Ground Support for Underground Mines

Ground Support for underground mines

Yves Potvin & John Hadjigeorgiou

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ii Preface

Recent years have seen considerable progress in In preparing the book, we have been fortunate to ground support technology and design techniques in have the support of many practising ground support underground mines, resulting in significant advances and mining engineers worldwide. Numerous in safety and productivity. This book came about as colleagues made important contributions to specific a direct recommendation from the sponsors of the chapters. In particular, Associate Professor Johan Ground Support Systems Optimisation (GSSO) Wesseloo, William Joughin and Joseph Mbenza, as research project conducted by the Australian Centre well as Philani Mpunzi and Denisha Sewnun, for Geomechanics and initiated in 2011. A strong contributed to Section 5 on ‘Probabilistic approach consensus emerged for a state-of-the-art book on to ground support design’. Professor Phil Dight ground support technology and design techniques contributed to Chapter 6 on ‘Rock stress data’, for underground mines. while Gordon Sweby co-authored Chapter 11: Ground Support for underground mines was ‘Numerical modelling for ground support design written to provide a comprehensive reference book – mining applications’. Emeritus Professor Dick for practising geotechnical and mining engineers Stacey, Dr Peter Mikula and Associate Professor faced with the task of designing ground support Johan Wesseloo reviewed the manuscript and systems in underground mines. In this context, made several insightful comments and suggestions the authors reviewed international best practices for improvement. The authors are indebted for and describe existing and novel ground support all their help and gratefully acknowledge their design methods. Throughout the book there is an contributions. We further acknowledge the many emphasis on both theory and practical tools to aid individuals, publishers and organisations who gave practising engineers in all steps of the ground support permission for the reproduction of data, figures design process. and photos. In practice, the selection of ground support Funding for the development of this book systems is not only dictated by the design process and was provided by the Ground Support Systems rock engineering criteria but also by other factors, Optimisation research project Phase 1. Major including workforce skill level, access to ground sponsors were: Minerals Research Institute of Western support supplies, local mining culture and corporate Australia (MRIWA), Codelco Chile, Glencore Mount risk tolerance. These can have a significant influence Isa Mines, IGO Limited, MMG Limited, and the on ground support strategies and installation Australian Centre for Geomechanics. Minor project practices. Nevertheless, given that ground support sponsors were: Atlas Copco Australia Pty Limited failures can have catastrophic consequences, every (now Epiroc), DSI Underground, Fero Strata Australia ground support system implemented in mines (now DSI Underground), Golder Associates Pty Ltd, should be justified by sound engineering design Geobrugg AG, and Jennmar Australia. principles. Ground Support for underground mines has been specifically written to assist practitioners in fulfilling this requirement. Yves Potvin & John Hadjigeorgiou

iii Industry Sponsors

The Australian Centre for Geomechanics gratefully acknowledges the industry sponsors who provided funding for the development of this book by supporting the ACG Ground Support Systems Optimisation research project Phase 1.

Major Project Sponsors

Minerals Research Institute of Western Australia

Minor Project Sponsors

iv Contents

Preface Industry Sponsors Contents Section 1 : Ground support selection and design considerations Chapter 1 : Introduction

1.1 The principal objective of ground support...... 3 1.2 Ground conditions...... 5 1.2.1 Normal conditions...... 6 1.2.2 High stress conditions...... 8 1.2.3 Large deformations...... 8 Chapter 2 : Rock mass behaviour and failure mechanisms

2.1 Introduction...... 15 2.2 Loading and failure mechanisms...... 15 2.2.1 Gravity failures driven by stress relaxation...... 18 2.2.2 Failures driven by compressive stress...... 19 2.2.2.1 Progressive or brittle failure (stiff loading) �� 20 2.2.2.2 Sudden and violent failure (soft loading) �� 21 2.3 Summary of typical failure mechanisms in underground mines...... 23 2.4 Stress path...... 23 Chapter 3 : Rock reinforcement behaviour, anchoring mechanisms and specifications

3.1 Introduction...... 27 3.2 Rock reinforcement behaviour...... 27 3.2.1 Strong versus weak rock reinforcement...... 28 3.2.2 Stiff versus soft reinforcement elements...... 28 3.2.3 Post-peak behaviour: brittle versus ductile ...... 29 3.2.4 Low versus high energy absorption rock reinforcement...... 30 3.2.5 Rock reinforcement and rock mass behaviour...... 30 3.3 Point anchor bolts...... 32 3.4 Continuous anchor bolts...... 35 3.4.1 Continuous chemically bonded anchor bolts...... 35 3.4.1.1 Resin anchor bolts �� 36 3.4.1.2 Cement-grouted continuous anchor bolts �� 37

v 3.4.2 Continuous friction bond anchor bolts...... 39 3.4.2.1 Friction rock stabilisers �� 39 3.4.2.2 Expandable bolts �� 41 3.5 High energy absorbing or yielding bolts...... 43 3.5.1 Debonded yielding reinforcement...... 43 3.5.2 Chemically bonded yielding anchors...... 44 3.5.3 Expandable bolt with a debonded point anchor...... 46 3.5.4 Hybrid bolts...... 46 3.5.5 Self-drilling bolts...... 47 3.6 Cable bolts...... 48 3.6.1 Prestressed cable bolts...... 49 3.6.2 Resin-grouted cable bolts...... 49 3.6.3 Debonded cable bolts...... 49 3.7 Surface fixtures...... 49 3.7.1 Rockbolt and plate interaction...... 49 3.7.2 Cable bolt surface fixtures...... 52 Chapter 4 : Surface support behaviour

4.1 Introduction...... 55 4.2 Mesh...... 55 4.2.1 Weldmesh...... 56 4.2.1.1 Weldmesh behaviour �� 57 4.2.2 Chainlink mesh...... 57 4.2.2.1 Chainlink mesh behaviour �� 57 4.3 Straps...... 58 4.3.1 Steel straps...... 58 4.3.2 Mesh straps...... 58 4.3.3 Osro straps...... 59 4.3.4 Cable (or rope) lacing...... 59 4.4 Shotcrete...... 60 4.4.1 Basic components of shotcrete...... 60 4.4.1.1 Cement �� 60 4.4.1.2 Aggregates �� 61 4.4.1.3 Admixtures �� 62 4.4.1.4 Cementitious products �� 62 4.4.2 Shotcrete reinforcement...... 63 4.4.2.1 Mesh-reinforced shotcrete �� 63 4.4.2.2 Fibre-reinforced shotcrete �� 63 4.4.3 Shotcrete thickness...... 65 4.4.4 Shotcrete stabilisation mechanisms...... 68 4.4.5 Reinforced shotcrete arches...... 68 4.4.6 Shotcrete pillars...... 69 4.4.7 Thin spray-on liners...... 73

vi Section 2 : Data required for ground support design Chapter 5 : Rock mass data

5.1 Introduction...... 77 5.2 Rock mass characterisation...... 77 5.3 Data collection...... 80 5.4 Exploration and geotechnical boreholes...... 80 5.5 Core drilling and logging...... 81 5.5.1 Total core recovery...... 82 5.5.2 Fracture frequency...... 82 5.5.3 Rock quality designation...... 82 5.5.4 Structure orientation from core...... 83 5.6 Mechanical properties of intact rock...... 85 5.6.1 Uniaxial compressive strength...... 87 5.6.2 Triaxial compressive strength...... 88 5.6.3 Tensile strength...... 90 5.6.4 Field estimates and index tests for intact rock...... 90 5.6.4.1 Schmidt hammer �� 91 5.6.4.2 Point load strength test �� 91 5.7 Mechanical properties of discontinuities...... 92 5.8 Structural mapping...... 96 5.8.1 Digital characterisation...... 97 5.9 Rock mass classifications...... 98 5.9.1 Rock Mass Rating system...... 99 5.9.2 Q-system...... 100 5.9.3 The mining or modified RMR (MRMR)...... 101 5.9.4 Geological Strength Index...... 101 5.10 Practical considerations in data collection for rock mass classification...... 103 5.10.1 Core logging for rock mass classification purposes...... 103 5.10.2 Surface exposure mapping for rock mass classification purposes...... 104 5.11 Interpretation and use of rock mass classifications...... 106 5.11.1 Rock mass classification as a design tool...... 106 5.11.2 Personnel...... 108 5.11.3 Scale...... 109 5.11.4 Reality check...... 110 5.11.5 Statistical correlations between rock mass classification systems...... 110 5.12 Large-scale rock mass properties...... 111 5.12.1 Modulus of elasticity...... 111 5.12.2 Estimating the strength of rock masses...... 111 5.13 Concluding remarks...... 114

vii Chapter 6 : Rock stress data

6.1 Introduction...... 117 6.2 Pre-mining stress regime and trends...... 117 6.2.1 Global stress regime trends...... 118 6.2.2 Stress regimes and trends in Canada...... 119 6.2.3 Stress regimes and trends in Australia...... 122 6.3 Techniques for assessing in situ stress...... 123 6.3.1 Overcoring...... 126 6.3.1.1 CSIRO HI Cell �� 127 6.3.1.2 The modified doorstopper �� 128 6.3.2 Stress memory techniques from core...... 129 6.3.3 Core discing...... 131 6.3.4 Borehole breakout method...... 132 6.4 Presentation and interpretation of stress measurement results...... 134 6.5 Understanding the stress at a mine site...... 134 6.6 The Final Rock Stress Model methodology...... 136 6.7 Monitoring of stress change...... 136 6.8 Concluding remarks...... 137 Chapter 7 : Ground support performance data

7.1 Introduction...... 141 7.2 Data for ground support subjected to static load...... 143 7.2.1 Laboratory static tests on reinforcement elements...... 143 7.2.2 In situ pull tests on rock reinforcement...... 151 7.2.2.1 Friction rock stabiliser tests �� 151 7.2.2.2 Grouted and expandable bolt tests �� 153 7.2.2.3 Indicative working capacity of rockbolts �� 154 7.3 Static tests on surface support...... 154 7.3.1 Static tests on mesh...... 156 7.3.2 Static tests on shotcrete...... 158 7.4 Data for ground support subjected to dynamic loading...... 162 7.4.1 Dynamic capacity of ground support estimated from drop tests...... 162 7.4.2 Compilation of published results from drop testing...... 164 7.4.2.1 Reinforcement results �� 164 7.4.2.2 Surface support results �� 168 7.4.3 Dynamic system testing...... 171 7.4.4 In situ dynamic drop tests...... 173 7.4.5 Dynamic capacity of ground support estimated from simulated rockbursts...... 173 7.5 Concluding remarks...... 175

viii Chapter 8 : Geomechanical data confidence and reliability

8.1 Introduction...... 179 8.2 Organising datasets into models...... 179 8.3 Geomechanical model confidence...... 180 8.3.1 Constructing geotechnical models...... 182 8.4 Statistical approaches to geomechanical data sampling...... 182 8.5 Structural data...... 183 8.6 Laboratory data...... 186 8.7 Practical implications...... 196 Section 3 : Ground support design methods Chapter 9 : Analytical ground support design methods

9.1 Introduction...... 199 9.2 Role of reinforcement and surface support...... 200 9.3 Reinforcement design...... 200 9.3.1 Closed form solutions...... 201 9.3.2 Convergence confinement or ground reaction curve method...... 201 9.3.3 Stress-induced buckling...... 203 9.3.4 Structural or limit equilibrium design...... 203 9.3.4.1 The beam concept for bedded rock �� 204 9.3.4.2 Rock reinforcement unit (RRU) �� 204 9.3.4.3 Reinforcement guidelines based on the rock arch concept �� 204 9.3.4.4 The parabolic arch concept �� 204 9.3.4.5 Two-dimensional wedge analysis �� 207 9.3.4.6 Three-dimensional wedge analysis �� 208 9.3.5 Discrete fracture network-based reinforcement analysis...... 211 9.4 Surface support...... 212 9.4.1 Design of mesh...... 213 9.4.2 Design of shotcrete...... 213 9.4.2.1 Shotcrete failure mechanisms �� 213 9.4.2.2 Two-dimensional wedge analysis for shotcrete support �� 214 9.4.2.3 Three-dimensional wedge analysis for shotcrete support �� 214 9.5 Concluding remarks...... 216 Chapter 10 : Empirical ground support design methods

10.1 Introduction...... 219 10.2 Empirical ground support design based on rock mass classification...... 219 10.2.1 Support pressure...... 220 10.2.2 Support recommendations based on RMR and MRMR...... 221 10.2.3 Ground support recommendations based on the Q-system...... 223 10.2.4 Issues related to the implementation of the Grimstad and Barton chart in mining...... 225

ix 10.3 New empirical ground support design guidelines for mining drives...... 227 10.3.1 Scope of the new guidelines...... 227 10.3.2 Methodology...... 228 10.3.3 Ground support design guidelines for mining drives...... 231 10.3.4 The limitations of the ground support guidelines...... 232 10.4 Concluding remarks...... 232 Chapter 11 : Numerical modelling for ground support design – mining applications

11.1 Introduction...... 237 11.2 Two-dimensional numerical analysis of tunnels...... 238 11.3 Instability zone approach...... 239 11.3.1 Analysis based on elastic modelling...... 240 11.3.2 Analysis based on elasto-plastic modelling...... 241 11.4 Comparative approach...... 242 11.5 Explicit modelling of ground support approach...... 246 11.5.1 Design specification...... 246 11.5.2 Rehabilitation scheduling...... 246 11.5.3 Residual capacity...... 248 11.6 Inputs and sensitivity...... 249 11.6.1 Rock mass strength and deformability...... 249 11.6.2 Discontinuity strength and deformability...... 251 11.6.3 Ground support strength and deformability...... 252 11.6.4 Sources of inherent variability...... 253 11.6.4.1 Actual versus design excavation geometry �� 253 11.6.4.2 Joint network geometry �� 255 11.6.5 Mesh dependence...... 258 11.7 Discontinuum models...... 258 11.7.1 Explicit representation of ground support in discontinuum models...... 260 11.7.2 Continuous versus discontinuous models...... 263 11.8 Confidence in numerical modelling esultsr ...... 263 11.8.1 Code verification...... 264 11.8.2 Model calibration and validation...... 264 11.9 Discussion...... 265 Section 4 : Extreme ground conditions Chapter 12 : Ground support in squeezing ground

12.1 Introduction...... 269 12.2 Squeezing mechanisms...... 269 12.3 Complete shear failure...... 270 12.3.1 Empirical approaches for complete shear failure mechanism...... 271 12.3.2 Numerical modelling of squeezing in civil engineering tunnels...... 273

x 12.4 Buckling failure mechanism: squeezing in foliated ground...... 273 12.4.1 Observations of buckling mechanism in underground mines...... 274 12.4.2 The hard rock squeezing index...... 279 12.4.3 Forecasting convergence in foliated squeezing ground...... 281 12.4.4 Numerical modelling of buckling failure mechanism in foliated squeezing ground...... 283 12.4.5 Explicit modelling of reinforcement in squeezing ground...... 286 12.5 Context for ground support application in civil and mining squeezing ground ...... 288 12.6 General approaches to manage squeezing ground in mines ...... 290 12.6.1 Support system concepts...... 291 12.6.2 Containing the excavation surface...... 291 12.6.3 Reinforcing squeezing ground...... 292 12.6.4 Ground support coverage...... 293 12.6.5 Connection between surface support and reinforcement...... 294 12.6.6 Deep anchoring of the reinforced shell...... 294 12.6.7 Rehabilitation...... 294 12.6.8 Umbrella arch method...... 294 12.6.9 Other ground support approaches in very weak and squeezing ground conditions... 297 12.7 Summary...... 297 Chapter 13 : Ground support design for rockburst prone conditions

