Cover

Contents

Preface and acknowledgements

About the editors

1 Introduction

1.1 Background

1.2 General descriptions and definitions for weak rocks

1.2.1 Cemented sediments

1.2.2 Weak sedimentary rocks

1.2.3 Saprolites: weathered rock and residual soil

1.2.4 Soft iron ores and leached rocks

1.2.5 Hydrothermally altered rocks

1.3 Slope design terminology

1.3.1 Slope configurations

1.3.2 Instability

1.4 Design implications

1.5 Design implementation

2 Field data collection and methodology

2.1 Introduction

2.2 General field descriptions

2.2.1 Field estimate of strength

2.2.2 Weathering and alteration

2.2.3 Moisture-sensitive weak rocks

2.2.4 Residual soils and weathered rocks

2.3 Field mapping

2.4 Core logging, sampling and sample preservation

2.4.1 Core boxing and photography

2.4.2 Core logging

2.4.3 Discontinuities and bedding plane shears

2.4.4 Sampling

2.5 Laboratory testing

2.5.1 Routine test methods

2.5.2 Characterisation tests

2.5.3 Strength testing

2.6 In-situ borehole characterisation

2.6.1 Borehole strength and stiffness methods

2.6.2 Geophysical methods

2.6.3 Geochemical testing

2.7 Summary

3 Weak rock strength models

3.1 Introduction

3.2 Geological and structural models

3.2.1 Lithology and alteration

3.2.2 Major structures

3.2.3 Structural fabric

3.2.4 Seismicity

3.3 Role of rock mass classifications systems

3.4 Shear strength and failure criteria of weak rocks in laboratory tests

3.4.1 Shear strength and failure envelopes

3.4.2 Stresses and stress path during open pit excavation

3.4.3 Factors affecting weak rock strength

3.4.4 Laboratory testing of weak rock

3.4.5 Weak rock behaviour

3.4.6 Anisotropic shear strength

3.4.7 Partially saturated strength

3.4.8 Coupled hydromechanical response during mining

3.5 Erodibility/degradability

4 The role of water in slope design for weak rocks

4.1 Introduction

4.2 Hydrogeological background

4.2.1 General classification

4.2.2 Discussion of basic parameters

4.2.3 Unconfined and confined groundwater

4.2.4 Pore pressure

4.2.5 Hydromechanical coupling

4.2.6 Piping

4.2.7 Fault gouge

4.3 Groundwater characterisation

4.3.1 General

4.3.2 Defining the goals

4.3.3 Data collection and compilation

4.4 Development of a conceptual hydrogeological model

4.4.1 General

4.4.2 Regional- and mine-scale model

4.4.3 Sector-scale model

4.5 Analysis and modelling of pore pressure

4.5.1 Defining the goals

4.5.2 Pore pressure input to the geotechnical analysis

4.5.3 Analysis of pore pressures

4.5.4 Planning of numerical models

4.5.5 Inclusion of hydromechanical coupling

4.5.6 Development of numerical models

4.6 Depressurisation of weak rocks

4.6.1 Importance of time

4.6.2 Importance of recharge

4.6.3 Methods for depressurisation of weak rock units

4.7 Characterisation of surface water

4.7.1 Sources of surface water

4.7.2 Estimating flow rates

5 Slope design considerations

5.1 Introduction

5.2 Modes of failure

5.2.1 Primary modes

5.2.2 Secondary modes

5.3 Instability mechanisms

5.3.1 Cohesion-loss processes

5.3.2 Weak planes in bedded deposits

5.3.3 Collapse in high porosity weak rocks

5.3.4 Summary of instability mechanisms

5.4 Stability analyses and predictions

5.4.1 Approaches

5.4.2 Seismic analysis

5.5 Limit equilibrium analyses

5.5.1 LEM methods

5.5.2 Role of Factor of Safety

5.5.3 Deformations and Factor of Safety

5.5.4 The postulated slip surface

5.5.5 Anisotropy

5.6 Numerical approaches

5.6.1 Role of numerical models

5.6.2 Continuum models

5.6.3 Discontinuum models

5.6.4 Simulating common behaviour modes in numerical models

5.6.5 Pore water pressures

5.6.6 Shear strength reduction

5.6.7 Application of numerical models

5.6.8 Summary of numerical approaches

5.7 Role of back analyses

5.8 Acceptance criteria

5.8.1 Limit-based design (pre-feasibility)

5.8.2 Performance-based design during operations

5.9 Summary

6 Cemented sediments

6.1 Introduction

6.2 General geological setting

6.2.1 Geology

6.2.2 Structural geology

6.3 Hydrogeology

6.3.1 Basin and range deposits

6.3.2 Cemented bedded sediments: Carlin Formation

6.4 General geotechnical properties

6.4.1 Cemented bedded sediments

