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Presented:

Date:

Location:

© SRK Consulting (UK) Ltd 2011. All rights reserved.

31 March 2014

Philipp Mohr

Minex Central Asia 2014 – Astana / Kazakhstan

Talk 6 – Development of an Integrated

Geotechnical Model for Open Pit

Design and its Impact on Slope

Design Angles.

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Case Study – Open Pit Project for

Narrow Vein Epithermal Gold Deposit

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Case Study – Open Pit Project for

Narrow Vein Epithermal Gold Deposit • Previously mined as UG operation, then abandoned.

• Initial studies by new owner concluded open pit mining potential.

• Project sensitive to slope angles due to pit location within deeply

incised valley.

• SRK commenced geotechnical pre-feasibility study building on

available but limited geotechnical data from scoping study.

• Hydrogeological information lacking.

• No pre-existing geological or structural 3D models

• Geotechnical study required input from geology, structural and

hydrogeology

• Scoping level optimised pit shell available

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Geological Setting • Gold mineralisation related to quartz veins and well confined hydrothermal

breccias of shallow epithermal, structurally controlled system.

• Mineralisation emplaced into tertiary basaltic-andesitic tuffs and andesitic-dacitic-

rhyolitic lava flows.

• Main quartz-gold veins act as extensional or shear vein faults as part of major

NW-SE striking steeply dipping graben structure.

• Complex structural setting with veins occurring in various orientations emplaced in

or intersected by numerous faults

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Scoping Study Gap Analysis • Complex structural setting but insufficiently understood and lack of

orientation data from rockmasses forming final pit slopes.

• Slopes sensitive to groundwater saturation; however no information on

groundwater depth.

• Resource holes drilled mainly within valley providing insufficient

intersections of footwall and hangingwall rockmass in which final slopes

will be formed.

• Poor quality ore rockmass affected by faulting, strong hydrothermal

alteration and weathering.

• Hangingwall and footwall rockmass outside of influence zone of faults

and hydrothermal fluids and thus likely to be significantly stronger.

• Scoping Study pit design based on ore rockmass thus being too

conservative.

• Upside potential for steeper slope angles motivated for integrated geo-

technical PFS level pit slope study.

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Approach to PFS Open Pit Slope Design

Geotechnical Domain Model

Failure Modes Strength Structure

Design Sectors

Stability Analysis

Final Design

Geology Model Structural

Model Rockmass

Model Hydrogeology

Model

Integrated Geotechnical Model

Kinematic Analysis for bench and inter-ramp stability

Overall and inter-ramp analysis

groundwater

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Pre-Feasibility Data Collection Programme

N mapped tunnel sections

Geotech boreholes with piezometers

Surface geotech mapping stations

Old Underground workings – no access

Resource holes extended into footwall

footwall

hangingwall

• UG and surface mapping (geology, structure and geotech).

• Rockmass and structural core logging

• Downhole televiewer imaging and core orientation for structure orientations

• Comprehensive rock mechanical laboratory testing programme

• Preliminary hydrogeological investigation (piezometer installation and permeability testing)

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Approach to Orientating Structures from

Drill Core for structural and rockmass model

• Potentially unstable borehole walls required using both downhole

televiewer imaging probes and core orientation tool.

• Using core orientation tool added data where televiewer tool could not

pass borehole blockages.

4% 5%

15%

40%

36%

Orientation Data Statistics

ACT2 - High confidence

ACT2 - Low confidence

ATV

OTV

No orientation

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Geological Modelling

• Large drillhole database used (>200 boreholes)

• Supplemented by geological surface mapping to constrain logged

lithologies.

• Modelling required re-coding of logged units into simpler but broader

units to improve wire frame continuity.

• Topographic surface used to interpret or guide some faults.

• Model contains:

o Overburden including residual soils and heavily weathered rock

o 5 modelled lithologies: felsic and intermediate tuffs, pyroclastic breccia, felsic

and intermediate lava flows.

o Lithologies further subdivided by weathering grade (UW-SW or MW)

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Geological Modelling

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Inputs to Structural Model

• Structural mapping (bedding, veins, fault planes and fault slickenlines).

• Structural lineament analysis based on high resolution topography map

• Regional structural analysis conducted by SRK

• Cross sections illustrating downhole Au values and lithological interpretations.

• Core photographs

• Geotechnical data from geotechnical drilling programme (RMR, RQD, fracture frequency, recovery, alteration etc).