13.1 Background...... 301 13.2 Ground support subjected to dynamic loading – a case of design indeterminacy...... 301 13.2.1 Scale–distance relationship and dynamic demand expressed as ppv...... 302 13.2.2 Radiation pattern...... 307 13.2.3 Reflection and refraction of the stress wave...... 307 13.2.4 The site effect...... 309 13.2.5 Effect of rock brittleness...... 311 13.3 Seismic events and rockburst damage mechanisms and demand on ground support...... 312 13.4 Dynamic capacity of ground support systems...... 314 13.4.1 The weakest link...... 315 13.4.2 Challenges in using drop test results for dynamic ground support system design..... 316 13.5 Dynamic ground support design...... 317 13.5.1 The Canadian Rockburst Handbook (CRH) approach...... 318 13.5.1.1 Static loading demand calculation �� 318 13.5.1.2 Displacement demand estimation �� 318 13.5.1.3 Dynamic displacement demand �� 319 13.5.1.4 Dynamic load or kinetic energy demand estimation �� 319 13.5.1.5 Typical ground support capacity requirements �� 320 13.5.1.6 Ground support capacity estimation �� 320 13.5.2 The rockburst damage potential approach...... 321 13.5.2.1 Static stress condition factor E1 �� 321 13.5.2.2 Ground support capacity factor E2 �� 322 13.5.2.3 Excavation span factor E3 �� 323 13.5.2.4 Geological structure factor E4 �� 323

xi 13.5.2.5 Rockburst damage scale (RDS) and rockburst damage potential (RDP) graph �� 323 13.5.2.6 Rockburst damage potential used as a design and risk management tool �� 323 13.5.2.7 LaRonde modified ockburstr damage potential �� 327 13.5.3 The Western Australian School of Mines (WASM) approach...... 327 13.5.4 Empirical charting...... 330 13.5.4.1 Acceptable (S0 to S2 – SC0 to SC2) �� 331 13.5.4.2 Tolerable (S3 – SC3) �� 331 13.5.4.3 Intolerable (S4 to S5 – SC4 to SC5) �� 331 13.5.5 Passive monitoring...... 334 13.6 Evolution of ground support selection for rockburst conditions...... 338 13.6.1 Strategy change at Creighton, Copper Cliff and Coleman mines...... 339 13.6.2 Kidd Mine ground support adjustments...... 339 13.6.3 Examples from other Canadian mines...... 340 13.7 Discussion...... 340 Chapter 14 : Long-term performance of ground support – corrosion

14.1 Introduction...... 345 14.2 Fundamentals of corrosion...... 347 14.2.1 Corrosion environments...... 347 14.2.2 Forms of corrosion...... 353 14.3 Ground support corrosion...... 353 14.3.1 Corrosion rates...... 358 14.3.2 General corrosion investigations using coupons...... 358 14.3.3 Friction rock stabilisers...... 359 14.3.4 Expandable bolts...... 361 14.3.5 Rebar...... 361 14.3.6 Cable bolting...... 362 14.3.7 Mesh...... 364 14.4 Mitigation of corrosion effects...... 365 14.4.1 Selection of ground support type...... 365 14.4.2 Material selection of ground support...... 367 14.4.3 Protective coatings...... 367 14.4.4 Installation practice...... 369 14.5 Characterising and monitoring the performance of ground support to corrosion...... 369 14.5.1 Characterisation of the environment...... 369 14.5.2 Visual monitoring of corroded support...... 370 14.5.3 Use of a borehole camera probe...... 370 14.5.4 Pull tests...... 370 14.5.5 Overcoring...... 370 14.5.6 Mesh...... 370 14.5.7 Fracture analysis...... 370

xii 14.6 Rehabilitation guidelines...... 371 14.7 Summary...... 373 Section 5 : Probabilistic approach to ground support design Chapter 15 : Probability, risk and design

15.1 Design acceptance criteria...... 377 15.1.1 Factor of safety as an acceptance criterion...... 378 15.1.2 Probability of failure as a design acceptance criterion...... 378 15.1.3 Risk as a design acceptance criterion...... 379 15.1.4 Factor of safety, probability of failure, and risk in the design process...... 379 15.2 Important concepts related to probabilistic and risk-based design...... 380 15.2.1 Statistical and probabilistic analyses...... 380 15.2.2 State of mind or state of nature...... 381 15.2.3 Frequency of occurrence and degree of belief...... 382 15.2.4 Uncertainty and variability...... 382 15.2.5 Correlations between factor of safety and probability of failure...... 383 15.2.6 Voluntary and involuntary risk...... 383 15.2.7 Corporate and individual risk...... 383 15.2.8 Evaluating total risk...... 384 15.2.9 Accuracy of risk assessment...... 384 15.3 Design acceptance levels...... 384 15.3.1 Economic risk...... 385 15.3.2 Safety risk...... 387 15.4 Safety risk acceptance levels...... 389 15.4.1 Accepted individual safety risk...... 390 15.4.2 Accepted societal safety risk...... 392 15.4.3 Concluding remarks...... 395 Chapter 16 : A risk-based approach to ground support design

16.1 Introduction...... 399 16.1.1 Risk-based ground support design in a mining context...... 399 16.2 Probabilistic stress damage analysis...... 400 16.2.1 Input data...... 401 16.2.2 Elastic superposition...... 403 16.2.3 Elasto-plastic analysis...... 409 16.2.4 Damage risk model (frequency and extent)...... 413 16.3 Probabilistic analysis of structurally defined failures...... 417 16.3.1 Input data...... 419 16.3.2 Ground support system...... 419 16.3.3 Analysis of structurally defined failures...... 420 16.3.4 Rockfall cumulative distributions...... 420 16.4 Economic risk evaluation...... 421

xiii 16.5 Safety risk evaluation...... 424 16.5.1 The safety risk model...... 424 16.5.2 Failure probability...... 425 16.5.3 Individual safety risk...... 427 16.5.3.1 Temporal coincidence �� 427 16.5.3.2 Spatial coincidence �� 427 16.5.3.3 Personnel failure coincidence for an individual �� 428 16.5.3.4 Exposure mitigation �� 428 16.5.3.5 Assessment of individual safety risk �� 428 16.5.4 Societal safety risk...... 428 16.5.4.1 Temporal and spatial coincidence �� 429 16.5.4.2 Exposure mitigation �� 429 16.5.4.3 Assessment of societal risk �� 429 16.5.4.4 Application of the safety risk model �� 429 16.6 Concluding remarks...... 433 Appendix 1 : Rock mass ratings

Ratings for the RMR-system parameters...... 437

Rock mass rating 1989 (RMR89)...... 438 Appendix 2 : Q-system

Ratings for the Q-system parameters...... 445 Appendix 3 : Techniques for probabilistic and risk calculations

A3.1 Introduction...... 453 A3.2 Monte Carlo method...... 453 A3.2.1 Random variate sampling...... 453 A3.2.2 Illustrated example...... 453 A3.2.3 Number of required samples...... 456 A3.2.4 More efficient sampling methods...... 456 A3.3 Point estimate method (PEM)...... 456 A3.3.1 Rosenblueth–Harr two-point estimate method...... 457 A3.3.1.1 Point estimates �� 457 A3.3.1.2 Point estimate method trials �� 458 A3.3.1.3 Weighting of results �� 458 A3.3.1.4 Calculation of the mean and standard deviation of the results �� 459 A3.3.2 Special case for independent and symmetric parameters...... 460 A3.3.3 PEM example: Hoek–Brown failure criterion...... 461 A3.3.3.1 Calculation of the Hoek–Brown rock mass parameter, s �� 461

A3.3.3.2 Calculation of the Hoek–Brown rock mass parameter, mb �� 462

A3.3.3.3 Calculation of the Hoek–Brown global rock mass strength, σ'cm �� 462 A3.3.3.4 Probability distributions of the rock mass parameters �� 464 A3.3.4 The estimation of higher order moments with the PEM...... 464 A3.3.5 Computational efficiency...... 464 xiv A3.4 The response surface method (RSM)...... 464 A3.4.1 Surface functions...... 466 A3.4.2 RSM design of experiment...... 467 A3.4.3 RSM example: Hoek–Brown failure criterion...... 468 A3.4.3.1 Calculation of the Hoek–Brown rock mass parameter, s �� 468

A3.4.3.2 Calculation of the Hoek–Brown rock mass parameter, mb �� 469

A3.4.3.3 Calculation of the Hoek–Brown ‘global rock mass strength’, σ'cm �� 470 A3.5 Response influence factor method (RIF)...... 470 A3.5.1 RIF design of experiment...... 473 A3.5.2 RIF example: Hoek–Brown failure criterion...... 474

A3.5.2.1 Calculation of the Hoek–Brown global rock mass strength, σ'cm �� 474 A3.6 A systematic approach to the design of experiment...... 475 A3.6.1 Optimising the design of experiment...... 476 A3.6.2 Staged execution of the design of experiment...... 476 A3.6.3 Targeting specific areas on the response surface...... 477 A3.7 Redundancy in calculations...... 477 A3.8 Decomposition techniques for assessing risk...... 479 A3.9 Fault trees...... 480 A3.9.1 Fault tree example: rockfall due to stress damage and excessive deformation...... 482 A3.10 Event trees...... 482 A3.10.1 Event tree example: consequence of drive deformation...... 485 A3.11 Concluding remarks...... 486

References...... 489

Index...... 513

xv Section 1 : Ground support selection and design considerations 1

Chapter 1 : Introduction Courtesy Brad Simser CHAPTER ONE 1 Introduction

1.1 The principal objective of The design of an effective ground support system ground support requires an appreciation of potential failure or instability mechanisms. These can be the result of relativelyhigh Ground support has been used to stabilise underground stress-to-strength ratios, inducing failure of intact excavations in rock since Roman times. In a review of the rock or rock mass. Alternatively, structural instability evolution of ground support and reinforcement, Brown is gravity driven and is a function of the geological (1999) refers to De Re Metallica (Agricola 1556), which describes timbering in shafts, tunnels and drifts used as structure. Rock mass instability can also result from a means of protection against collapse and risk of injury. a combination of stress or structurally driven failure Stabilisation of the immediate boundary of a rock mass modes. In addition, other factors, such as mining- surrounding an excavation is often referred to as ‘local induced seismicity, can aggravate existing conditions, support’. Until the 1950s, timbering remained one of the and trigger failure. main means of local support. Since then, it has gradually A primary effect of constructing an excavation in been replaced by internal reinforcement techniques, rock is the resulting displacement of surrounding rock such as dowels installed inside a drilled hole. and the potential for structurally defined rock blocks Advances in ground support techniques, critical to to slide into the excavation. ‘Improvement’ provided by both safety and economic success in modern mining, reinforcement comes primarily from resisting rock mass have gathered pace since the 1980s. The terminology deformation at the excavation boundary. associated with ground support has also evolved (Brady Maximum displacement is most likely to occur & Brown 2006). Although there are few universally at the weakest part of the rock mass, along natural accepted definitions, it is convenient to distinguish discontinuities, exhibiting a shearing or sliding between support and reinforcement, as put forward mechanism. A conceptual model of displacement is by Windsor and Thompson (1993): “Support is the shown in Figure 1.1. The green arrows show the initial application of a reactive force at the face of the excavation.” natural response of the rock mass at the boundary of And… “Reinforcement is considered to be an improvement the excavation. As a result of this initial movement, of the overall rock mass properties from within the rock dilation along discontinuities (blue arrows) becomes mass and will therefore include all devices installed possible, and this enables sliding of blocks along in boreholes.” discontinuities (red arrows). The behaviour of The application of both surface support and discontinuities under load has been investigated reinforcement to stabilise an excavation in rock extensively in rock mechanics. This has resulted in constitutes the ground support system. a series of representative failure criteria capturing Surface support is generally installed on the surface the influence of different parameters such as joint of excavations (roofs and walls) to catch rock material roughness, asperities and scale effects; for example that may detach from the boundary, hence maintaining Patton (1966), Barton (1976) and Bandis et al. (1983). its integrity and limiting deformation or ‘bulking’ of the The impact of reinforcement on individual surface. The timbering described in Agricola (1556), as discontinuities is complex. It is influenced by the well as in early mining textbooks such as Peele (1941), discontinuity characteristics, the properties of the was in fact a form of surface support. In modern mining, reinforcement system and loading conditions. This mesh and shotcrete have replaced timbering. has been the topic of several investigations: laboratory, In this book, the term ‘ground support’ is used to refer to analytical and numerical. Bjurström (1974) undertook both surface support and reinforcement. some of the earliest laboratory tests to investigate 3 This book is available from the ACG store https://acg.uwa.edu.au/shop/gsso/ --

...... This book is available from the ACG store https://acg.uwa.edu.au/shop/gsso/-- a e

• Section 1 : Ground support selection and design considerations 2

Chapter 2 : Rock mass behaviour and failure mechanisms Courtesy Mount Isa Mines, A Glencore Company Courtesy Mount Isa Mines, A Glencore CHAPTER TWO 2 Rock mass behaviour and failure mechanisms

2.1 Introduction Identifying the behaviour and potential failure function of the ratio of the maximum far field stress to mechanisms of the rock mass around underground the unconfined compressive strength (σ1/σc). Rock mass excavations is critical to the selection and design of quality was assigned based on rock mass rating (RMR) appropriate support systems. An important objective values (Bieniawski 1989). of ground support is to resist rock mass deformation The inherent assumption behind most of these tunnel resulting from mining activities and changes in stress instability mechanisms is that the rock mass is isotropic. conditions. It is therefore essential that the rock mass In reality, rock mass anisotropy is quite common and can deformation process is understood. have a significant impact on ground behaviour andfailure Rock mass instability is an ever-present threat to mechanisms. Anisotropy implies that rock properties both the safety of people and equipment in the mine. vary with direction. Rock mass anisotropy is observed It can be very intricate and may involve multiple in several rock types such as schists, slates, phyllites complex processes. In Chapter 1, a simple grouping was and gneisses. Anisotropic behaviour is also observed in proposed whereby mining geomechanics environments regularly jointed and bedded rock masses. Specifically, were distinguished as ‘normal’, ‘high stress’ and ‘large when a rock mass has one of its discontinuity sets with deformations’. All three types may occur in different a spacing much smaller than other discontinuities, the places in one mine or even in the same place at different rock mass discontinuity fabric will have a laminated times. The failure mechanisms associated with these shape. Colloquially, such a rock mass can be referred to environments generally are: as bedded, foliated or laminated rock. The most frequent • discrete gravity-driven wedge failure failure mechanism in laminated rock masses is flexural • stress-driven rock failure exhibiting progressive failure (buckling of layers) as shown in Figure 2.2(a), fracturing and accumulated damage (spalling and which can result in gravity fall or slabbing (Figure 2.2b). crushing) or sudden violent failure (rockburst) Anisotropic rock masses exhibit different behaviour • stress-driven large deformations defined by a when loaded or unloaded in different directions. weak rock mass or the presence of discontinuities Therefore, the relative orientation of the layers compared (squeezing ground). to the excavation surface and the stress field influence Conceptual models of typical failure mechanisms the potential for and severity of failure. The thickness of related to underground excavations in hard rock have the layers also has a significant influence on the stability been proposed by Stillborg (1994), Hoek et al. (1995), of anisotropic rock masses. Kaiser et al. (1996) and Palmström and Stille (2007). This chapter extends some of the existing concepts to present a simple framework to categorise typical ground behaviour 2.2 Loading and failure mechanisms and failure mechanisms in underground mines. To understand rock mass failure mechanisms, it is Hoek et al. (1995) provided a conceptual basis for important to examine the driving forces that act on the identifying potential tunnel instability modes as a rock. In Section 1.2, three generic mining geomechanics function of in situ stress (low or high) and rock mass environments are defined based on relative stress and quality (massive, jointed and heavily jointed rock). rock mass conditions and, in Figure 2.1, rock behaviour Martin et al. (1999) expanded this to a 3 × 3 matrix is divided into categories also based on stress levels and by including an intermediate in situ stress component rock mass quality. This is a logical basis for considering (Figure 2.1). They further defined the stress levels as a loading and failure mechanisms. The fundamentals 15 Chapter 2 : Rock mass behaviour and failure mechanisms