6.4.2 Cemented gravels

6.5 Slope design considerations

6.6 Goldstrike Betze-Post open pit, Nevada: instability in the Carlin Formation

6.6.1 Introduction

6.6.2 Geology

6.6.3 East wall development history

6.6.4 Engineering geology

6.6.5 Hydrogeology

6.6.6 Laboratory testing and material and strength properties

6.6.7 North-east layback slope analysis and design

6.6.8 Numerical modelling of deep-seated slope deformation

6.6.9 Carlin Formation instability

6.6.10 Continued instability, monitoring, and remediation

6.6.11 Control of slope movement in the Carlin Formation

6.6.12 Conclusions

6.7 Nine Points slope failure at Newmont’s Gold Quarry open pit

6.7.1 Introduction

6.7.2 Geology

6.7.3 Hydrogeology

6.7.4 Midway slope instability

6.7.5 April 2009 Nine Points slope instability

6.7.6 Post-April 2009 Nine Points instability behaviour

6.7.7 December 2009 failure event

6.7.8 Geological investigation and model update

6.7.9 Initial slope modelling results

6.7.10 Geotechnical drilling results

6.7.11 Hydrogeology results

6.7.12 Material strength results

6.7.13 Slope remediation design

6.7.14 Summary of lessons learned

6.8 Overview of open pit experience in cemented (alluvium) gravels found in south-western United States

6.8.1 General

6.8.2 Geological setting for cemented gravels of the south-western United States

6.8.3 Material properties characterisation

6.8.4 Hydrogeology

6.8.5 Pit slope performance

6.8.6 Design considerations

6.8.7 Operational considerations

6.8.8 Slope performance

6.9 Ministro Hales Mine, Codelco: bench failure in massive gravels

6.9.1 Background

6.9.2 Characterisation

6.9.3 Failure description

6.9.4 Back analysis

6.9.5 Proposed change in bench geometry

6.9.6 Conclusions and recommendations

Acknowledgements

7 Weak sedimentary mudrocks

7.1 Introduction

7.2 General geological setting

7.2.1 Geology

7.2.2 Structural geology

7.3 Hydrogeology

7.3.1 Mudrocks

7.3.2 Borates

7.4 General geotechnical properties

7.4.1 Terminology

7.4.2 Micro-fabric, macro-fabric, fissures and bedding plane shears

7.4.3 Mineralogy and plasticity

7.4.4 Strength, modulus and moisture

7.4.5 Swelling, softening and time-dependent deformations

7.4.6 Classification parameters

7.5 Slope design considerations

7.5.1 Failure kinematics

7.5.2 Design process

7.6 Voorspoed Mine, South Africa: open pit diamond mining in weak mudrock

7.6.1 Introduction

7.6.2 Geological setting

7.6.3 Slope performance

7.6.4 Feasibility study and current geotechnical domains

7.6.5 Laboratory testing database and core logging information

7.6.6 Precipitation and groundwater

7.6.7 Hypothesised failure mechanisms

7.6.8 Summary of Voorspoed failure mechanisms and consequence on design

7.6.9 Risk management strategy

7.6.10 Future design and mining strategy

7.6.11 Conclusions

7.7 Rio Tinto Minerals Boron operation: design considerations for weak lakebed sedimentary rocks

7.7.1 Introduction

7.7.2 Geology

7.7.3 Hydrogeology

7.7.4 Rock mass strengths and design considerations

7.7.5 Design considerations

7.7.6 Conclusions

Acknowledgements

8 Weak sedimentary coal, chalk and limestone

8.1 Introduction

8.2 General geological setting

8.2.1 Geology

8.2.2 Structural geology

8.2.3 Material properties

8.3 Slope design considerations

8.3.1 Typical failure modes

8.3.2 Slope designs

8.4 Weak coal measures

8.4.1 Overview

8.4.2 Derivation of empirical rock mass shear strength models

8.4.3 Interpretation of groundwater conditions with coal measures rock masses

8.4.4 Large-scale open pit mining in low-strength rock masses at the PT Kaltim Prima Coal Sangatta and Bengalon Projects

8.4.5 Geotechnical conditions for mining the coal measures rocks of the Western Canada Sedimentary Basin

8.5 Chalk and weak limestones

8.5.1 Introduction

8.5.2 General geology and classification

8.5.3 Sampling

8.5.4 Material properties

8.5.5 Hydrogeology

8.5.6 Case histories

8.5.7 Slope designs

8.5.8 Design implementation

9 Saprolite: weathered rock and residual soil

9.1 Introduction

9.2 Terminology

9.2.1 Saprolith

9.2.2 Pedolith

9.3 Weathering processes and geology

9.3.1 Chemical weathering

9.3.2 Physical weathering