• Underground fault mapping data

• 3D wireframe of Vein model;

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Structural Model – Logged Faults

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Structural Model

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Structural Model • NNW - NW striking structures: normal faults sub-parallel to the main

mineralised vein trend

• NE – WNW striking structures: represent reactivated basement controlled weaknesses. No structural control on the mineralising system

Vein Fault Group

WNW trending Fault Group

NE trending Fault Group

Vein Faults

NE striking Faults WNW

striking Faults

HW Faults

NE striking Faults

WNW striking Faults

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Rockmass Model – Data Collected • Rockmass Logging (Laubscher’s MRMR) using natural (not drill induced)

discontinuities only. Confirmed from televiewer imaging and cross

checked against core logging.

• UG Mapping confirmed logged structure orientations.

• Large number of on-site Point Load Testing carried out to determine

intact rock strength calibrated against UCS laboratory testing.

• Direct shear testing to obtain peak and basic shear strength from natural

joints and saw cut surfaces to determine joint shear strength..

• Triaxial compression testing to determine material constant Mi for input to

Hoek-Brown failure criterion.

• Shear box testing of fault gouge to obtain cohesion and friction angle.

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Rockmass Characterisation

Andesite Lapilli Tuff

Moderately weathered Unweathered Rock most

common Worst Best

most common

Worst Best

Intact Rock Strength (MPa) 23 2 85 46 34 130 Fracture Frequency (FF/m) 1.53 1.53 1.53 1.84 20.00 0.40

Joint Spacing (m) 0.46 (moderate)

0.02 (very close)

1.45 (wide)

0.54 (moderate)

0.05 very close)

2.5 (very wide)

Micro Roughness smooth

undulating smooth planar

rough undulating

smooth undulating

smooth planar

rough undulating

Macro Roughness straight straight

slightly undulating straight straight

slightly undulating

Joint Infill Clay Clay Clean Clay Clay Clean

Weighted MRMR 25 (poor) 13 (very

poor) 46 (fair) 33 (poor) 8 (very poor) 75 (good)

Weighted RMR 30 (poor) 16 (very

poor) 53 (fair) 40 (fair) 10 (very

poor) 92 (very

good)

N

S

EW 1m 1m

2m

2m

3m

3m

4m

4m

Symbol Scatter

1 Pole Vectors

2 Pole Vectors

3 Pole Vectors

4 Pole Vectors

5 Pole Vectors

Color Density Concentrations

0.00 - 1.00

1.00 - 2.00

2.00 - 3.00

3.00 - 4.00

4.00 - 5.00

5.00 - 6.00

6.00 - 7.00

7.00 - 8.00

8.00 - 9.00

9.00 - 10.00

Maximum Density 9.31%

Contour Data Pole Vectors

Contour Distribution Fisher

Counting Circle Size 1.0%

Color Dip Dip Direction Label

Mean Set Planes

1m 84 91

2m 87 62

3m 83 37

4m 36 198

Plot Mode Pole Vectors

Vector Count 346 (346 Entries)

Hemisphere Lower

Projection Equal Angle

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Rockmass and Intact Rock Strength

Lithology Weighted RMR Weighted MRMR Average Stdev Max Min Average Stdev Max Min

Andesitic lapilli tuffs (PPMI)

CW 30 14 56 16 25 11 46 13

MW 30 -- 30 30 24 -- 24 24

FR 40 14 92 10 33 11 75 8

Felsic (rhyolitic) pyroclastic breccias (PPBF)

CW 30 12 48 9 24 10 39 8

MW 34 16 86 11 27 13 70 9

FR 42 12 74 14 34 10 60 11

Felsic lavas (VF)

MW 42 7 53 30 34 6 43 24

FR 43 14 74 8 35 11 60 6

Porphyritic andesite lavas (VIA)

CW 38 -- 38 38 31 -- 31 31

MW 47 15 51 19 38 12 42 16

FR 53 14 65 23 43 11 53 18

Lithology Intact Rock Strength (MPa)

Average Stdev Max Min

Andesitic lapilli tuffs

CW 22 31 85 2

MW 34 -- 33.6 34

FR 46 34 130 0.1

Felsic (rhyolitic) pyroclastic breccias

CW 12 19 51.36 1

MW 18 16 50 1

FR 25 20 75 0.5

Felsic lavas

MW 36 31 100 4

FR 72 40 150 0.1

Porphyritic andesite lavas

CW 23 -- 22.8 23

MW 38 30 69.3 0

FR 58 29 90 6.2

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Joint Shear

Strength

• Saw cut shear tests (basic friction angle) show similar shear strength than natural joint surfaces (peak strength) which are commonly straight and rough planar.

• Most natural joints contain also clay coatings which often reduces joint wall contact.

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Properties of Modelled Faults • Modelled faults divided into two categories:

o Faults comprised of broken and crushed rock

o Faults containing clayey gouge

(c) High plasticity clayey gouge consists of illite and minor portions of swelling clays; c = 21 kPa ; phi = 13 degrees (1 shear test)

Crushed and intensively fractured fault rock; c = 50 kPa ; phi = 30 degrees (estimated)

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GROUNDWATER • Prior to PFS no groundwater information available.