2.3 Summary of typical failure loading and soft loading conditions. The matrix shows mechanisms in underground the probable failure mechanisms resulting from the mines combination of loading conditions and rock masses. Based on the scenarios involving different rock masses and loading conditions discussed in this chapter, we have 2.4 Stress path developed a matrix to summarise and categorise the One of the major differences between civil tunnelling potential failure mechanisms commonly encountered and mining excavation is that, in civil works, the in underground hard rock mines. The new matrix is stress fields tend to remain constant throughout the shown in Figure 2.13. serviceable life of the tunnel. In mining excavation, Rock masses are divided into isotropic and anisotropic the stress fields may change significantly, depending materials. Like the Martin et al. (1999) matrix on their proximity to production mining where large (Figure 2.1), the isotropic category is subdivided according excavations are created. The difference is illustrated to competency into massive, moderately fractured in Figure 2.14, which suggests that in civil tunnelling, (jointed) and heavily fractured (jointed) categories. A excavation-induced stresses peak at a distance of about block size scale is provided as a rough guide to assist in three tunnel diameters behind the tunnel while mining- categorising the rock mass competency. Anisotropic rock induced stresses can continue to increase. masses are subdivided according to the relative angle of Ground support systems in mining must therefore the foliation compared to the excavation surface (parallel cater for the potential range of variations in the stress or perpendicular) and subdivided again according to the field. In some instances, the magnitude may be more spacing between the foliation planes. These are the two than double the original stress, creating high compressive main parameters controlling the failure mechanisms. stress conditions, and subsequently decrease to virtually The loading conditions are divided into low stress and no stress, creating a low confinement environment. The high stress. The high stress is further subdivided into stiff principal stresses can also experience severe rotation at

Low stress High stress

Stiff loading Soft loading

Massive N/A Spalling Rockburst rock mass 100+

Large 50 Jointed Gravity wedge Crushing/shearing Rockburst rock mass shakedown Small 10 Isotropic rock mass rock Isotropic Heavily Unravelling Block size scale (cm) jointed Squeezing Squeezing 1 shakedown rock mass

90° Thick layers >10 N/A Spalling Rockburst

Thin layers <1 Buckling Crushing Crushing/buckling Foliation spacing (cm )

>10 Gravity slabbing Thick layers Buckling Rockburst shakedown Anisotropic rock mass rock Anisotropic

Relative angle of foliation Ψ Unravelling Squeezing Squeezing/buckling 0° Thin layers <1 shakedown Foliation spacing (cm )

FIGURE 2.13 Rock mass behaviour and failure mechanism matrix

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• Section 1 : Ground support selection and design considerations 3

Chapter 3 : Rock reinforcement behaviour, anchoring mechanisms and specifications Courtesy Glencore Canada Courtesy Glencore CHAPTER THREE 3 Rock reinforcement behaviour, anchoring mechanisms and specifications

3.1 Introduction Chapter 2 focused on the behaviour of a rock mass around excavations and potential failure mechanisms. A wide variety of ground support products is available This chapter describes the load–displacement behaviour to the mining industry worldwide. The list of new products is constantly expanding but, at the same time, of rock reinforcement, providing the background for certain products fall out of favour and are withdrawn selecting the elements that meet the rock mass demand. from the market. For example, as patent protection Anchoring mechanisms and specifications of available expires, more companies sell different versions of some reinforcement elements are summarised. bolts and so the presentation of a definitive list cannot be included. In the introductory chapter, we divided 3.2 Rock reinforcement behaviour ground support elements into two broad categories: The behaviour of rock reinforcement elements can be reinforcement applied internally to the rock mass and quite complex. For practical purposes, it is usual to test a surface support applied externally. This chapter focuses on reinforcement elements. Surface support will be reinforcement element by subjecting it to an axial tensile discussed in Chapter 4. force and recording the displacement to produce a force–displacement (or load–displacement) graph Reinforcement elements in underground mines (Figure 3.1). In this context, ‘working capacity’ is the can be subdivided into short and long bolts. Rockbolts load on the reinforcement element at which significantly generally aim at stabilising a shallow zone within 2–3 m increasing displacement begins and ‘ultimate capacity’ is of the excavation surface. Cable bolts are designed for the maximum load sustained by the element. deeper reinforcement, from about 3–15 m and beyond, depending on the application. The use of multiple connectable bolt segments to form a longer rockbolt fall under the deep reinforcement category. Since rock reinforcement is installed internally, it is intimately coupled to the rock mass. The behaviour of Ultimate capacity the reinforcement and, more specifically, its deformation Working capacity characteristics when subjected to load, must therefore match the deformation in the rock mass. Otherwise decoupling will occur, triggering failure of the rock reinforcement bond or, if the bond is strong enough, Load failure of the reinforcement element itself. Consequently, it is important to understand the behaviour of rock reinforcement in the context of its interaction with the rock mass. In Chapter 1, we proposed the following three broad Displacement geomechanics environments or ground condition categories, based on the relative level of stress compared to the strength of the rock mass and on the deformation characteristics of the rock mass: ‘normal’, ‘high stress’, FIGURE 3.1 Typical load versus displacement and ‘large deformation’. behaviour of a rockbolt pull test (after ASTM D4435–13) 27 This book is available from the ACG store https://acg.uwa.edu.au/shop/gsso/

I IV Section 1 : Ground support selection and design considerations

Axial stress in bolt σ0 ess Shear/axial str

Shear stress on bolt

σ0

FIGURE 3.10 Stress distributions along the length of a point anchor bolt when subjected to a pull load at the bolt head (after Li et al. 2014)

In the past, mechanical anchor bolts were widely used point anchor bolt overcomes many of the mechanical in underground mines throughout the world. In recent anchor quality control issues but it has its own resin years, due to a number of limitations and quality control mixing issues. The low stiffness of thepoint anchor bolt issues with the shell anchor, their use has been reduced is considered a disadvantage in normal conditions (i.e. a significantly. The shell anchoring mechanism tends low-stress environment and stiff rock masses). to loosen with time and blasting-induced vibrations. Consequently, these bolts generally require repeated retorquing to maintain their capacity. Furthermore, the mechanical shell anchor does not grip well in very hard Bail rock and can over-expand in very soft rock. If faceplate support is lost, the bolt may perform inefficiently. Also, Wedge corrosion may degrade these anchors. Point anchoring can also be achieved by grouting a section of a rebar or a thread bar at the toe of the borehole Segments

(Figure 3.9b). A fast-setting resin cartridge is inserted at Sleeve the toe of the hole. The fast-setting resin usually takes less than 30 seconds to set, after which it is possible to install the surface fixture and tension the bolt using a nut and plate, in a similar way to the shell anchor. In most Friction ares applications, a resin cartridge is used for grouting but cement cartridges are also available.

A long curing time makes cement anchored bolts FIGURE 3.11 Components of an expansion shell impractical for point anchoring applications. The resin anchor: the bail, wedge, segments, sleeve and friction flares 34 Section 1 : Ground support selection and design considerations 4

Chapter 4 : Surface support behaviour CHAPTER FOUR 4 Surface support behaviour

4.1 Introduction takes advantage of the strength and yielding properties of different support elements. However, success depends on Reinforcement and surface support elements interact the connection between the reinforcement and surface to form an integrated ground support system. support being strong enough to transfer and share the Independently, they each have a role to play based load as a fully integrated support system. on their mode of interaction with the rock mass. Reinforcement elements penetrate the rock mass, resist In this chapter, we discuss the three main types internal rock movement and preserve the integrity of of surface support products: steel mesh, straps and the rock mass as a structural material. Surface support shotcrete. We describe their behaviour, specifications links reinforcement elements together at the rock and applicability to different ground conditions. surface to resist surface deformation and contain rock Further details of surface support in underground fragments from falling or ejecting. mining, including case studies, can be found in Potvin et al. (2004a) and Hadjigeorgiou and Potvin (2011a). Steel mesh was the principal means of surface support The fundamentals of shotcrete are described in ACI in underground mines until the 1990s when the use 506R-16 (American Concrete Institute 2016) and of of shotcrete became more widespread. The mesh was fibre-reinforced shotcrete in ACI 506.1R-08 (American intended to retain smaller pieces of loose rock between Concrete Institute 2008). the reinforcement elements. Essentially, rockbolts were designed to support larger wedges or rock blocks while mesh was installed in the roof of mine drives to prevent 4.2 Mesh fallout of smaller rock between the rockbolts. Given Following the rapid mechanisation of mines in the that reinforcement patterns generally vary from about late 1970s, the size of mine equipment has increased 1 × 1 m to 1.5 × 1.5 m, the maximum weight of a rock significantly and so has the requirement for larger prism that can detach from between bolts is about 20 kN. underground excavations. Consequently, the probability Therefore, mesh commonly used in underground mines of rockfalls has increased due to the larger spans has a load-bearing scope capable of holding that weight. involved. At the same time, the ability of mine workers In this context, mesh acts as passive support. to manage this risk by regularly inspecting and scaling The introduction of shotcrete in underground mines possible loose ground has been hampered by limited has extended the capacity of surface support to provide visual and manual reach to high backs. This has led to an a more active and immediate reaction to rock movement increased need for surface support. Nowadays, in most and to preserve the self-supporting capability and mining jurisdictions, it is mandatory that miners are confinement of the rock mass. never exposed to unsupported ground because it is seen In poor ground conditions, especially where stress as an unsafe practice. exceeds rock mass strength, the concept of larger and The modern way to manage the risk of smaller smaller wedges is no longer the primary concern for rockfalls in large excavations is by systematically designers because in these situations the rock mass often installing surface support to the back of drives that deforms significantly as a volume of material instead exceed 3.5 m in height. Mesh (also referred to as ‘screen’ of as discrete block failure. Reinforcement and surface in North America) is the main type of surface support support must then work together as a system to contain applied worldwide in underground mines. Depending the volumetric deformation. Combinations of surface on the ground conditions and the mandatory support elements, such as shotcrete and mesh or mesh and requirements, the installation of mesh is often extended straps, are often used in these situations. This approach down to the shoulder of the excavation (3.5 m from 55 Section 1 : Ground support selection and design considerations conversely, a drive with a high profile factor that would The application of shotcrete in mining is different. probably require > 50 mm of shotcrete. First, the vertical walls allow bending in the layer. Consider a case where the ground condition factor Second, the liner thickness is relatively rarely extended to the floor. In fact, in many instances, the lower wall is fair, the drill and blast process is also fair, and a is not covered (Figure 4.20). Therefore, the shotcrete 75 mm shotcrete liner is proposed. The excavation support reaction in mining relies on adhesion to the profile roughness factor as read from Table 4.4 is 1.61. rock surface to prevent rock mass deformation and Moving to Table 4.5, a roughness factor combined with a interlocking rock mass joints, and on the transmission 75 mm liner implies a roughness factor of 1.10 and a of the load from the surface to the reinforcement. volume increase factor of 1.8. Stacey (2001) considered in some detail the Several Australian mines establish the roughness factor interaction between deforming rock masses and the based on site-specific visual observations as illustrated in stabilisation mechanisms of liners, including shotcrete Figure 4.18. The roughness factor varying from 1.3 to 1.9 and thin spray-on liners (TSL). He proposed several can be applied as a multiplier of the volume of shotcrete stabilisation scenarios that are regrouped here into five required to cover a flat surface. main mechanisms and summarised in Figure 4.21. As an alternative, a rule of thumb for increasing wet Stacey (2001) also proposed a pictorial description of shotcrete volume in Australian mines based on ground loading and failure scenarios for liners that are regrouped conditions is 1.3 for excellent conditions, 1.5 for average here under three main mechanisms and summarised in conditions and 1.7 for very poor conditions. Figure 4.22. Note that some of the diagrams of failure mechanisms in Figure 4.22 are similar to the stabilising 4.4.4 Shotcrete stabilisation mechanisms mechanism shown in Figure 4.21.

In civil tunnelling and shaft applications where 4.4.5 Reinforced shotcrete arches excavations are circular, shotcrete is often applied as a closed ring designed to ensure it acts in compression. In very poor ground conditions, or when dealing Even when a horseshoe-shaped civil tunnel is used, with very high stress, the common combination of which is much closer to a mining drive shape, shotcrete reinforcement and surface support may not be sufficient is applied all the way down to the floor and the floor to keep deformation under control. Shotcrete arches itself is sometimes covered. Clements (2009) used simple have been used with some degree of success in such diagrams (Figure 4.19) to illustrate how the vertical load conditions (Gaudreau et al. 2004; Ferland & Fuller 2011). is transferred from the overlying rock mass to the floor, The construction of reinforced shotcrete arches is through the arched shotcrete liner, with the arch shape rapid as it utilises a variety of prefabricated steel frame promoting compression within the liner. modules made, for example, of 9.5 mm (#3) rebars and

CL CL CL CL

1.3 1.5 1.7 1.9

FIGURE 4.18 Guide for assessing drive profileroughness factor to estimate the volume of shotcrete required

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• Section 2 : Data required for ground support design 5

Chapter 5 : Rock mass data CHAPTER FIVE 5 Rock mass data

5.1 Introduction convenient to characterise independently the intact rock, individual discontinuities and the rock mass. In recent Ground support design requires an understanding of the years, it has also been recognised that the presence of relationship between the size and shape of an excavation, veins in an intact rock matrix merits individual attention the stress regime, the surrounding rock mass and the for caving applications. Rock mass characterisation is strength and displacement capacity of the ground not entirely based on quantitative measurements or support system. There is no unique approach to the qualitative observations, but on a combination of both. design of ground support. An effective design requires the availability of adequate and representative rock mass A comprehensive rock mass characterisation process data and an understanding of the potential loading and should contain all information necessary to enable failure mechanisms and their interaction with the ground future desktop classification of the rock mass, using any support system. This allows the treatment of ground of the popular classification systems. It should provide support design as a capacity and demand problem. In information relevant to all the likely failure mechanisms, practice, it is often difficult to quantify the capacity of, so that these and their appropriate failure criteria can be and demand on, an engineered structure (in this context, taken into account in the support design. an excavation in rock). This has led to the development The rock mass characterisation process should be of a range of empirical tools for the design of ground independent of the design process and, as a result, a given support in parallel with analytical approaches. Both the rock mass volume has a unique rock mass characterisation. analytical and empirical approaches require knowledge Consequently, sound rock mass characterisation should of the intact rock and rock mass strength. This chapter provide information on the rock mass character at different provides an overview of rock mass properties taken into scales. For example, tunnel-scale characterisation should consideration in the design of ground support. not ignore the larger scale structures spaced at intervals that are greater than tunnel scale. Such structures may have It is useful to make the distinction between rock a significant impact on the design of larger engineering mass characterisation and classification. Rock mass structures, such as open stopes, but are not represented characterisation is the process of identifying features or appropriately in the rock mass classification systems that parameters of importance for an engineering project. were originally developed for tunnelling design. From a This involves measuring and/or describing these rock ground support perspective, rock mass characterisation properties and assigning values or ratings based on can provide the necessary input parameters to rock mass their structure, composition properties and mechanical classification schemes, androck strength and failure behaviour. Rock mass classification on the other hand criteria used in the analysis and design of ground support. is the process of assigning rock mass properties into classes with the purpose of reaching a better overview The fundamental distinction of rock compared to and understanding of a set of data for applying empirical other engineering materials is the presence of fractures design methods. or discontinuities. Brady and Brown (2006) describe the intact rock between discontinuities as rock material, and the total in situ medium containing bedding planes, 5.2 Rock mass characterisation faults, joints, folds and other structural features as the Rock mass characterisation focuses on characterising rock mass. The International Society for Rock Mechanics the intact rock properties, the intensity, orientation, (ISRM 2007) identified 10 parameters that can be used to persistence of natural fractures (joint sets) and the characterise a rock mass, and Hudson (1989) provided the conditions of each joint set. For engineering purposes, it is conceptual representation illustrated in Figure 5.1. 77 --

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• Section 2 : Data required for ground support design

and of fractures and other defects along the borehole. Televiewer images can be used as a means of quality control and assurance by providing a reconciliation e axis α-angle of the recorded images to core. Figure 5.7(a) provides Drillcor an example of sample optical and acoustic televiewer images in a jointed zone resulting in broken core while Figure 5.7(b) shows the photographed core box interval between 48.5 and 52 m, showing the broken core zone. β-angle Ellipse long axis Although constructing reliable structural models from

ence line core is not a trivial subject, it is often not done with the Refer required level of detail and precision unless explicitly e Bottom of core requested by the design team and management. A further Drillcoraxis concern is that most methods for determining the orientation of planar structures from drillcore are designed to work with full core, as retrieved directly from a drillhole. FIGURE 5.6 Measurement of orientation of discontinuity in oriented drillcore In most mining and exploration circumstances, mass quality from boreholes. The ISRM has proposed a critical intervals of the core are cut in half for assay method for rock fracture observations using a borehole purposes, as soon as possible after drilling. There are digital optical televiewer (Li et al. 2015). limited approaches to determine orientation from half The use of optical and acoustic geo-cameras or core, for example, the method by Blenkinsop and Doyle televiewers provides a valuable tool in obtaining 360° (2010), and it is difficult to establish how these methods digital colour images of the walls inside a borehole are applied.