9.3.3 Weathering rates

9.3.4 Influence of parent rock

9.3.5 Weathering profile examples

9.4 General geotechnical properties

9.4.1 Weathering descriptions

9.4.2 Composition and structure withdepth

9.4.3 Effect of weathering on strength

9.5 Hydrogeology

9.5.1 Typical hydrogeology profiles

9.5.2 Dewatering

9.5.3 Hydrogeology observations at Rosebel Gold Mine

9.6 Slope design considerations

9.6.1 Failure kinematics

9.6.2 Design process

9.6.3 Design implementation

9.7 Cowal Gold Mine: back analysis

9.7.1 Introduction

9.7.2 Geology

9.7.3 Slope stability assessments (pre-mining)

9.7.4 Mining

9.7.5 East wall instability

9.7.6 Back analysis

9.7.7 Discussion of back analysis results

9.7.8 Conclusions

9.8 Newmont Boddington Gold: slope design optimisation in oxide/saprolite

9.8.1 Introduction

9.8.2 Site condition

9.8.3 Oxide material at NBG

9.8.4 Historical slope failures in oxide slopes at NBG

9.8.5 Laboratory testing of oxide/saprolite at NBG

9.8.6 Groundwater conditions

9.8.7 Geotechnical assessment for oxide/saprolite slope design

9.8.8 Slope design optimisation for oxide/saprolite slopes at NBG

9.8.9 Surface drainage in oxide slopes

9.8.10 Summary

Acknowledgements

10 Soft iron ores and other leached rocks

10.1 Introduction

10.2 Background

10.3 Soft iron ores: geology

10.3.1 Lithology

10.3.2 Alteration

10.3.3 Structure

10.4 Soft iron ores: geotechnical characteristics

10.4.1 Definitions

10.4.2 Field characterisation

10.4.3 Laboratory characterisation and testing

10.4.4 Conclusions

10.5 Weathered country rocks

10.5.1 Weathering and strength

10.5.2 Characterisation

10.5.3 Strength parameters

10.6 Hydrogeology of soft iron ores and associated rocks

10.6.1 Hydrogeology in the Iron Quadrangle of Brazi

10.6.2 Dewatering and slope depressurisation

10.6.3 Carajás hydrogeology

10.7 Leached quartzites and quartzitic sediments

10.8 Slope performance and case histories

10.8.1 Introduction

10.8.2 Patrimônio: back analysis

10.8.3 Carajás: failures of the south and south-east walls of the N4E pit

10.8.4 Pau Branco Mine – Iron Quadrangle, State of Minas Gerais: weathered phyllites associated with soft iron ores

10.8.5 Pico Mine: an assessment of the mechanism of flexural toppling in weak phyllite

11 Hydrothermally altered rocks

11.1 Introduction

11.2 General geological setting

11.2.1 Igneous deposits

11.2.2 Epithermal deposits

11.2.3 Structural geology

11.3 Geotechnical properties

11.3.1 Description and classification

11.3.2 Strength and deformation

11.4 Hydrogeology

11.5 Slope stability and engineering geology at the Pierina Mine

11.5.1 Introduction

11.5.2 Engineering geology

11.5.3 Slope stability experience

11.5.4 Stability analysis and disturbance factor (D)

11.5.5 Surface runoff and hydrogeology

11.5.6 Pit slope monitoring

11.5.7 Results and conclusions

11.6 Instability in weak rocks, El Tapado Pit north wall, Yanacocha Operation

11.6.1 Introduction

11.6.2 Site conditions

11.6.3 Phase 2 El Tapado Pit

11.6.4 Deep-seated instability in north wall

11.6.5 Completion of mining

Acknowledgements

11.7 Lihir Open Pit Mine in argillic materials

11.7.1 Introduction

11.7.2 Engineering properties of argillic materials

11.7.3 Gw28 slope

11.7.4 Western Stockpile

11.7.5 Conclusions

12 Design implementation and operational considerations

12.1 Introduction

12.2 Mine planning

12.2.1 Pre-mining (Levels 1 and 2)

12.2.2 Feasibility level and detailed design

12.3 Design implementation

12.3.1 Excavation and scaling

12.3.2 Blasting

12.4 Surface water control

12.4.1 Surface water diversion

12.4.2 Collection of runoff water on catch benches and haul ramps

12.4.3 Control of recharge

12.4.4 Managing large surface flow volumes

12.4.5 Maintenance of surface water management systems

12.4.6 Case history

12.5 Slope protection

12.6 Performance assessment andmonitoring

12.6.1 Geotechnical model validation

12.6.2 Slope performance

12.6.3 Slope movement monitoring

12.6.4 Mine water monitoring

12.7 Ground control management plans

12.8 Mine closure

12.8.1 Slope stability considerations

12.8.2 Hydrogeological considerations

12.8.3 Post-closure monitoring

References

Index