GUIDELINES FOR OPEN PIT SLOPE DESIGN EDITORS: JOHN READ, PETER STACEY # & CSIRO J x PUBLISHING S
Contents Preface and acknowledgments xiii 1 Fundamentals of slope design 1 Peter Stacey 1.1 Introduction 1 1.2 Pit slope designs 1 1.2.1 Safety/social factors 2 1.2.2 Economic factors 2 1.2.3 Environmental and regulatory factors 3 1.3 Terminology of slope design 4 1.3.1 Slope configurations 4 1.3.2 Instability 4 1.3.3 Rockfall 6 1.4 Formulation of slope designs 6 1.4.1 Introduction 6 1.4.2 Geotechnical model 6 1.4.3 Data uncertainty (Chapter 8) 8 1.4.4 Acceptance criteria (Chapter 9) 8 1.4.5 Slope design methods (Chapter 10) 9 1.4.6 Design implementation (Chapter 11) 10 1.4.7 Slope evaluation and monitoring (Chapter 12) 10 1.4.8 Risk management (Chapter 13) 11 1.4.9 Closure (Chapter 14) 11 1.5 Design requirements by project level 11 1.5.1 Project development 11 1.5.2 Study requirements 12 1.6 Review 12 1.6.1 Overview 12 1.6.2 Review levels 14 1.6.3 Geotechnically competent person 14 1.7 Conclusion 14 2 Field data collection 15 John Read, Jarek Jakubec and GeoffBeale 2.1 Introduction 15 2.2 Outcrop mapping and logging 15 2.2.1 Introduction 15 2.2.2 General geotechnical logging 17 2.2.3 Mapping for structural analyses 19 2.2.4 Surface geophysical techniques 22 2.3 Overburden soils logging 23 2.3.1 Classification 23 2.3.2 Strength and relative density 26 2.4 Core drilling and logging 26
vi Guidelines for Open Pit Slope Design 2.4.1 Introduction 26 2.4.2 Planning and scoping 26 2.4.3 Drill hole location and collar surveying 27 2.4.4 Core barrels 27 2.4.5 Downhole surveying 27 2.4.6 Core orientation 28 2.4.7 Core handling and documentation 29 2.4.8 Core sampling, storage and preservation 31 2.4.9 Core logging 32 2.4.10 Downhole geophysical techniques 39 2.5 Groundwater data collection 40 2.5.1 Approach to groundwater data collection 40 2.5.2 Tests conducted during RC drilling 42 2.5.3 Piezometer installation 44 2.5.4 Guidance notes: installation of test wells for pit slope depressurisation 47 2.5.5 Hydraulic tests 49 2.5.6 Setting up pilot depressurisation trials 51 2.6 Data management 52 Endnotes 52 3 Geological model 53 John Read and Luke Keeney 3.1 Introduction 53 3.2 Physical setting 53 3.3 Ore body environments 55 3.3.1 Introduction 55 3.3.2 Porphyry deposits 55 3.3.3 Epithermal deposits 56 3.3.4 Kimberlites 56 3.3.5 VMS deposits 57 3.3.6 Skarn deposits 57 3.3.7 Stratabound deposits 57 3.4 Geotechnical requirements 59 3.5 Regional seismicity 62 3.5.1 Distribution of earthquakes 62 3.5.2 Seismic risk data 65 3.6 Regional stress 66 4 Structural model 69 John Read 4.1 Introduction 69 4.2 Model components 69 4.2.1 Major structures 69 4.2.2 Fabric 75 4.3 Geological environments 76 4.3.1 Introduction 76 4.3.2 Intrusive 76
Contents vii 4.3.3 Sedimentary 76 4.3.4 Metamorphic 77 4.4 Structural modelling tools 77 4.4.1 Solid modelling 77 4.4.2 Stereographic projection 77 4.4.3 Discrete fracture network modelling 79 4.5 Structural domain definition 80 4.5.1 General guidelines 80 4.5.2 Example application 80 5 Rock mass model 83 Antonio Karzulovic and John Read 5.1 Introduction 83 5.2 Intact rock strength 83 5.2.1 Introduction 83 5.2.2 Index properties 85 5.2.3 Mechanical properties 88 5.2.4 Special conditions 92 5.3 Strength of structural defects 94 5.3.1 Terminology and classification 94 5.3.2 Defect strength 94 5.4 Rock mass classification 117 5.4.1 Introduction 117 5.4.2 RMR, Bieniawski 117 5.4.3 Laubscher IRMR and MRMR 119 5.4.4 Hoek-Brown GSI 123 5.5 Rock mass strength 127 5.5.1 Introduction 127 5.5.2 Laubscher strength criteria 127 5.5.3 Hoek-Brown strength criterion 128 5.5.4 CNI criterion 130 5.5.5 Directional rock mass strength 132 5.5.6 Synthetic rock mass