(a)

Depth Geology Optical televiewer Acoustic televiewer Caliper Televiewer structureTeleviewer tadpolesJCore logged tadpoles t ship Joint 1m:25m 0° 90° 180° 270° 0° 0° 90° 180° 270° 0° 3.75 4.5 0° 90° 180° 270° 0° 0° 90° 0° 90° Rough Alt’n

48.0 UN,SM 4 UN,RO 1

Zone of broken core PL,RO 1 49.0 UN,RO 4

Granite UN,RO 3

50.0

PL,RO 4 UN,RO 3

(b)

FIGURE 5.7 (a) Optical and televiewer images reconciled with photographed core; and (b) Photographed core for the same length as in (a) (courtesy Golder Associates)

84 Section 2 : Data required for ground support design 6

Chapter 6 : Rock stress data

J Hadjigeorgiou, PM Dight and Y Potvin Courtesy Brad Simser CHAPTER SIX 6 Rock stress data

6.1 Introduction the influence of mining-induced stresses and the resulting rock mass behaviour around excavations. Still Excavations in rock redistribute the in situ or virgin further analysis will estimate the potential load and stresses around an opening, inducing a new set of displacement demand to which the ground support will stresses. Knowledge of the orientation and magnitude be subjected. In effect, good geomechanical practices of these stress fields contribute to an understanding of require an understanding of both in situ and induced ground behaviour and potential failure mechanisms stress fields. of the rock mass. This was illustrated in the developed ground behaviour matrix introduced in Chapter 2. The Hudson et al. (2003) provide a roadmap for developing in situ, or pre-mining, stress field is usually estimated an improved understanding of the in situ stress field. The or measured, while mining-induced stress is calculated objective of their incremental approach, as summarised by analytical solutions or determined by stress analysis in Table 6.1, was to build up knowledge of the rock software, which facilitates the introduction of multiple stress tensor (a tensor is a quantity with magnitude excavations and more complex geometries. Thein situ and direction acting across a plane). The table includes stress field is an important component in assessing the respective steps in the important tasks of hydraulic underground excavation design, since in many cases, fracturing and overcoring techniques. In practice, both the strength of the rock is exceeded and the resulting for practical and economic reasons, not all of these steps instability can have serious consequences on the are used to construct stress models at a specific mine site. behaviour of the excavations. Selected elements of the steps presented in Table 6.1 are discussed further in this chapter. A simplifying assumption (and not valid in all Before commencing a campaign of stress estimation, environments) is that vertical stresses (σv) at depth are directly related to the weight of the overlying rock it is important to establish the objectives (e.g. mine or overburden. It is also convenient to discuss the activity, productivity, costs) and acknowledge specific site conditions, such as the presence of large-scale magnitude of horizontal stresses (σh) as a function of structures and rock mass heterogeneity. Further, any the vertical stress, where k = σh/σv. The principal stresses are the normal stresses in the directions where the shear interpretation of the results of stress measurements should comment on the reliability of these results with stress is zero. They are represented by the major (σ1), reference to intrinsic and natural uncertainty, as well intermediate (σ2) and minor (σ3) principal stresses. From an engineering perspective, any discussion of as the reliability associated with analysis of the data. stress should recognise the inherent variations of stress Amadei and Stephansson (1997) warn that “an exact as a function of scale. This includes the regional stresses, prediction of the in situ stresses in rock and their spatial the mine-site scale and the excavation scale, all of which variation is very difficult and for all practical purposes are important for the design of ground support. At the impossible, as the current stress state is the end product of same time, it is important to acknowledge the influence a series of past geological events and the superposition of of stress variations at the borehole scale and its impact on stress components of several diverse types”. stress measurements. Selecting an appropriate ground support system is a 6.2 Pre-mining stress regime and progressive process. A first step is determining whether trends a geotechnical domain qualifies as a high or low stress Rock stress measurements are required as input zone, is based on the depth of mining, the local ‘k’ ratio information for design. For a given project, pre-mining

(σ1/σ3) and an estimate of rock mass strength. stresses can be determined in two ways. One is to Subsequent (more detailed) analysis will consider undertake a direct stress measurement campaign; the 117 Section 2 : Data required for ground support design

The elastic modulus and Poisson’s ratio are measured extreme temperatures, either low or high, or due to core onsite, with least delay time, using a biaxial cell. disking, overcore breakout, or in poor/highly fractured Generally, several measurements, taken close to each ground conditions). other, are made in the same or adjacent boreholes to investigate the inherent variability of the results and 6.3.1.2 The modified doorstopper obtain higher confidence in the data. Challenges may The original CSIR doorstopper equipment and procedure arise when some measurements differ from others, as it is has been described by Leeman (1971). A borehole is sometimes difficult to identify whether the difference is drilled into the rock and the strain cell is glued on the due to a local stress effect or simply a poor measurement flattened end of the borehole. The rock carrying the cell result. is overcored, thus relieving the stresses present on the The CSIRO HI Cell has been widely used in mines flattened and ground smooth end of the borehole. The for at least 40 years. Like all overcoring methods, it strains are recorded before and after overcoring, and the has significant limitations. An important limitation is stresses at the end of the hole are determined based on that the measurements site needs to be more distant the elastic properties of the rock. from a mine access than a radius of about three times The latest version of themodified doorstopper cell and the opening width (i.e. 15 m away for a 5 m drive). wireless datalogger by Corthésy et al. (2016), as shown During the feasibility stage of a greenfield project, where in Figure 6.10, provides continuous monitoring of the pre-mining stress data are required for mine design strain gauges, of a resistance temperature detector (RTD) purposes, such access rarely exists. Another significant and of a reference gauge. It can be attached to the rock limitation is the underlying assumption in transforming in water-filled boreholes. A stress reduction procedure strain into stress using the elastic properties of intact suggested by Corthésy et al. (1994) allows the calculation rock. This assumption implies that the rock is continuous, homogeneous, isotropic and has perfectly elastic of four stress components from only one measurement behaviour. Unless these assumptions are met at the scale in a single borehole, and the complete 3D stress tensor of the core, an error is introduced in the interpretation can then be obtained from measurements performed of the measurement data. The error and suitability in two non-parallel or non-perpendicular boreholes. of overcoring is a function of the nature of the rock Recent improvements to this procedure, as discussed by (e.g. anisotropy, non-linearity, non-homogeneity). Corthésy et al. (2016) and Corthésy & Leite (2017), allow Therefore, one of the most important criteria in selecting the simultaneous consideration of rock anisotropy and an area suitable for measurements is the quality of the progressive rock failure during the stress relief process. core and absence of structures. An advantage of the modified doorstopper, compared Based on anecdotal evidence and discussions with to other overcoring methods, is that the overcore length practitioners, a success rate of about 70% can be expected does not have to be long, making it very expedient in with HI Cell measurements in normal conditions. weak and fractured ground in highly stressed rock Invalid measurements can occur when the glue does not masses. An example of a recovered rock core in a highly cover the strain gauge properly (due to air bubbles or fractured rock mass is reproduced in Figure 6.11.

FIGURE 6.10 The modified doorstopper cell (after Corthésy & Leite 2017)

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• Section 2 : Data required for ground support design 7

Chapter 7 : Ground support performance data Courtesy Natural Resources Canada Courtesy Natural Resources CHAPTER SEVEN 7 Ground support performance data

7.1 Introduction Numerical modelling methods can model ground support explicitly or non-explicitly. As with the limit The choice of a particular approach to ground support equilibrium approach, non-explicit design will generally design has significant implications for the type of input require only load and/or displacement capacity of ground data required. While previous chapters have focused on support to compare with the weight of a calculated failed stress and rock mass data collection, this chapter reviews zone or deformation of a failed rock mass. Explicit the various techniques for determining ground support modelling of ground support is a more complex approach data that characterise reinforcement and surface that requires intricate data on the performance of ground support behaviour. support elements. There are three main approaches in ground support For the more commonly used analytical methods and design: (a) empirical, (b) analytical, and (c) numerical non-explicit numerical modelling, the data requirements modelling. Input data requirements are different for each generally focus principally on static, and sometimes, method. dynamic load and displacement capacity of ground Empirical, and in particular, rock mass classification support elements. These are the main topics covered in methods, do not usually rely on any ground support this chapter. performance data for design. They typically provide The performance of ground support elements is ground support recommendations based on a rock generally investigated using standard laboratory tests mass classification rating and the excavation span. In or in situ (often underground) testing. Laboratory tests this way, they bypass capacity and demand calculations are useful because they are performed using standard and usually make no distinction on the type of bolt or procedures with reliable equipment and tend to produce energy capacity required. An exception to this is the repeatable results. However, their capacity to simulate use of support pressure estimates as a function of a classification system rating. For example, Barton et al. real conditions has known limitations. (1974) proposed relationships for support pressure, In situ tests allow the assessment of the performance p, in MPa as a function of the rock mass rating Q. of ground support elements under real conditions, and in Hutchinson and Diederichs (1996) suggested that this an evolving environment throughout the mine life. Many support pressure can be loosely related to the installed aspects of interactions between, for example, rockbolts, bolt capacity per unit area of excavated rock face or to an bonding agent and the rock mass, or the friction capacity equivalent cable bolt spacing. of anchors in different ground conditions, can only be Analytical limit equilibrium methods typically require investigated with in situ testing programs. further information on ground support capacity and A significant number of tests have been proposed demand. The demand is based on the dead weight of and used over time, both for laboratory and in situ failed ground or wedges and is compared to ground conditions. It is useful to separate testing programs of support capacity. As such, some form of assessment data ground support according to the objective of the test. for load bearing ground support performance is required The first and most common objective is to perform tests when using a limit equilibrium approach. However, for the purpose of quality assurance/quality control the use of limit equilibrium analysis is problematic for (QA/QC). Figure 7.1 provides a classification tree of dynamic support as it is extremely difficult to estimate different QA/QC testing methods that may be applied the dynamic demand on the ground support system as to reinforcement and surface support elements under well as the dynamic capacity of the system. laboratory and in situ conditions. 141 Section 2 : Data required for ground support design

Chen and Li (2014) used the same apparatus as Stjern (1995) to further investigate the influence of loading 30 under a combination of pull and shear, by applying 28 Resin-grouted 22 mm diameter breglass rod 26 both a lateral shear displacement (Ds) and an axial 24 pull displacement (Dp) to the bolt (Figure 7.10). They 22 Resin-grouted 20 mm defined the angle between these two displacements as 20 diameter steel rebar Cement-grouted the displacing angle α: 18 20 mm diameter 16 steel rebar Ds 14 αa= rctan 7.1 EXL Swellex dowel Dp 12 Load (tonnes) to 150 mm 10 Expansion shell anchored 17.3 mm diameter 8 rockbolt The loading angle was denoted as the angle between 6 the lateral shear load (Fs) and the axial pull load (Fp): 4 to 150 mm Type SS 39 2 Split Set stabiliser F θa= rctan s 7.2 0 Fp 0 10 20 30 40 50 60 Displacement (mm) In a series of tests on cement-grouted rebars, at a range of displacing angles (α) from 0° (pure pull) to 90° (pure FIGURE 7.6 Load–displacement curves obtained shear), they demonstrated that ultimate displacement from laboratory pull tests on a variety of commonly used at failure decreases with an increase in the displacing rockbolts (based on Stillborg 1994 and compiled by Hoek et al. 1995) angle. Chen and Li (2015) undertook further tests using

Shear cube

Bolt

Tension cube

Loadcell LVDT

WP WP Shear cylinder (500 kN) WP

WP Wire potentiometer Tension cylinder LVDT Linear variable differential transducer (2 × 300 kN)

FIGURE 7.7 Schematic illustration of full-scale Norges Tekniske Hogskøle (NTH) test rig (after Stjern 1995)

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I IV Section 2 : Data required for ground support design 8

Chapter 8 : Geomechanical data confidence and reliability Courtesy MMG Dugald River Mine CHAPTER EIGHT 8 Geomechanical data confidence and reliability

8.1 Introduction in both the geological and the geomechanical models. Read and Stacey (2009) refer to a rock mass model as a The data required for the design of ground support subcomponent of the geomechanical model. In practice, under static loading conditions essentially comprises there is no standardised way to organise mine design parameters characterising the rock mass combined with data but it is generically convenient to construct a series the stress field to assess the demand on ground support, and laboratory and/or field testing data to assess the of databases or models. capacity of the support elements. The construction of a mine geomechanical model The specific data required to undertake an analysis that was briefly discussed in Chapter 5 was undertaken is necessarily a function of the support design method during the scoping and feasibility study and evolved selected. For example, numerical modelling may as the mining project developed into an operating require different data thanempirical design approaches. mine. Commonly, the mining industry refers to five A probabilistic approach will need to meet certain data stages of mine design using the following standardised sampling protocols while rules of thumb can be applied terminology (Table 8.1). on more limited and simplified datasets. Nevertheless, TABLE 8.1 Stages of mining project development there exists a common basis of raw geomechanical data (after Read 2009) that are used as input by most ground support design methods. The purpose of this section is to present relevant Mining project (or mine design) stages information from the literature to help practitioners 1 Conceptual (or scoping) study obtain meaningful datasets for ground support design. 2 Prefeasibility study 8.2 Organising datasets into models 3 Feasibility study Ground support design is generally one of many 4 Detailed design and construction tasks to be completed within a general mine design 5 Operations context. Mine design is a complex process and relies on numerous sources of data. The datasets used for mine Most models, and geotechnical models in particular, design work, including ground support design, are become increasingly populated from Stages 1 to 5. It is often organised into models. Some of the commonly generally assumed that the level of confidence in the used models include: quantity and quality of geotechnical data should increase • geological models focusing on orebody grade from one project stage to another throughout the timeline • structural models displaying major structures of a mine operation. Hence, an important consideration such as faults and shear zones and containing when undertaking data collection for design purposes is information on their characteristics the quantity and variability of geotechnical data required • geomechanical models containing intact rock to meet the target levels of data confidence (TLDC) properties and rock mass characterisation data. relevant to each stage of the mine project. The TLDC is an important concept for the interpretation of data. A model may be defined as a representation of the environment, built to show assumed important indicators Although uncertainty, input data variability, reliability of reality. The boundaries between different models and confidence level are terms that have different and the content of each model are not consistent from meanings, they are sometimes used interchangeably. mine to mine. For example, structures may be included Baecher and Christian (2003) distinguish two types of 179 Chapter 8 : Geomechanical data confidence and reliability

The practical implication is that it may be beneficial to select a higher p resulting in a slightly lower bound for 2.2 CI instead of collecting additional data to increase CI. Average UCS = 18.4 MPa 2.0 Furthermore, in their analysis of UCS data from Cl = 86% a South African mine, Fillion and Hadjigeorgiou 1.8 (2017) illustrated the significant difference in the minimum number of specimens required for the same

1.6 confidence interval in different geotechnical domains

Cl = 61% (Figure 8.9). For a CI of 90%, the minimum number of

Precision index (p) Precision 1.4 specimens needed ranged from four in the kimberlite Cl = 47% domain to 127 in the Vryheid siltstone, sandstone and