• PFS commenced with a scoping level GW study to determine GW

surface, developing into a full PFS hydrogeological study as study

progressed.

• Only results of preliminary hydrogeological study available for input to

slope stability modelling.

• Well defined groundwater surface available for input to PFS slope

stability analysis based on piezometer installation in the valley slopes

and valley floor.

• Falling head tests as first pass hydrogeological testing showed rockmass

having a moderate hydraulic conductivity of 2.2E-3 to 4.5E-3 m/d.

• Joints are predominantly steeply dipping indicating a high vertical

hydraulic anisotropy allowing for both rapid recharge but less horizontal

permeability.

• Rockmass will at least partially drain freely.

• Active drainage using vertical pumping wells or horizontal drainage holes

should be successful to drain groundwater away from the slope faces to

increase slope stability.

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Geotechnical Domain Model

Fault Traces

(purple)

• Domain definition a

combination of:

• Rockmass strength

variation,

• Structural domains,

• Weathering style

• Pit sector orientation

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Geotechnical Domain Rockmass Design

Input Values (Footwall)

DO

MA

IN I

D

WA

LL

SL

IDE

SE

CT

ION

DO

MA

IN W

EA

TH

DO

MA

IN L

ITH

Domain Lith Description

WT

D I

RS

(M

Pa

)

Std

De

v I

RS

(M

Pa

)

WT

D I

RS

- 0

.5 S

TD

EV

(M

Pa)

DE

SIG

N I

RS

(M

Pa

)

WT

D R

MR

Std

De

v R

MR

WT

D R

MR

- 0

.5 S

TD

EV

DE

SIG

N R

MR

(W

TD

RM

R -

5)

Mi

De

ns

ity

(t/

m3

)

Slo

pe

He

igh

t (m

)

C_

rm (

kP

a)

[D=

0.6

]

ph

i_rm

(d

eg

) [D

=0

.6]

C_

rm (

kP

a)

[D=

0.5

]

ph

i_rm

(d

eg

) [D

=0

.5]

C_

rm (

kP

a)

[D=

0.3

]

ph

i_rm

(d

eg

) [D

=0

.3]

an

iso

tro

py

dip

an

gle

(d

eg

)

C_

j (k

Pa

) [D

=0

.6]

ph

i_j

(de

g)

[D=

0.6

]

C_

j (k

Pa

) [D

=0

.5]

ph

i_j

(de

g)

[D=

0.5

]

C_

j (k

Pa

) [D

=0

.3]

ph

i_j

(de

g)

[D=

0.3

]

1-4 ALL ALL CW ALL Overburden / HW-CW rock

5 15 2.4 25 55 25 55 25 55 25 -- -- -- -- -- --

1 FW F1 MW VF Felsic Lava Flow 68 10 63 30 36 10 31 25 15 2.4 50 160 32 175 34 225 39 77 +/- 10 52 23 55 23 60 24

1 FW F1 SW VF Felsic Lava Flow 58 30 42 45 32 13 26 27 15 2.4 50 215 37 235 39 270 42 77 +/- 10 63 24 67 24 74 25

2 FW F2 MW VF Felsic Lava Flow 51 24 39 60 32 10 27 27 15 2.4 50 240 39 260 41 300 45 77 +/- 10 72 26 70 27 90 29

2 FW F2 SW VF Felsic Lava Flow 88 36 71 75 44 16 36 39 15 2.4 50 315 47 405 48 465 51 77 +/- 10 130 30 139 30 140 32

3 FW F3 MW PPMI Andesite Lapilli Tuff 36 19 27 30 30 7 26 25 15 2.4 50 172 33 200 35 240 38 77 +/- 10 55 24 60 25 72 26

3 FW F3 SW PPMI Andesite Lapilli Tuff 64 30 49 50 36 9 31 31 15 2.4 50 250 40 275 41 320 44 77 +/- 10 81 27 90 27 105 29

4 FW F4/5 MW PPBf Rhyolite Pryoclastic Breccia

11 9 8 7 30 7 27 25 10 2.4 50 90 21 100 23 120 26 77 +/- 10 27 22 30 23 36 24

4 FW F4/5 MW PPMI Andesite Lapilli Tuff 6 10 5 5 24 9 20 20 10 2.4 50 185 33 200 35 240 38 77 +/- 10 55 24 60 25 72 26

4 FW F4/5 SW PPMI Andesite Lapilli Tuff 32 32 16 25 16 6 13 15 10 2.4 50 270 40 300 41 350 44 77 +/- 10 81 27 90 27 105 29