model 138 6 Hydrogeological Geoff Beak model 141 6.1 Hydrogeology and slope engineering 141 6.1.1 Introduction 141 6.1.2 Porosity and pore pressure 141 6.1.3 General mine dewatering and localised pore pressure control 146 6.1.4 Making the decision to depressurise 148 6.1.5 Developing a slope depressurisation program 151 6.2 Background to groundwater hydraulics 151 6.2.1 Groundwater flow 151 6.2.2 Porous-medium (intergranular) groundwater settings 154 6.2.3 Fracture-flow groundwater settings 156 6.2.4 Influences on fracturing and groundwater 161 6.2.5 Mechanisms controlling pore pressure reduction 163
viii Guidelines for Open Pit Slope Design 6.3 Developing a conceptual hydrogeological model of pit slopes 166 6.3.1 Integrating the pit slope model into the regional model 166 6.3.2 Conceptual mine scale hydrogeological model 166 6.3.3 Detailed hydrogeological model of pit slopes 167 6.4 Numerical hydrogeological models 168 6.4.1 Introduction 168 6.4.2 Numerical hydrogeological models for mine scale dewatering applications 169 6.4.3 Pit slope scale numerical modelling 173 6.4.4 Numerical modelling for pit slope pore pressures 175 6.4.5 Coupling pore pressure and geotechnical models 179 6.5 Implementing a slope depressurisation program 180 6.5.1 General mine dewatering 180 6.5.2 Specific programs for control of pit slope pressures 6.5.3 Selecting a slope depressurisation method 192 6.5.4 Use of blasting to open up drainage pathways 192 6.5.5 Water management and control 192 6.6 Areas for future research 195 6.6.1 Introduction 195 6.6.2 Relative pore pressure behaviour between high-order and loworder fractures 195 6.6.3 Standardising the interaction between pore pressure and geotechnical 181 models 196 6.6.4 Investigation of transient pore pressures 197 6.6.5 Coupled pore pressure and geotechnical modelling 197 7 Geotechnical model 201 Alan Guest and John Read 7.1 Introduction 201 7.2 Constructing the geotechnical model 201 7.2.1 Required output 201 7.2.2 Model development 202 7.2.3 Building the model 202 7.2.4 Block modelling approach 205 7.3 Applying the geotechnical model 206 7.3.1 Scale effects 206 7.3.2 Classification systems 210 7.3.3 Hoek-Brown rock mass strength criterion 210 7.3.4 Pore pressure considerations 211 8 Data uncertainty 213 John Read 8.1 Introduction 213 8.2 Causes of data uncertainty 213 8.3 Impact of data uncertainty 213 8.4 Quantifying data uncertainty 215 8.4.1 Overview 215 8.4.2 Subjective assessment 215
Contents 8.4.3 Relative frequency concepts 216 8.5 Reporting data uncertainty 216 8.5.1 Geotechnical reporting system 216 8.5.2 Assessment criteria checklist 219 8.6 Summary and conclusions 219 9 Acceptance criteria 221 ]ohan Wesseloo and John Read 9.1 Introduction 221 9.2 Factor of safety 221 9.2.1 FoS as a design criterion 221 9.2.2 Tolerable factors of safety 223 9.3 Probability of failure 223 9.3.1 PoF as a design criterion 223 9.3.2 Acceptable levels of PoF 224 9.4 Risk model 225 9.4.1 Introduction 225 9.4.2 Cost-benefit analysis 226 9.4.3 Risk model process 228 9.4.4 Formulating acceptance criteria 232 9.4.5 Slope angles and levels of confidence 234 9.5 Summary 235 10 Slope design methods 237 Loren Lorig, Peter Stacey and John Read 10.1 Introduction 237 10.1.1 Design steps 237 10.1.2 Design analyses 238 10.2 Kinematic analyses 239 10.2.1 Benches 239 10.2.2 Inter-ramp slopes 244 10.3 Rock mass analyses 246 10.3.1 Overview 246 10.3.2 Empirical methods 246 10.3.3 Limit equilibrium methods 248 10.3.4 Numerical methods 253 10.3.5 Summary recommendations 263 11 Design implementation 265 Peter Williams, John Floyd, Gideon Chitombo and Trevor Maton 11.1 Introduction 265 11.2 Mine planning aspects of slope design 265 11.2.1 Introduction 265 11.2.2 Open pit design philosophy 265 11.2.3 Open pit design process 267 11.2.4 Application of slope design criteria in mine design 268 11.2.5 Summary and conclusions 276