1.2 conglomerate domain. Cl = 23% The different steps in the use of the methodology 1.0 for determining the minimum number of specimens 12 14 16 18 20 22 24 26 28 Uniaxial compressive strength (UCS) required for laboratory testing to reach a targeted level of confidence and precision index on the confidence

FIGURE 8.8 Example of the influence of the precision interval is outlined in Table 8.9, based on Fillion and index p on the resulting confidence interval for lognormally Hadjigeorgiou (2017). distributed UCS data (after Fillion & Hadjigeorgiou 2017) (adapted by permission from Springer Nature Customer Service Centre GmbH: Springer, Rock Mechanics and Rock Engineering)

300

250 Vryheid siltstone, sandstone and conglomerate Kimberlite Dolerite 200

150

100 Minimum number of samples needed

0 70 75 80 85 90 95 100 Condence interval Cl (%)

FIGURE 8.9 Minimum number of specimens needed as function of confidence intervals for different geotechnical domains (after Fillion & Hadjigeorgiou 2017) 191 --

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• Section 3 : Ground support design methods 9

Chapter 9 : Analytical ground support design methods Courtesy John Hadjigeorgiou CHAPTER NINE 9 Analytical ground support design methods

9.1 Introduction In previous chapters, we established that the data required for design of ground support is related to the The design of underground excavations in rock has design approach to be implemented. A number of design been reviewed in multiple publications, textbooks and methods are available to practitioners. Groupings of these handbooks. Bieniawski (1992) has written extensively methods seem to vary in the technical literature. For on design for rock engineering, suggesting that the example, Choquet and Hadjigeorgiou (1993) identified following principles should be an integral part of design methods as empirical, rational and observational. all design processes: clarity of design objectives and functional requirements; minimum uncertainty • Empirical – which quite often involves rock mass classification schemes accompanied by a of geological conditions; simplicity of design set of design recommendations or employing components; state-of-the-art practice; optimisation; sets of rules based on acceptable practice. and constructability. Stacey (2004) explored the link between the design process in rock engineering and • Rational – which make use of analytical solutions and numerical modelling to predict the code of practice to mitigate rockfall and rockburst the influence of different support designs on the accidents and suggested a distinction between defining overall stability of an excavation. and executing the design. • Observational – which call for the In a mining context, there is a need to construct instrumentation of the excavation and the both geological and geomechanical models, employ implementation of support as the design is classification systems, build structural models and assess developed. This is demonstrated in the new the necessary input data into 2D and 3D numerical stress Austrian tunnelling method (NATM) and the analysis models. All these processes have different data ground reaction curve (GRC) or convergence requirements. Unless these needs are understood and confinement methods. communicated to the personnel responsible for data Bieniawski (1992) refers to analytical, observational collection, there is a risk that the data collected will be and empirical methods while Stille and Palmström (2003) inadequate for the purpose of support design. prefer the following groupings: empirical and classification In conceptual design diagrams, data collection usually methods; numerical analyses and other calculations; and precedes analysis and design. This sequential process may observational methods. Although many authors suggest be appropriate and applicable in civil engineering projects the use of observational methods, the application of such but it is not necessarily the case in mining projects methods implies a reliance on instrumentation feeding (Hadjigeorgiou 2012). In a mining context, what is more back into the design during the development of the applicable is a continuous process where several steps are infrastructure. In a mining context, this is not a common run either in parallel or through several iterations. practice. Instead, as the mine matures and encounters A further issue, sometimes overlooked, is that the more demanding ground conditions, an enhanced ground engineering design process has become tool driven. support design is implemented. This is based on visual Therefore, unless there is a clear understanding, a priori, observations of the ground support system performance, of the input data needed to apply these tools, the required but generally without the use of instrumentation. As data will invariably not be collected. As a result, further such, it does not qualify as an ‘observational method’ as data collection campaigns may become necessary or, described in literature. alternatively, the gaps may be tentatively filled by other In this book, the following three categories are means, such as extrapolating available data or using used: analytical, empirical and numerical modelling default values suggested in different software. methods. The current chapter describes analytical 199 Chapter 9 : Analytical ground support design methods

recent years, the trend has been to use 3D wedge analysis Type B length: L = D/2 + 3 9.6 software Unwedge (RocScience 2012) to assess ground support requirements in large spans and intersections. K Rosengren (personal communication, 2018) suggested that this method provides conservative 9.3.4.5 Two-dimensional wedge analysis estimates of ground support. He also provided One of the most common failure mechanisms is fall of design charts for wide spans (Figure 9.11) and for blocks through sliding or falling from the back of an intersections (Figure 9.12). excavation. Figure 9.13 summarises a limit equilibrium Based on extensive application in Australian solution for a block or wedge susceptible to slide and underground mines of this design concept during the a wedge vulnerable to fall. It is important to reiterate past 50 years, the method appears to mitigate the majority some of the inherent assumptions. The first one is that of gravity/structurally driven roof failures. However, in the potentially unstable wedges are clearly delineated by the dominant structures. In this respect, the role of rock bridges, which may be beneficial, is ignored. Another assumption is that defined wedges are rigid (i.e. non-deformable). A further implicit assumption is that the shear strength of the weakness planes or joints is mobilised simultaneously along the full length of the slip surface. This is an important assumption in that if the shear strength drops post-peak, the mobilised shear strength may be lower than the peak strength. The choice of a factor of safety can differ significantly to account for the consequences of failure. Therefore, the traditional values for wedges at the back of a drive susceptible to fall under the effect of gravity is significantly higher (2 to 5) than for wedges sliding along the sidewall of a drive (1.5 to 2). In these circumstances, the type of reinforcement element used also influences the factor FIGURE 9.8 Compression zone formed by tensioned of safety. The assumption being that the long-term bolts (after Schach et al. 1979) performance of point anchored bolts may be lower

L a L a L a

L L L a = 2.0 a = 1.6 a = 1.33

Broad Narrow None compression zone compression zone compression zone

FIGURE 9.9 Establishing a prestressed arch (after Schach et al. 1979)

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• Section 3 : Ground support design methods 10

Chapter 10 : Empirical ground support design methods ) Source: BHP Source: ( Reproduced with the permission of BHP Reproduced CHAPTER TEN 10 Empirical ground support design methods

10.1 Introduction mass classification systems were reviewed in Chapter 5, Section 5.9. The current chapter focuses on empirical The traditional definition of empiricism is the practice ground support design methods and how they apply to of relying on observation and experiment. The mining underground mining. industry has relied heavily on experience to address mining challenges, including ground control. This experience is often transcribed as ‘rule of thumb’. 10.2 Empirical ground support design based on rock mass classification In an earlier compilation of the Hard Rock Miner’s Handbook (de la Vergne 2003), the following definition Empirical ground support design techniques based on for a rule of thumb was used: “An easy to remember guide rock mass classification systems rely on a systematic that falls somewhere between an engineered solution and approach and on an extensive database of observations. an experienced guess.” They enable users to account for not only the span and In a mining context, they suggested that: “… a Rule of the function of the excavation but also for variations of Thumb is an empirical standard. It is further defined as a ground conditions. pragmatic guideline or “norm” related more to the art than Rock mass classification systems originated from civil the science of mining. A Rule’s main roles are to provide engineering with a strong emphasis on tunnelling, but the perspective required to ensure practical concepts and they are used widely in rock engineering for a variety designs, and to facilitate finding pragmatic solutions for of applications. Over the years, the mining industry has operating problems.” tended to apply the results of rock mass classification Over the years, several rules of thumb have been systems for a variety of purposes well beyond the proposed to determine reinforcement patterns and lengths limits of their constitutive databases. At the same time, of ground support elements. Charette and Hadjigeorgiou numerous publications have identified the limitations of (1999) reviewed the rules employed for mining classification systems, the pertinence of their constitutive applications. These rules, which are still employed today, parameters, the weight assigned to the different have been derived from work in Canada, the United States, parameters and even the concept of an index or a ‘unique the United Kingdom, Australia and South Africa. Several value’ that captures the rock mass behaviour. rules of thumb applicable to the selection of reinforcement Nevertheless, rock mass classification systems are the type and dimensions also exist; for example, rules from link between rock mass quality and the choice of support. Laubscher (1984), Farmer and Shelton (1980) and the US An important assumption of these systems is that the Army Corps of Engineers (1980). classification adequately captures rock mass conditions. It should also be noted that most empirical rules are Terzaghi’s (1946a) work is of interest in that it was the based on past practice, which is often influenced by a first classification system proposed and it recognised the variety of non-technical factors such as legislation and importance of geological structure: “From an engineering site preferences. In fact, some of these rules are simply point of view, knowledge of the type and intensity of the rock based on geometry (e.g. the length of a roof bolt can defects may be much more important than the type of rock be one-third of the span). As it is difficult to justify a which will be encountered.” Terzaghi’s system provided ground support design that ignores ground conditions, guidelines for estimating the loads to be supported by the rules of thumb for ground support design in mines steel arches in tunnels. In the 1970s, a number of new have been to a large degree superseded by empirical and more elaborate classification systems were developed methods based on rock mass classification systems. Rock (Table 10.1) with similar objectives: 219 Chapter 10 : Empirical ground support design methods

Despite the small database, the general trend of using show the ground support stopping around mid drift. thicker reinforced shotcrete layers in poorer ground When ground conditions are fair or good (Q74 > 4.0), it is observed. In most cases where Q74 > 1, a 50 mm is more common to have the wall support reaching only thickness was employed. When Q74 < 1.0, either 75 or the shoulder of the drive, leaving about 3 m or more of 100 mm shotcrete thickness was applied. Although the unsupported wall height. data is scarce, 100 mm layers tend to be used when Q < 0.2. 74 10.3.3 Ground support design guidelines Figure 10.8 shows the extent of reinforcement and for mining drives surface support coverage applied to walls, expressed as the distance between ground support and the floor (or Interpretation of the data described in the previous the height of the unsupported wall). Three coverage section has been compiled and rounded up to produce categories are defined: the guidelines shown in Figure 10.9. These guidelines • floor – when the ground support coverage provide recommendations for the preliminary design extends down to within 1 m of the floor of reinforcement and surface support for drives in • mid-drift – when theground support coverage metalliferous mines. terminates around the mid height of the drive, These are general guidelines intended only as a ‘first 1–3 m from the floor pass’ design at the early stages of mine life (pre-feasibility, • shoulder – when the ground support coverage feasibility studies and early mine development). They terminates more than 3 m from the floor. reflect safe practices documented in the GCMPs of many

When Q74 < 1.0, the reinforcement and surface Australian and Canadian mines. The support design support coverage is often extended to near the floor. is likely to be refined as experience in local ground

In poor ground (1.0 < Q74 < 4.0), the majority of cases conditions is gained.

Extremely poor Very poor Poor Fair Good Very good

Minimum bolt density 0.85 0.70 0.65 0.55 with mesh (bolts/m2)

Minimum bolt density with reinforced 0.65 0.50 0.45 0.40 shotcrete (bolts/m2)

Reinforced shotcrete thickness 100 mm 75 mm 50 mm

Wall support coverage To floor Mid drift Shoulder

0.01 0.04 0.1 0.2 0.4 1410 40 100

RQD J J Rock mass quality Q = ××r w ()Jn ((Ja ))SRF

FIGURE 10.9 Ground support guidelines for mine drives of 4–6 m span (Potvin & Hadjigeorgiou 2016)

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• Section 3 : Ground support design methods 11

Chapter 11 : Numerical modelling for ground support design – mining applications

G Sweby, J Hadjigeorgiou, J Wesseloo and Y Potvin CHAPTER ELEVEN 11 Numerical modelling for ground support design – mining applications

11.1 Introduction analyses can be performed easily as post-processing exercises. Hence, elastic analysis is very useful for Desktop-based numerical analysis tools have been estimating the size and shape of a zone of potential available to practitioners of mining rock mechanics instability. This ability will be discussed further in for nearly 30 years. Examples include software suites this chapter. from Itasca Consulting Group Inc., Rocscience Inc. and When calibrated, elastic estimates of the shape and zone Map3D International Ltd. Early versions of software of potential instability are generally reliable. However, in provided the means to study rock mass behaviour and cases where the potential yielding zone is quite large, and potential instability, either perfectly elastic or with the where adjacent excavations allow interaction between inclusion of rock mass yielding, predominantly in 2D yielding zones, elasto-plastic analysis will be more (plane strain, plane stress or axisymmetrical analyses). useful. With 2D modelling, the underlying assumptions With advancement in computing power came the are related to geometry, stress field and emergingfailure ability to model larger and more complex geometries mechanisms, as these are limited to the analysis plane. in 3D. More recently, the introduction of ground Analyses in 2D are therefore easier to set up and quicker support elements (rockbolts and liners) expanded the to run, and the results are simple to display. Thus, functionality of computer codes still more to include 2D analysis is well suited for ‘what-if’ analyses and analysis of ground support/rock mass interactions. testing numerous scenarios but is generally not able to adequately represent the real 3D mine geometry, stress A wide range of modelling approaches and software field, failure and deformation mechanisms. packages is available. In this chapter, we refer to various commercially available software packages that are Continuum models consider the rock mass as a commonly used in mining; however, ground support continuous material that can deform in all directions. designers may choose to use or have access to different Conversely, the rock mass in discontinuum models is software to achieve the same results. represented as an assemblage of blocks that can deform and interact with neighbouring blocks, enabling the Analysts have the choice of 2D or 3D simulations, simulation of complex rock mass failure mechanisms. complex local or mine-wide geometries and elastic and Table 11.1 (Stead et al. 2006) provides a summary elasto-plastic solutions, with or without ground support of advantages and limitations of continuum and in addition, and a wide range of possible rock mass discontinuum modelling. The first part of this chapter constitutive models. will focus on continuum approaches. Discontinuum In the context of ground support, numerical modelling approaches will be discussed in more detail later in approaches can be classed into three broad categories, the chapter. each of which requires a different methodology and Ground support standards in mines are not designed different types and levels of data. These categories are: on a cut-by-cut basis but rather on a geotechnical • elastic or elasto-plastic domain basis. The prevailing conditions in different domains are therefore crucial. To delineate zones of • 2D or 3D similar ground support requirements within a mine, • continuum or discontinuum. 3D modelling on a mine scale may be needed to define Compared to elasto-plastic analysis, elastic analyses the boundary conditions of smaller scale models. After are quick to run and require only elastic parameters the zones of similar ground support requirements have as inputs. Strength factor calculations occur been identified, further small-scale analysis, potentially post-processing; and sensitivity analysis and ‘what-if’ including smaller scale 2D and 3D models, can be run, 237 Section 3 : Ground support design methods given in Table 11.7. For comparison purposes, the liner the as-built tunnel profile, theyield zone is appreciably was modelled using finite elements rather than by the larger than for the design profile. formulation for the built-in liner element. Comparing rockbolt loading, the design profile As a basis for comparison, the size of the yield zone is (left-hand wall) indicates higher maximum loading shown in Figure 11.25. It can be seen that in the case of than the as-built profile, with the distribution of

TABLE 11.6 Summary of shotcrete strength gain with time

Time Strength (MPa) Elastic modulus (GPa)

7 hours 6 12

1 day 10 15

3 days 21 22

7 days 33 27

21 days 40 32

(a) Design profile – yielded zone (b) As-mined profile – yielded zone

FIGURE 11.25 The yielded zone indicated by plastic volumetric strain contours is more extensive in the as-mined case on the right

(a) Design profile –rockbolt loads (b) As-mined profile –rockbolt loads

FIGURE 11.26 Rockbolt loads represented as filled bar charts plotted along bolt axes. Significant differences in the load distribution are evident 254 This book is available from the ACG store https://acg.uwa.edu.au/shop/gsso/

I IV Section 4 : Extreme ground conditions 12

Chapter 12 : Ground support in squeezing ground Courtesy Agnico Eagle CHAPTER TWELVE 12 Ground support in squeezing ground