1-4 ALL ALL CW CRUZ Fault - Crushed 2 50 35

1-4 ALL ALL CW GOUGE Fault with clayey gouge 2 100 25

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Approach to PFS Open Pit Slope Design

Geotechnical Domain Model

Failure Modes Strength Structure

Design Sectors

Stability Analysis

Final Design

Geology Model Structural

Model Rockmass

Model Hydrogeology

Model

Integrated Geotechnical Model

Kinematic Analysis for bench and inter-ramp stability

Overall and inter-ramp analysis

groundwater

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Failure Modes – Overall and Inter-ramp

Slopes

• Used Limit Equilibrium Method - appropriate for PFS level

• Non-circular step-path failure type assumed for footwall

• Circular failure type assumed for hangingwall

Failure Mode:

Footwall – potential failure surface controlled by shear strength of both unfavourably orientated discontinuities and intact rock bridges separating discontinuities. Typical ratio of rock bridges to joint length: 20 - 40%. Hangingwall – potential failure surface controlled by strength of jointed rockmass assuming circular shape. Hoek-Brown failure Criterion

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Stability Analysis of Overall Slopes Hangingwall Generalised Hoek-Brown Failure Criterion – Input parameters

• Intact Rock Strength – UCS (MPa);

• Geological Strength Index – GSI (taken from RMR);

• Material Constant – mi;

• Disturbance Factor – D; and

• Density and height of rock material above potential slip surface.

Footwall • Step-path failure – equivalent shear strength by failure through intact rock bridges

and coplanar joints (assumed coplanar failures as worst case scenario)

• Rock bridges = H&B rockmass strength

• Discontinuities = joint shear strength

• Equivalent shear strength depends on

ratio rock bridges to joint length.

a

b

lj

lr

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Stability Analysis of Overall Slopes

Location of Section

Models for Analysis • Highest Slopes • Weakest rockmass • Large scale structures

behind pit wall

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Approach to Slope Stability Analysis

Stability Analysis

Final Design

Limit equilibrium

groundwater

Overall Slope

Inter-ramp Slope

Bench Face

Kinematic Analysis

Probabilistic Wedge Analysis

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Stability Analysis of Overall and Inter-

Ramp Slopes

Overall slope stability analysis

Inter-ramp slope stability analysis

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Bench and Inter-ramp Slope Stability

wedge failure planar failure toppling failure

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Kinematic Analysis of Structurally

Controlled Failures

• Define orientation of slope orientation

• Define frictional properties (angle of internal friction) for each structure set

• kinematic analysis for wedge, planar, and toppling type instabilities

• Analysis run for all pit slope dip directions

planar failure wedge failure toppling failure

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

Pole friction cone Daylight envelope Critical Pole vector zone where failure is possible

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Kinematic Analysis - Results

% unstable to total

potential slip surfaces Risk

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Probabilistic Wedge Analysis

Input parameters to probabilistic wedge

analysis

Maximum bench width required

according to probabilistic wedge

analysis

mean min max mean min max

1 83 95 20 Major

2 77 50 20 Major

3 75 359 20 Major

4 57 306 20 Minor

5 36 174 20 Minor

150 20 2.4

density

(t/m3)

2 0.2 6 10 5

Persistence (m)Joint Spacing (m)Joint

Set

Dip

(deg)

Dip Dir

(deg)

Range

(deg)

J set

Significance

C

(kPa)

Phi

(deg)

Maximum bench width required for

fitting berm design into inter-ramp

design

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Final Design

Pit Slope Domain

ID

Design Bench Face

Angle (°)

Bench Height

(m)

Bench Width

(m)

Max Bench Stack height

(m)

GT Berm/Ramp Width (m)

Design IRA (°)

Max. Overall Slope Angle

(°)

Footwall

1 75 20 10 100 30 52 43

2 75 20 8 100 30 57 48

3 75 20 10 100 30 52 44

4 75 20 11 100 30 50 44

Hanging-wall

5 75 20 14 100 30 46 40

6 75 20 10 100 30 52 44

7 75 20 9 100 30 54 45

8 75 20 11 100 30 51 44

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Conclusions • High sensitivity to slope angles for an open pit gold project motivated to

undertake a detailed geotechnical study with the aim to increase slope

angles and reduce stripping ratio.

• Complex structural conditions of epithermal gold narrow vein system

emplaced in brittle environment required integration with geological and

structural modelling as well as hydrogeology.

• The study provided a sound characterisation of rockmass properties,

large scale structures, distribution and extent of lithologies and

hydrogeological conditions which lead to a higher confidence in the

derived slope design compared to the scoping study.

• Overall slope angles increased from 40-42 degrees (Scoping Study) to

40-48 degrees (Pre-feasibility) improving overall project economics.

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Thank You!