X Guidelines for Open Pit Slope Design 11.3 Controlled blasting 276 11.3.1 Introduction 276 11.3.2 Design terminology 277 11.3.3 Blast damage mechanisms 278 11.3.4 Influence of geology on blast-induced damage 279 11.3.5 Controlled blasting techniques 282 11.3.6 Delay configuration 292 11.3.7 Design implementation 294 11.3.8 Performance monitoring and analysis 296 11.3.9 Design refinement 299 11.3.10 Design platform 305 11.3.11 Planning and optimisation cycle 306 11.4 Excavation and scaling 310 11.4.1 Excavation 310 11.4.2 Scaling and bench cleanup 312 11.4.3 Evaluation of bench design achievement 313 11.5 Artificial support 313 11.5.1 Basic approaches 313 11.5.2 Stabilisation, repair and support methods 314 11.5.3 Design considerations 315 11.5.4 Economic considerations 316 11.5.5 Safety considerations 317 11.5.6 Specific situations 317 11.5.7 Reinforcement measures 318 11.5.8 Rockfall protection measures 325 12 Performance assessment and monitoring 327 Mark Hawley, Scott Marisett, Geoff Beale and Peter Stacey 12.1 Assessing slope performance 327 12.1.1 Introduction 327 12.1.2 Geotechnical model validation and refinement 327 12.1.3 Bench performance 329 12.1.4 Inter-ramp slope performance 337 12.1.5 Overall slope performance 339 12.1.6 Summary and conclusions 342 12.2 Slope monitoring 342 12.2.1 Introduction 342 12.2.2 Movement monitoring systems 343 12.2.3 Guidelines on the execution of monitoring programs 363 12.3 Ground control management plans 370 12.3.1 Introduction 370 12.3.2 Hazard management plan 371 13 Risk management 381 Ted Brown and Alison Booth 13.1 Introduction 381 13.1.1 Background 381 13.1.2 Purpose and content of this chapter 381 13.1.3 Sources of information 382
Contents : xi 13.2 Overview of risk management 383 13.2.1 Definitions 383 13.2.2 General risk management process 383 13.2.3 Risk management in the minerals industry 384 13.3 Geotechnical risk management for open pit slopes 385 13.4 Risk assessment methodologies 389 13.4.1 Approaches to risk assessment 389 13.4.2 Risk identification 389 13.4.3 Risk analysis 391 13.4.4 Risk evaluation 395 13.5 Risk mitigation 396 13.5.1 Overview 396 13.5.2 Hierarchy of controls 398 13.5.3 Geotechnical control measures 398 13.5.4 Mitigation plans 399 13.5.5 Monitoring, review and feedback 400 14 Open pit closure 401 Dirk van Zyl 14.1 Introduction 401 14.2 Mine closure planning for open pits 403 14.2.1 Introduction 403 14.2.2 Closure planning for new mines 403 14.2.3 Closure planning for existing mines 403 14.2.4 Risk assessment and management 405 14.3 Open pit closure planning 405 14.3.1 Closure goals and criteria 405 14.3.2 Site characterisation 407 14.3.3 Ore body characteristics and mining approach 408 14.3.4 Surface water diversion 409 14.3.5 Pit water balance 409 14.3.6 Pit lake water quality 409 14.3.7 Ecological risk assessment 410 14.3.8 Pit wall stability 410 14.3.9 Pit access 412 14.3.10 Reality of open pit closure 412 14.4 Open pit closure activities and post-closure monitoring 412 14.4.1 Closure activities 412 14.4.2 Post-closure monitoring 412 14.5 Conclusions 412 Endnotes 413 Appendix 1 415 Groundwater data collection Appendix 2 431 Essential statistical and probability theory
xii Guidelines for Open Pit Slope Design Appendix 3 437 Influence of in situ stresses on open pit design EvertHoek, Jean Hutchinson, Kathy Kalenchuk and Mark Diederichs Appendix 4 447 Risk management: geotechnical hazard checklists Appendix 5 459 Example regulations for open pit closure Terminology and definitions 462 References 467 Index 487