12.1 Introduction criterion for squeezing cannot reasonably be established solely on ground conditions, since there is always Squeezing is a term used to describe the behaviour of interaction between the deforming ground and the tunnels that undergo large deformations over time. These installed support. conditions are encountered in tunnelling and in mining drives in weak or poor quality rock and in structurally In a mining context, squeezing ground conditions are defined rock masses. encountered when the excavation convergence, even with Terzaghi (1946b) provided one of the earliest ground support, exceeds 2% (Hadjigeorgiou et al. 2013). definitions of squeezing rock behaviour with respect In general, this suggests that the total displacement of the to tunnelling: “Squeezing rock slowly advances into the drive closure will reach at least tens of centimetres within tunnel without perceptible volume increase. Prerequisite the life expectancy of a supported drive. In general, of squeeze is a high percentage of microscopic and sub- mine drives are designed to be in operation for up to microscopic particles of micaceous minerals or of clay 18 months to two years. Furthermore, it is also implied minerals with a low swelling capacity.” Furthermore, he that in squeezing ground conditions, the resulting loads distinguished between squeezing rock at moderate depth will be greater than the capacity of a ‘stiff’ support system. and squeezing rock at great depth to provide estimates of This often results in significant failure of ground support the resulting rock loads on the roof of tunnels. Terzaghi and necessitates extensive rehabilitation work. made a clear distinction between squeezing rock and swelling rock, which is limited to rocks that contain clay 12.2 Squeezing mechanisms minerals such as montmorillonite. Squeezing ground has been observed in a range of ground Barla (1995) proposed a widely cited definition: conditions including massive (weak and deformable) “Squeezing of rock is the time dependent large deformation rocks and in highly jointed rock masses with large-scale which occurs around the tunnel and is essentially defects such as joints, foliation and bedding. Aydan et al. associated with creep caused by exceeding a limiting shear (1993) has provided a phenomenological description of stress. Deformation may terminate during construction squeezing in rocks, distinguishing between three types of or continue over a long time period.” Einstein (1996) failure mechanisms (Figure 12.1). proposed another definition: “Time dependent shearing of the ground, leading to inward movement of the tunnel A complete shear failure (Figure 12.1a) implies that perimeter.” This is similar to the definition for swelling the shearing of the rock mass is of sufficient magnitude as a “time dependent volume increase of the ground, to destroy or seriously endanger the tunnel structure. leading to inward movement of the tunnel perimeter” . The This is observed in continuous ductile rock masses or definitions are similar but swelling is usually associated what is often referred to as ‘weak’ rock masses. Gao et with ‘a combination of physical–chemical reaction al. (2015) reported cases of complete shear failure of the involving water and stress relief’ while squeezing is rock around excavations in coal mines. a mechanical process. Although both swelling and Buckling failures (Figure 12.1b) are observed in squeezing can result in an inward movement of the metamorphic rocks (i.e. phylitte, mica schists) and tunnel periphery over time, most occurrences of swelling thinly bedded ductile sedimentary rocks (i.e. mudstone ground are associated with argillaceous (clay-rich) soil shale, siltstone, sandstone, evaporitic rocks) (Aydan et or rock. al. 1993). These types of failures have been observed None of the proposed squeezing definitions are in several underground hard rock mines (Potvin & universally accepted. Schubert (2015) has argued that a Hadjigeorgiou 2008). 269 Section 4 : Extreme ground conditions

No squeezing Low squeezing – bolts take the load

Moderate squeezing – convergence Extreme squeezing

Rehabilitated drift Purged drift FIGURE 12.18 Examples of drifts subjected to squeezing at the Lapa and LaRonde mines (after Hadjigeorgiou et al. 2013) in FLAC3D using an explicit representation of foliation. damage, the squeezing mechanism cannot be well The model captured failure along the foliation, indicated represented. To better replicate the mechanism, foliation by separation of joints, but did not correctly predict the should be modelled explicitly to allow block rotation and location and degree of stress-induced fracturing. buckling. Continuum modelling using finite element or Numerical modelling in squeezing ground using the finite difference methods with explicit representation GSI for reducing intact rock parameters has similar of foliation can provide an improved representation of limitations in anisotropic conditions. Although the squeezing mechanisms observed in hard rock mines. modelled deformation in these cases may match observed However, the foliated squeezing mechanism is still not 284 This book is available from the ACG store https://acg.uwa.edu.au/shop/gsso/

I IV Section 4 : Extreme ground conditions 13

Chapter 13 : Ground support design for rockburst prone conditions CHAPTER THIRTEEN 13 Ground support design for rockburst prone conditions

13.1 Background further as a combination of excavation vulnerability and workforce exposure, expressed as follows: The trend towards exploiting deeper mineral resources is a natural evolution of underground mining as near Excavation Seismic risk = Seismic ××vulnerability Workforce 13.1 surface reserves become progressively depleted. The hazard potential exposure deepest mines in the world are currently operating at depths of 3–4 km below the surface. The challenges where: associated with deep mines are numerous but perhaps • seismic hazard is the probability of initiation of the prime risk and concern in many of these mines is a seismic source to produce a certain magnitude rockbursting. Intense rockbursts may cause death or event. injury to workers and significant loss of assets. Rockburst • excavation vulnerability potential is the probability is a difficult risk to manage due to the unpredictable of damage occurring at each excavation site of interest and involves local site characteristics and nature of seismic hazards. the proximity of the seismic hazard. In addition to the moral obligation to protect • workforce exposure is the probability of people workers’ safety, Potvin and Wesseloo (2013) argue exposure to harm as a result of a seismic event. that mine seismicity and, more specifically, the In this context, the concept of seismic risk is not a possibility of experiencing a seismic event resulting single point estimate but a range of values reflecting the in one or multiple fatalities has become the most probability of harm to people. important financial risk in underground hard A number of measures can be implemented to rock mines operating in developed countries. The mitigate seismic risks by reducing the seismic hazard, financial consequences of a fatal accident resulting excavation vulnerability and workforce exposure. This from a seismic event is likely to involve a long-term chapter focuses on reducing excavation vulnerability shutdown; this cost alone will generally far exceed the through the design and implementation of dynamic total cost associated with any other type of fatality in resistant ground support. underground mines. For example, two separate rockburst fatal accidents 13.2 Ground support subjected to in Australian mines (at Beaconsfield in 2006 and Big dynamic loading – a case of Bell in 2000) closed operations for well over one year to design indeterminacy allow mining methods and sequences to be completely redesigned. In addition to the tragic loss of human It is important to recognise that the design of ground lives, the financial costs resulting from these accidents support for seismically active conditions does not replace were extremely high. the design requirements for static loads. Dynamic loading of ground support occurs as a result of mine-induced From time to time, seismic risk has also resulted in seismicity – repeated discrete seismic events of varying significant loss of mineral reserves. Numerous mines magnitude and location. In between seismic events, and deep orebodies around the world have been and in addition to the seismic loads, a ground support abandoned as unminable due to unmanageable seismic system must continue to fulfil its role of maintaining risk, among other factors. the integrity of an excavation while subjected to static The definition of risk as a function of the likelihood loading. In effect, the design for seismic conditions is of a hazard occurring and the consequence is well additional or complementary to the design process for established. Heal (2010) decomposed the consequence static conditions. 301 --

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• Chapter 13 : Ground support design in rockburst prone conditions

Distance Twin Cone 0 Gauge SS * Rebar * WWM * SS * Rebar * WWM * MN SFRS m cables Yield Lok strap <1 m2 <1 m2 <1 m2 >2 m2 >2 m2 >2 m2 > 50

< 1 10–50

< 10

> 50

1–2 10–50

< 10

> 50

2–3 10–50

< 10

> 50

3+ 10–50

< 10

SS = Split Set WWM = Welded wiremesh SFRS = Steel fibre-reinforced shotcrete * Density expressed as square metres per tendon. > or < inferred as ‘or equal to’.

# Mesh without secondary reinforcement can fail at low MN resulting in tendon stripping.

FIGURE 13.35 Kidd Mine’s ground support survivability chart for events from 0.5 to 3.8 by support type (after Counter 2017) (courtesy Glencore Canada Corporation, Kidd Mine Operations)

• accidents, including work stoppages • work stoppages not associated with accidents • poor quality excavation, blasting and support, leading to less stable excavations • clean-up costs and rehabilitation costs • loss of production in operations directly affected by the damage • loss of production in areas more remote from the damage, owing to loss of access, such as blockages of tunnels, damage to roadways, and damage to ventilation • reassignment of crews • loss of ore • difficult-to-quantify factors such as public FIGURE 13.36 Example of low displacement perception, reduction of mining company share compatibility between the stiff reinforcement and soft price, reduced worker morale and labour unrest. surface support (courtesy Brad Simser) It was argued that full quantification of these risk financial terms. The argument was made that rockburst considerations would almost certainly justify the damage can be the cause of significant direct and indirect extra cost of employing extensive dynamically capable costs associated with: rock support. 341 Section 4 : Extreme ground conditions 14

Chapter 14 : Long-term performance of ground support – corrosion Courtesy Jean-Francois Dorion CHAPTER FOURTEEN 14 Long-term performance of ground support – corrosion

14.1 Introduction • short term (less than two years), e.g. crosscuts and temporary openings, including big stopes The selection, design and implementation of a ground support strategy considers the ground conditions, the • medium term (2–5 years), e.g. exploration drifts significance of the excavation and the anticipated service • long term (5–10 years), e.g. level accesses and life of the mine. The effectiveness of a support system can ventilation drifts change over time due to a multitude of external factors. • life of the mine (more than 10 years), e.g. main Experience has been that any reinforcement or surface accesses, ramps, shafts and garage crusher support element is susceptible to degradation, potentially stations. leading to failure (Hadjigeorgiou 2016). This process can From the moment a reinforcement or support element be triggered or accelerated by the following factors: achieves its initial performance level, it is susceptible to • mine-induced seismicity degradation. The rate and severity of degradation can • drive convergence resulting in loading of be influenced by increased demand or damage to the individual reinforcement or support elements support system (Table 14.1). A conceptual representation • blast damage associated with explosive gases of the degradation of two ground support systems is and flyrock presented in Figure 14.1. • material quality and the presence of In the first case, the system degrades until it reaches manufacturing flaws a critical performance level when it is considered to have failed. In the second case, the system degrades • installation issues, such as bolt orientation, without reaching the critical performance level during grout quality, damage to protective coatings its service life. or galvanisation This system has experienced loss of performance • damage to reinforcement or support caused but for practical purposes has not failed. In reality, the by equipment degradation process of ground support may be more • corrosion of support systems. complex, with multiple factors interacting. For example, In this context, the service life of an operation imposes ground support may substantially degrade due to further constraints on the choice of ground support. corrosion and then fail when loaded by a seismic event, The following guidelines provide an indication of the resulting in either an impact load or further degradation anticipated service life of mining excavations: until failure (Figure 14.2).

TABLE 14.1 Causes for degradation of support

Increased demand Damage to support

Changes in stress Loading: static and dynamic

Rock mass degradation Corrosion of support

Increase in excavation dimensions Blast damage (flyrock)

Mine-induced seismicity Equipment damage

Loading: static and dynamic

345 Section 4 : Extreme ground conditions

Level 1: Negligible corrosion

Level 2: Localised corrosion

Level 3: Surface corrosion

FIGURE 14.9 Visual assessment of corrosion levels of ground support (modified from Dorion & Hadjigeorgiou 2014)

356 This book is available from the ACG store https://acg.uwa.edu.au/shop/gsso/ Section 5 : Probabilistic approach to ground support design 15

Chapter 15 : Probability, risk and design

J Wesseloo and WC Joughin Courtesy LKAB Kiruna CHAPTER FIFTEEN 15 Probability, risk and design

15.1 Design acceptance criteria it will be with sufficiently high reliability. A design acceptance criterion focused on stability is appropriate The geomechanical engineer is tasked with designing for civil engineering structures designed to be stable and managing geomechanical structures in spite of for long periods and with the public having access to being faced with incomplete and inadequate data and these structures. In mining, the safety of personnel and knowledge. By its nature, geomechanical engineering optimised economic value, rather than exclusive stability, is subject to uncertainty and variability that need to are the aims. be taken into account in the design process and in Improved safety can be achieved not only by the management of geomechanical risks. Amid this increasing stability, but by monitoring of behaviour uncertainty, the engineer faces the most fundamental and management of personnel exposure. Personnel questions in engineering design: “When is my design exposure can be managed and significantly reduced good enough and how will I know? What confidence can with the effective use of monitoring, or even eliminated, I have in my design?” by using remotely controlled or autonomous These questions are not trivial and the answers equipment. Under the latter circumstances, a design are highly dependent on the non-technical mining acceptance criterion focused on stability is not environment. Often the engineer applies widely used optimum and should be replaced with a criterion that design acceptance levels without evaluating their quantifies the financial risk associated with the design. applicability to the actual situation he or she is required In other circumstances, economic risk and safety risk to design for – for example, by choosing a factor of safety need to be evaluated in parallel. of 1.5 (FS = 1.5) as a design acceptance criterion without If one considers the fact that mining is about considering whether a design based on the available managing the risk–reward balance for the information will be sufficiently reliable at FS = 1.5. shareholders without endangering personnel, it Three types of acceptance criteria can be used in is clear that design acceptance criteria in mining mining geomechanics—namely, factor of safety (FS), should be based on risk and not exclusively on FS probability of failure (PF) and risk. FS is defined as the or PF. In this context, quantitative risk-based design ratio of capacity over demand: is a continuation of the probabilistic design process taken to its natural conclusion. Capacity The common use of the word ‘risk’ includes a wide FS = 15.1 Demand variety of meanings and sometimes with different meanings in different industries (Baecher & Christian The use of FS has been a standard approach in many 2003). In the engineering definition, risk is the branches of engineering for more than a century. FS product of the likelihood and the consequence of a as a design acceptance criterion is used even when the particular event: other two acceptance criteria (PF and risk) are employed. PF is used to quantify the reliability of a design when Risk = (Probability of an event) • (Consequence of the event) faced with uncertainty and variability in the design Risk = P[X] • C[X] 15.2 parameters. It is commonly defined as the probability of FS < Stability Limit, generally accepted as FS < 1. where: Both the FS and PF focus on stability – that is, defining P[X] = probability of event X occurring. a criterion to ensure that when a design is accepted C[X] = consequence of the occurrence of X. 377 --

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• Chapter 15 : Probability, risk and design in a fraction of the mine being analysed (SR) of about 40 criteria applicable to a range of different lengths is shown and a design acceptance criterion of about C = 1.5 × 10-3. in Figure 15.11. These two figures illustrate the concept of evaluating total risk discussed in Section 15.2.8. If The design acceptance criterion applicable to a a tunnel section of 10 m is evaluated in isolation, the 500 m length is shown in Figure 15.10 whilst the same acceptance level needs to be lower for the total risk associated with a longer length of tunnel to fall below societal risk acceptance levels. 1.00E-02 It is interesting to note the similarity between this Unacceptable; proposed criterion and the design acceptance level Acceptance level 500risk m (β cannot = 0.1) be justified 1.00E-03 Hong Kong Planning Department prescribed by the Hong Kong Planning Department for slope stability along highway/freeway development 500 m top of ALARP 1.00E-04 (Hong Kong Government Planning Department 1994). The Hong Kong Planning Department’s criterion is a e fatalities per year Acceptance level 500 m (β = 0.01) risk neutral criterion proposed for rock engineering- 1.00E-05 Hong Kong Planning Department related aspects only and is applicable to a 500 m section of road. The criteria derived here and the 500 m bottom of ALARP ALARP 1.00E-06 acceptance level prescribed by the Hong Kong Planning

obability of N or mor Department, both for an applicable length of 500 m, are Pr shown in Figure 15.10. In the figure, the dashed lines 1.00E-07 110 100 1000 are the acceptance levels prescribed by the Hong Kong Estimated number of fatalities for scenario under consideration Planning Department. In lieu of a government-prescribed risk acceptance FIGURE 15.10 Safety risk acceptance levels derived level, the method described here can be used to develop a for Australian mining industry based on a national risk site-specific risk acceptance level. Figures 15.10 and 15.11 acceptance level shown with the criterion for the Hong summarise a general risk acceptance level applicable to Kong Government Planning Department (1994) Australian mines.

