm zone size)

Model with structures

• Larger model

• Same resolution

Material Properties

• Parameter values estimated from

laboratory tests, logging & experience

• Properties defined for dominant rocks:– RL = Red leptite

– GL = Grey leptite

– BI = Biotite

• Properties weighted by rock type:

FLAC_p = RL(%) * RL_p + GL(%) * GL_p + BI(%) * BI_p

Representative Results

Orepass – Yielding

(b)(a)

(b)(a)

Mohr-Coulomb

• Perfectly plastic

• 50% RL, 50% GL

CWFS

• Cohesion weak-

ening, frictional

strengthening

• 50% RL, 50% GL

Orepass – Yielding

Comparison

with fallouts

• CWFS

• 100% GL

Observed fallout

depth

(a) (b)

Influence of nearby

structures

• CWFS, 100% GL

• Structure simulated

as weak zone, c=0,

f=20 (Mohr-

Coulomb)

Ventilation Shaft

(c)(a) (b)

Dennewitz

orebody

Ventilation Shaft

Öde E8

Ore pass

BS225

Level 1076

Cross-section orientation

Dip = 30⁰ ; Dip dir. = 35⁰

Ore pass

BS225

Ventilation shaft

Öde E8

y’x’

z’

• CWFS

• Fine-tuning of rock mass strength

parameters

Validated Strengths (CWFS)

Rock mass c [MPa] f [°] Plastic strain limits [%]

IBeps tm

[MPa]Initial Residual Initial Residual eps_Coh eps_Fric

55-60% RL and

40-45% GL 55.0 6.2 0 46.2 0.2 0.4 1 0.95

60% RL

40 % GL

Conclusions

• Brittle material model (CWFS) required

to replicated notch-shaped fallouts &

spalling failure

• Strength values representative for

stress-induced orepass failures

• Large-scale structures influence orepass

stability – but only when in close

proximity to the boundary (< 10 m).

Design Considerations

Future Orepass Design

• Analysis of different orepass locations

and orientations for potentially deeper

mining

• Application the Alliansen-Printzsköld

orebody and the Fabian orebody

(two major future production areas)

• CWFS material model

• Orepass "groove" (wear effect)

Analysed Cases

Alliansen-

Printzsköld

Fabian

Analysed Cases

FLAC (Version 7.00)

LEGEND

16-Jul-14 17:09

step 88436

-1.000E+00 <x< 1.000E+00

5.000E-01 <y< 2.500E+00

Boundary plot

0 5E -1

Grid plot

0 5E -1

0.600

0.800

1.000

1.200

1.400

1.600

1.800

2.000

2.200

2.400

-0.900 -0.700 -0.500 -0.300 -0.100 0.100 0.300 0.500 0.700 0.900

JOB TITLE : Channel and slits geometries

Itasca Consultants AB

Sweden

12 m

12 m

Orepass with

"groove"

Destressing slot

0

20

40

60

80

100

120

140

160

180

0 500 1,000 1,500 2,000

Sig

ma D

[M

Pa]

X-axis [m]

Pre-mining

Step14

Step20

Step22

Step24

Step34

Printzsköld

Level 1350

Stress at Orepass Location

Effect of Orepass Location (& Strength)

Middle location East location East location &

lower strength

Stage

C

D

E

F

Stage

G

H

I

Progressive Failure

Design Recommendations (I)

• Influencing factors:

– Rock mass strength

– Geographical location (stress state)

– Orepass geometry

– Orepass orientation

• East location (for Alliansen-Printzsköld)

is more advantageous)

• Parallel orientation is (slightly) preferable

Decreasing

importance

Design Recommendations (II)

• De-stressing slot not recommended;

deconfinement leads to increased rock

mass damage near the orepass

• Progressive geometrical changes due to

wear may lead to more extensive spalling;

must be considered in future work

• 3D stress model of the orepass should be

considered

The funding by LKAB is gratefully acknowledged

Special thanks to: Jimmy Töyrä (LKAB)