1.00E-02 15.4.3 Concluding remarks

Acceptance level 1000 m A design acceptance criterion for safety was developed 500 m 1.00E-03 in this chapter in a transparent manner. It is important 200 m to note that the development of design acceptance levels 100 m for safety risk is not a moral issue nor does it impose 1.00E-04 any level of risk on society. Rather it is an attempt to

e fatalities per year quantify the level of risk already accepted by society as

1.00E-05 being reasonable. It is clear that the last word on this subject has not been written and much more can, and 50 m should, be done to improve these guidelines. We do, 1.00E-06 20 m however, provide a start that is meant to enable the

obability of N or mor 10 m

Pr use of a risk-based approach for geomechanical design

1.00E-07 in underground mines. The lack of official guidelines 110 100 1000 Estimated number of fatalities should not prevent an engineer from performing a for scenario under consideration quantitative risk-based design. The safety risk design acceptance levels developed here are proposed for use FIGURE 15.11 Upper ALARP safety risk acceptance where no other guidelines are available. levels derived for the Australian mining industry (solid line) and Hong Kong Planning Department criterion The process ofrisk-based design is discussed with a (dashed line) safety risk example application in Chapter 16. 395 Section 5 : Probabilistic approach to ground support design 16

Chapter 16 : A risk-based approach to ground support design

WC Joughin, J Wesseloo, J Mbenza, P Mpunzi and D Sewnun Courtesy Brad Simser CHAPTER SIXTEEN 16 A risk-based approach to ground support design

16.1 Introduction installed ground support is not sufficient and the drive may experience damage. This book has addressed all elements of ground support design in underground mines. The previous chapter Two potential failure modes are considered – excessive discussed the importance of risk-based design and deformation due to stress driven damage (Figure 16.2a) risk-based decision making. This chapter provides an and structurally controlled failure (Figure 16.2b). If example of the risk-based design process for ground a rockfall occurs, some rehabilitation effort will be support in an underground mine. It illustrates the use of required, depending on the size of the rockfall. The quantitative risk-based design methods by focusing on original ground support system will be able to cater for a two mechanisms – excessive deformation due to stress limited amount of stress damage. When the deformation damage of the drive and structurally controlled failure. exceeds the capacity of the ground support, or the drive It should be recognised that there is room for converges so much that mobile equipment cannot pass, improvement on each element presented in this remediation will be necessary. The cost of rehabilitating discussion. This is the case for any design as the designer the drive will be a function of the extent of the damage is faced with having to make simplifying assumptions. or size of the rockfall. However, the loss of revenue As with any design process, testing the sensitivity of while the drive is being rehabilitated is often far greater the outcome of these assumptions is also part of the than the cost of rehabilitation. The potential impact on design process. For example, if the width of an assumed revenue will depend on the location of the damage and distribution for one of the parameters has little influence the purpose of the excavation. on the final outcome of the design, there is no reason Injury to personnel and damage to mining equipment to spend more resources on improving the knowledge may also occur if the time and location of rockfalls are or data about that particular parameter. Of course, the coincident with the presence of personnel or mining opposite is also true. Resources need to be assigned to equipment. Injuries, fatalities and damage to equipment better understand and analyse those components where may have severe financial implications but are generally uncertainty has a large influence on the final design. less significant than production losses, because they only The risk-based approach provides a way of assigning occur when there is spatial and temporal coincidence. a cost to information. In some circumstances, the cost While the financial implications of injuries may of conservatism might be less than the cost of improved be less significant, companies are morally and legally information and improved simulation and analysis. responsible for the safety of personnel. Safety risk acceptance criteria are discussed in Chapter 15, where 16.1.1 Risk-based ground support design a method of estimating acceptance levels for individual in a mining context and societal risks is presented.

We describe the risk-based ground support design As discussed in Chapter 15, reduction of risk to process using simple examples of drives supported with personnel does not necessarily relate to a reduction rockbolts and mesh (Figure 16.1). When ground support in the probability of failure. Risk mitigation strategies systems are appropriate for the prevailing stress state form part of the design, and accepting a higher and ground conditions and correctly installed, the drive probability of failure may result in a more economical will function effectively and there will be no disruption design if the risk to personnel is mitigated – for to mining operations. However, due to variability example, by the use of remote-controlled or automated and uncertainty, conditions may occur for which the vehicles or by preventing exposure using appropriate 399 Chapter 16 : A risk-based approach to ground support design

1.E+00

Probability 1.E-01 of failure 50%

1.E-02 20% 10% 1.E-03 5% Pr (≥k) 2% 1.E-04 1% 0.5% 1.E-05 0.2% 0.1% 1.E-06 0010 15 20 25 30 35 40 45 50 Length of damage (m)

FIGURE 16.26 Reverse cumulative probability damage length distributions for lp = 45 m and values of p from 0.1 to 50% (Joughin 2017) for the decline, only the total length that services the 16.3 Probabilistic analysis of sublevel was considered. structurally defined failures The reverse cumulative distribution of normalised Several software tools are available to assess the stability expected frequency of damage length are presented in of structurally controlled failure modes. For the purpose Figure 16.27. The likelihood intervals are shown on the of this example, an analysis of structurally defined right-hand axis for reference purposes. Note that the failures was performed using the software JBlock (JBlock 2017) originally applied in a risk-based design method expected frequency normalised with time, FT, values for narrow tabular slopes in South Africa (Joughin are highest for the crosscuts because they have the et al. 2012a; Joughin et al. 2012b). JBlock was designed greatest N. However, the maximum length of damage to create and analyse geometric blocks or wedges, based ld is limited by lp. While the decline has the greatest on collected data in the form of joint orientations, trace ld values, it is expected that p will be relatively low lengths, joint conditions and friction angles. The blocks because it is further away from the stopes. In fact, it are formed by the intersection of joints or faults in the is essential to ensure that p is low, because the decline excavation roof, which can fail by sliding or falling into affects all of the potential production from the sublevel. the excavation. This frequency damage model is used to evaluate the Although JBlock is being further adapted for economic risk in Section 16.4. application in drives, its ability to handle 3D drive

TABLE 16.7 Specific input parameters for the frequency and extent of damage analysis

Parameter Decline Footwall drive Crosscuts

Total length (L) 500 m 225 m 900 m

Potential affected length l( p) 42 m 30 m 15 m

Number of potentially affected lengths N( ) 12 8 60

Number of segments (n) 14 10 5

417 --

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• Appendix 1 : Rock mass ratings Courtesy Mount Isa Mines, A Glencore Company Courtesy Mount Isa Mines, A Glencore APPENDIX ONE A1 Rock mass ratings

Ratings for the RMR-system parameters

TABLE A1.1 Rock Mass Rating (RMR76), Bieniawski 1976

Parameter Ranges of values

Point-load For this low range – strength index > 8 MPa 4–8 MPa 2–4 MPa 1–2 MPa uniaxial compressive Strength of test is preferred intact rock 1 material Uniaxial > 200 100–200 50–100 25–50 10–25 3–10 1–3 compressive MPa MPa MPa MPa MPa MPa MPa strength

Rating 15 12 7 4 2 1 0

Drill core quality RQD 90–100% 75–90% 50–75% 25–50% < 25% 2 Rating 20 17 13 8 3

Spacing of joints > 3 m 1–3 m 0.3–1 m 50–300 mm < 50 mm 3 Rating 30 25 20 10 5

Very rough Slightly rough Slightly rough Slickensided surfaces surfaces surfaces surfaces Soft gouge > 5 mm Not Separation Separation OR thick continuous < 1 mm < 1 mm Gouge OR No separation Hard joint Soft joint wall < 5 mm Joints open > 5 mm Condition of joints Hard joint wall wall rock rock thick Continuous joints 4 rock OR Joints open 1–5 mm Continuous joints

Rating 25 20 12 6 0

Inflow per 10 m < 25 25–125 None > 125 litres/min tunnel length litres/min litres/min

Joint water pressure Groundwater Ratio 0 0.0–0.2 0.2–0.5 > 0.5 5 Major principal Stress

Moist only Water under General Severe water Completely dry (interstitial moderate conditions problems water) pressure

Rating 10 7 4 0

Bieniawski, ZT (1976) ‘Rock mass classifications in rock engineering’, inProceedings of the Symposium on Exploration for Rock Engineering, ZT Bieniawski (ed.), A.A. Balkema, Rotterdam, pp. 97–106.

437 Appendix 2 : Q-system APPENDIX TWO A2 Q-system

where: RQD J J r w A2.1 Q74 = ×× RQD = rock quality designation. Jn Ja SRF74 Jn = joint set number. Jr = joint roughness number. RQD J J J = joint alteration number. r w A2.2 a Q93 = ×× Jn Ja SRF93 Jw = joint water reduction factor. SRF74 = stress reduction factor (Barton et al. 1974)

SRF93 = stress reduction factor (Grimstad & Ratings for the Q-system parameters Barton 1993)

TABLE A2.1 Rock quality designation

RQD

A Very poor 0–25

B Poor 25–50

C Fair 50–75

D Good 75–90

E Excellent 90–100

Note: Where RQD is reported or measured as ≤ 10 (including 0), a nominal value of 10 is used to evaluate Q. RQD intervals of 5, that is, 100, 95, 90, etc. are sufficiently accurate.

TABLE A2.2 Joint set number

A Massive, no or few joints 0.5–1.0

B One joint set 2.0

C One joint set plus random joints 3.0

D Two joint sets 4.0

E Two joint sets plus random joints 6.0

F Three joint sets 9.0

G Three joint sets plus random joints 12.0

H Four or more joint sets, random, heavily jointed, ‘sugar cube’, etc. 15.0

J Crushed rock, earthlike 20.0

Note: For intersections, use (3.0 x Jn); for portals, use (2.0 x Jn).

445 Appendix 3 : Techniques for probabilistic and risk calculations

J Wesseloo and J Mbenza Courtesy LKAB Kiruna APPENDIX THREE A3 Techniques for probabilistic and risk calculations

A3.1 Introduction A3.2.1 Random variate sampling

This appendix focuses on practical methods for Random variate sampling refers to the sampling of probabilistic calculations. Each method is illustrated random numbers from a given distribution, which by a brief theoretical discussion followed by simple results in the sampled values approximating the given examples. The discussion is aimed at providing enough distribution. This is illustrated in Figure A3.2 where understanding so that the methods can be used with 1,000 values are sampled from a normal distribution confidence. Due to space constraints, our discussion is with mean = 0 and standard deviation = 0.1, where limited to an introduction to the subject. For an in-depth the frequency distribution of the sampled numbers discussion on these methods, interested readers are approximates the original distribution. referred to Baecher and Christian (2003). Many computational software programs used in engineering have built-in functions for random A3.2 Monte Carlo method variate sampling. However, this sampling process can easily be performed on any distribution for which the Any available deterministic analysis can be used to inverse cumulative distribution function is available. perform probabilistic analysis with the use of the The process relies on a pure random sampling (i.e. Monte Carlo method, the only requirement being uniform distribution) of probabilities between 0 and 1. to repeat the deterministic computation many times A set of random variate sampled values can then be with different inputs. obtained by calculating the parameter values for every With modern computing power, Monte Carlo analysis sampled probability value. This process is illustrated is often the obvious first approach for probabilistic in Figure A3.3. analysis as additional assumptions are not required. However, for computationally intensive problems – for A3.2.2 Illustrated example example, a finite element analysis – Monte Carlo analysis For this example, consider the following problem where is generally still impractical. the objective is to calculate the mean and standard The Monte Carlo method, in essence, consists of deviation for the cross-section area of a rectangular multiple deterministic analyses with varying input tunnel. The width of the tunnel is 5 m with a standard values. The set of values used in the different analyses deviation of 0.1 m; the height of the tunnel is 5 m with for each parameter represents the desired probability a standard deviation of 0.2 m. For this example, let distribution. If enough trial analyses are performed, the us assume the height and width are both adequately set of results will approximate the true distribution of described by a normal distribution. the output. This simple application of the Monte Carlo process A Monte Carlo analysis consists of three basic can easily be performed with a spreadsheet, as shown in components (Figure A3.1): Figure A3.4 where Figure A3.4(a) shows the formulas in the cells and Figure A3.4(b) shows the results of the 1. Random variate sampling of the input parameters. calculation performed in each cell. Note that the actual 2. Performing multiple deterministic analyses for numbers in each of the cells will be different for each new each Monte Carlo trial. calculation but, if one performs enough Monte Carlo 3. Investigating the distribution of the results of the trials, the final statistical results will be reliable and will calculations. not differ significantly for different Monte Carlo runs. 453 system response can be calculated and a surface function • Perform probabilistic assessment using, for can be fitted through the results, which provides an example, the Monte Carlo method. approximation of the true, but unknown system response. A multidimensional surface function is applicable to A3.4.1 Surface functions problems with more than two stochastic parameters. The number of coefficients in the surface function Probabilistic assessment using the response surface that need to be obtained determines the number of method (RSM) follows these steps: analyses required. For higher order surface functions, • Perform selected analyses to obtain points more trial analyses are required than for lower order distributed through the response space to best functions. capture the response with varying inputs. The simplest surface function is a first order polynomial • Fit a multidimensional surface function though function which can be written as follows: the result points. This surface approximates the true system response and provides an RSd=1 = β0 + β1 • x1 + β2 • x2 + β3 • x3 + • • • + βn • xn approximate closed form solution of the A3.17 unknown system response.

Fitted response surface approximation of the system response Response Trial responses

Input parameter 2

Input parameter 1

Trial points: point for which the response was evaluated

FIGURE A3.9 Conceptual illustration of the response surface methodology for two stochastic parameters

466 This book is available from the ACG store https://acg.uwa.edu.au/shop/gsso/ References Courtesy LKAB Kiruna References

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510 Index Courtesy LKAB Kiruna Index

A CT-Bolt 38 D-Bolt 44 60, 62 accelerators debonded cable bolt 49 accepted risk 379, 389, 394 expandable bolt 46 individual safety risk 379, 389, 394 hybrid bolt 46 societal safety risk 379, 389, 394 modified cone bolt 44, 336–338 acoustic emissions 85 Omega-Bolt 46 acoustic televiewers 132 Paddle Bolt 37 active spans 203, 379, 389, 394 point anchor bolt 32–34, 39, 204, 247 actual profile 256 Posimix bolt 37, 38 49 adhesive failure 215 resin grouted cable bolt self-drilling bolt 47 admixtures 62 Solid Dynamic Bolt 45 aggregates 60–62, 67 Wiggle Bolt 37 analytical ground support design 197, 199 Yield Lok bolt 45 anchoring mechanisms 27 bolt density 228, 229, 232, 241, 331 angular limit 184, 185 bolt length 204–206, 210, 222, 223, 241, 242, 361, 369 anisotropic rock 15, 17–19, 108, 110, 131, 249 bond strength 41, 43, 48, 49, 64, 144, 145, 152 anisotropy 15, 92, 101, 108, 118, 125, 128, 136, 138, 313 borehole breakout 118, 132, 133, 135, 136 aperture 35, 56, 57, 107, 156, 158, 169, 232, 419, 440 Brazilian test 182, 401 aqueous corrosion 348, 359 brittle 8, 20–22, 29, 30, 35, 41, 43, 60, 63, 88, 113, 144, 153, arithmetic sample mean 186, 192, 195 201, 248, 250, 253, 259, 263, 306, 311–314, 353 ASTM standard tests 142 brittle failure 8, 20, 153, 201, 263, 306 atmospheric corrosion 347, 348, 359 buckling 11, 15, 17, 21, 107, 203, 265, 273–276, 278, 283–286, ATVs (acoustic televiewers) 132 289, 293, 297, 298, 313 axial force 263 buckling failure mechanism in foliated squeezing ground 283 axial pull displacement 146 buckling mechanism 274, 285, 286, 298 axial pull load 146 bulbed strand 48 axial strain 88, 261, 263 bulking 3, 8, 22, 29, 32, 162, 250, 263, 274–276, 297, 313–315, 318, 319, 323, 332, 400, 405, 407, 411 B bulking factor 318, 319, 405, 407, 411 barrel and wedge 52, 363 bulking of the tunnel 263 Barton-Bandis failure criterion 91, 95 beam concept 204 C bearing capacity of bolt 205, 210 cable bolts bedding planes 77, 132, 205, 270 debonded 49 bias prestressed 49 censoring bias 185 resin grouted 49 orientation bias 185 cable lacing 59 size bias 185 Cable-Lok Shell 48, 49 truncation bias 185 capacity, ground support 141, 143, 175, 315, 320, 323, 325 birdcaged strand 48 cement 32–38, 41, 44, 47–49, 60–62, 73, 146, 148, 149, 296, block size 23, 104, 203, 211 297, 362, 366 bolt cement grouted continuous anchor 37 cone bolt 44, 166, 336–338 censoring bias 185 513 chainlink mesh 56–58, 158–161, 168, 169, 172 criterion, Hoek-Brown 103, 112, 113 chemically bonded anchor 35 criterion, Mohr-Coulomb 89, 92, 94, 95, 419 chemically bonded yielding anchor 44 CSIRO HI Cell 126–129 closed form solutions 201 CT-Bolt 38 coarse aggregate 62 coefficient of variation 186, 189, 192, 195 D cohesion 5, 92, 206, 250, 253, 260, 297 damage risk model 413 compressive stress 17–19, 20, 22, 23, 134 data confidence 177, 179–182, 189, 401 conceptual design 114, 118, 199, 380 data reliability 186 conceptual models 15, 259 D-bolt 147 condition of joints 99, 437 dead weight 5, 8, 28, 30, 49, 141, 143, 162, 205, 212, 232, 238, conductivity 353 239, 241, 314 cone bolt 44, 166, 336–338 debonded cable bolts 49 confidence coefficient 183, 192–195 debonded yielding reinforcement 43 confidence intervals 187, 188, 191 decomposition techniques 480 confinement 4, 8, 19, 22, 23, 29, 48, 55, 60, 113, 149, 199, event trees 480 201–203, 213, 276, 278, 293, 294, 297, 313, 322, 323, 409 fault trees 4 connection between surface support and reinforcement 294 deconfinement 294 continuity of joints 99 deep anchoring degree of belief 382 continuous anchor 32, 35, 37–39, 43, 48, 204 density 80, 81, 91, 96, 114, 118, 184, 187, 193, 194, 205, 206, continuum models 237, 400 228, 229, 232, 241, 242, 253, 270, 291, 292, 331, 358, 401, convergence 6–8, 10, 30, 144, 199, 201–203, 213, 238, 261, 403, 415, 429, 449 262, 265, 269, 273, 274, 277–279, 281, 283, 284, 289–291, 279, 318, 319, 324, 400, 405, 410–413 293, 294, 297, 298, 327, 345, 409 depth of failure design acceptance criteria 377 convergence confinement 199, 201–203, 213 design profile 254–256 core discing 125, 131, 132, 136 design specifications 246 core logging 80–82, 96, 97, 103, 104, 106, 180, 182 detailed design and construction 179 corporate risk 383, 384 DFN (discrete fracture network) 81, 97, 184, 186, 211, 212, corrodants 347, 348 419 corrosion environments 361, 370 dip 17, 81, 83, 96, 97, 99, 100, 183, 184, 403 corrosion, forms of dip direction 17, 96, 183, 184, 403 aqueous corrosion 348 atmospheric corrosion 347, 348, 359 direct shear 214, 215 crevice corrosion 353 direct shear failure 214, 215 electrochemical corrosion 347 discontinuity spacing 5, 183 galvanic or bimetal corrosion 353 discontinuum models 258 microbiologically influenced corrosion 353 displacement demand 117, 302, 319 uniform or semi-uniform corrosion 353 dissolved oxygen 348, 352, 363 corrosion, fundamentals of 347 DOE (design of experiment) 403, 404, 411, 467, 473, 475–480 corrosion, galvanic or bimetal 353 dog-earing 132 corrosion levels 356, 357 borehole breakout 118, 132, 133, 135, 136 corrosion of support 345 domed plate 51 corrosion rate 347, 352, 353, 355, 358, 359, 361–363, 373 donut plate crack 35, 36, 43, 63, 64, 85, 291, 353, 362, 368 embossed plate 51 cracking 20, 63, 64, 85, 159, 353, 366 double shear test 148 crevice corrosion 353 DRA (displacement rate analysis) 126, 129, 131 criteria, design acceptance 377 drive closures 279 criterion, extension strain 113 drop test 143, 162, 163, 173, 316 514 Index dry mix 60–62 F ductile 29, 30, 32, 41, 46, 57, 63, 144, 269, 289, 293, 311, 371 face crush 22, 313 dynamic capacity 162, 173, 314 factor of safety 144, 207, 208, 242, 302, 316, 318, 319, 377, dynamic displacement 319 383 dynamic ground support design 317 failure, brittle 8, 20, 153, 201, 263, 306 dynamic load 141, 302, 315 failure criterion 58, 59, 63, 64, 142, 162, 168, 170, 171, 174, dynamic loading Barton-Bandis 91, 95 175, 226, 232, 301, 302, 315–318, 330, 332, 334, 340 Hoek-Brown 401, 461, 468, 474 59, 323 dynamic resistant surface support failure, crushing 20 171 dynamic system testing failure, depth of 279, 318, 319, 324, 400, 405, 410–413 E failure, flexural 15, 17 failure mechanisms 13, 15, 23, 24, 27, 31, 68, 71, 77, 117, 200, economic risk 377, 379, 385, 386, 389, 400, 401, 407, 417, 207, 213, 214, 232, 237, 258, 260, 269, 274, 278, 324, 330, 421, 424, 433 479 economic risk model 421 fatal incident risk profile 430 economic risk profile 425–427 faults 77, 79, 120, 134, 135, 179, 259, 312, 323, 324, 417, effective normal stress 95 480–482 ejection 20, 22, 32, 162, 302, 313–315, 318, 320, 321, 339 fault slip 21, 134 elastic modelling 240, 322, 400, 403 fault tree 480 elastic modulus 254 feasibility study 118, 179, 183, 216, 225, 228 elastic rock mass properties 242 fibre-reinforced shotcrete 55, 60, 63, 159, 161, 168, 214, 223, elastic superposition 403, 409 228, 229, 275, 292, 295, 298, 314, 332, 333, 341 60, 61, 67 elasto-plastic analysis 237, 243, 249, 403, 409 fine aggregates 22, 313 elasto-plastic modelling 241, 400 first motion Fisher’s constant 185 electrochemical corrosion 347 fissures 90 embedment length 36, 49, 145, 149, 164, 203, 204 flexural bending 214 empirical charting 318, 330–334 flexural failure 215 empirical design 77, 109, 179 floor heave 71 energy absorption 8, 30–32, 63, 64, 148, 158, 162, 164–171, fly ash 63 175, 302, 316, 330–333, 340 focal mechanisms 118 energy demand 45, 174, 302, 316, 317, 319, 320 foliated squeezing ground 281, 283 epoxy coatings 368 foliation 23, 107, 108, 203, 269, 273, 274, 276–286, 297, 327 equipment damage 63, 385 forepole umbrella 275, 276 ESR (excavation support ratio) 227 fracture 22, 69, 81–85, 87, 90, 97, 98, 110, 113, 182, 184, 186, 203 Euler’s formula 211, 212, 262, 264, 270, 274, 286, 311–313, 359, 371, 419, evaluating total risk 395 441 event tree 479, 480, 482, 484–486 fracture analysis 370 EVP (excavation vulnerability potential) 321, 323, 326, 327 fracture frequency 82, 182, 441 excavation span 141, 208, 210, 220, 221, 227, 321, 323, 324, fracture initiation criterion, extension strain 113 335 fractures 20, 21, 77, 79, 82, 84, 101, 104, 113, 118, 125, 132, expandable bolt 51 180, 188, 211, 263, 264, 297, 312 exposure frequency of occurrence 382 mitigation 428, 429 friction angle 92, 95, 242, 250, 260, 419, 420 parameters 429 friction bond 32, 39, 41, 166 extension strain criteria 113 friction plate 51 extreme conditions 156, 232, 274 friction rock stabilisers 39, 145, 166, 293, 359 515 G in situ test drop 173 367 galvanised coating pull 145, 151, 154, 157, 286, 288, 370 204, 222 GCMPs (ground control management plans) instability zones 240, 241, 248 179 geological models internal support pressure 276 182 geomechanical data sampling involuntary risk 383, 392 180 geomechanical model confidence IRS (intact rock strength) 98, 101, 110, 182, 280, 402, 439 geomechanical models 179, 180, 199 geotechnical domains 181, 182, 189–191 J global stress 118 joint G-plate 51 alteration 100, 103, 445 gravity failures 18 alteration number 100, 103, 445 GRC (ground reaction curve) 199, 202 amplitude 105 95 Grimstad and Barton chart 225, 227 compressive strength condition 101, 108 ground conditions 5 continuity 103 ground support density 96 141, 143, 175, 315, 320, 323, 325 ground support capacity fillings 447 231 ground support coverage orientation 77, 78, 83–84, 96, 99, 101, 102, 183–186, 280, ground support subjected to dynamic loading 162, 302, 417, 419 317, 340 profile 103 ground support subjected to static load 143 roughness coefficient 95 ground support survivability chart 341 roughness number 100, 103, 221, 445 groundwater 81, 99, 101, 348, 359, 362, 363 set number 100, 103, 445 grout shear modulus 252 water reduction factor 100, 445 joint separation 99, 284 grout shear stiffness 245, 252, 255 JRC (joint roughness coefficient)95, 104, 242 grout water-cement ratio 152 GSI (Geological Strength Index) 99, 101–103, 110–113, 272, K 284, 401–405, 409–411, 414, 461–463, 468–472, 474, 476, knowledge uncertainty 382 477 H L laboratory static tests 143 hard rock squeezing index 278, 279, 281, 297 large deformations 8 heterogeneous rock 186 lateral shear high stress 3, 5–8, 11, 15, 23, 27, 30, 68, 205, 226, 232, 241, displacement 146 243, 273, 283, 311, 318, 335, 337, 339 load 146 high tensile chainlink mesh 58 loading 147 Hoek-Brown criterion 103, 112, 113 level of confidence 88, 118, 179–185, 189, 191, 192, 196, 378 386 homogeneous rock 101, 186, 249 levels of risk LiDAR 97, 183 horizontal stresses 117, 119, 277, 402 limit equilibrium design 200, 203 hybrid bolts 46 lining stress controllers 289 hydraulic fracturing 117, 118, 125 load-displacement curves 28, 144, 145, 153 hydrogeological model 180 longitudinal deformation profile 202 loss of adhesion 214, 216 I loss of capacity 365, 366, 372, 373 individual safety risk 390, 424, 428 infilling 98, 99, 107 M input energy 162, 168, 171 magnetic particle imaging 355 in situ stress 15, 16, 117, 118, 123, 126, 127, 129, 132, 136, maximum practical support limits 321 137, 271, 401, 402 mesh-reinforced shotcrete 72 516 Index mesh straps 58, 59, 314 PEM (point estimate method) 409–415, 456–458, 460–465, MIC (microbiologically influenced corrosion) 353 467, 470, 475–479 micro silica 63 persistence 77, 104, 106, 107, 183, 440 mine design, stages of 103, 123, 179, 180, 227 personnel failure coincidence 428 minimum number of specimens 188, 190–195 pH 348, 349, 351–353, 359, 361–363, 368 mining-induced deformations 24 photoelastic model 309 mining-induced stress 8 photogrammetry 97, 183 mining projects, stages of 180, 183 pillar burst 21, 22, 313 mobilised energy 331, 332 plate butterfly 51 113, 264 model calibration domed 51 model inputs 242, 253, 255 donut 51 modified cone bolt 44, 336–338 embossed 51 friction 51 modified doorstopper 126, 128 G-plate 51 modified RMR (rock mass rating) 101, 222 push-on 51 modulus point anchor 32–34, 38, 39, 44, 46, 49, 201, 204, 247 elastic 33, 128, 131, 253 point anchor groutable bolt 39 grout shear 242, 253, 255 point load strength test 91 28, 111, 113, 253 of elasticity Poisson’s ratio 88, 113, 128, 242, 250, 255, 260 shear 252 porosity 187 Young’s 88, 89, 148, 242, 245, 250, 255, 260, 286 Posimix bolt 37, 38 Mohr circles 85 post-peak behaviour 19, 30, 32, 41, 43, 46, 57, 311 Mohr-Coulomb criterion 89, 92, 94, 95, 419 ppa (peak particle acceleration) 302 momentum transfer 162 ppv (peak particle velocity) 302–306, 323 453, 456, 466, 472, 477 Monte Carlo method near-field ppv 302–304, 313, 332 83, 85, 101, 220–223 MRMR (modified rock mass rating) precision index 187, 188, 190–195 N prefeasibility study 131 pre-mining stress 17, 122, 128, 129, 134, 138, 249 natural variability 137, 180, 202, 253, 382, 414 pressure arch 200 non-yielding reinforcement 166, 170 prestress 49 non-yielding surface support 171 prestressed cable bolts 49 normal stiffness of defects 251 primary wave 307 normal stress, effective 95 principal stress 5, 17, 20, 90, 92, 113, 118, 122–124, 137, 242, number of sets 78 255, 270, 321, 322, 335, 438 number of specimens, minimum 188, 190–195 probabilistic analyses 380, 403, 404, 409, 477 Nuttli magnitude 303 probabilistic approach 375, 397 O probabilistic stress damage analysis 400 probability of event 377 observational design 199 probability of failure 377–379, 381, 383, 399, 401, 425, 472, 46 Omega-Bolt 477 80 orientation bias protective coatings 345, 364, 365, 367, 368, 373 59, 292, 294, 314 Osro straps pull tests 28, 41, 145, 146, 148, 151, 153, 154, 157, 158, 252, P 286, 288, 361, 362, 370 continuous tube 149 Paddle Bolt 37 split tube 149 parabolic arch concept 204 punching shear failure 214, 215 parting 79 push-on plate 51 passive monitoring 318, 334, 337 PVC-based coatings 368 peak strength 5, 19, 21, 24, 30, 85, 93, 168, 207, 311 P-wave 307 517 Q classifications 103, 106, 107, 110, 111 degradation 345 103, 143, 151 QA/QC program quality 8, 15, 16, 67, 83, 111, 219, 220, 224, 228– 230, 271 100, 103, 104, 106, 108, 109, 223, 225, 227, 271, Q-system rock reinforcement 27, 28, 30, 144, 151, 162, 171, 200, 204, 272, 443, 445, 449 212, 214, 220, 223, 265, 273, 327, 328 R rock strength 29, 77, 98, 101, 131, 132, 182, 280, 401, 402, 439 rock testing program 186 radiation pattern 306–310, 320 roof beam building 200 random variate sampling 453, 455 rope lacing 59, 169 RBS (rock block strength) rating 101 Rosenblueth-Harr 457 RDP (rockburst damage potential) 303, 318, 321, 323, roughness 3, 67, 68, 93–96, 98, 100, 103–105, 221, 242, 325–328, 335 445, 446 RDP (round determinate panel) 64, 143, 158–160 RQD (rock quality designation) 8, 82, 83, 97, 99–101, RDS (rockburst damage scale) 323, 325 103–106, 108, 182, 220, 437–439, 441, 445 reaction pressure 330 RSM (response surface method) 409, 464, 466–471, 474, 476, 307 reflection and refraction of the stress wave 477 rehabilitation scheduling 246 reinforced shotcrete arches 68, 297 S 292 reinforcing squeezing ground safety factor 144, 204, 205, 210, 211, 241 134 representative elementary volume (REV) safety risk 387, 389, 395, 399, 424 residual friction angle 95 safety risk model 424, 429 residual rockbolt capacity 248 sample standard deviation 186, 192 resin anchor 36 scale-distance relationship 302 resin cartridges 36 scaling law 320, 321, 327, 330, 335 resin-grouted cable bolts 49 scanline 78, 96, 97, 183 resin injection systems 36 Schmidt hammer 91, 95 Richter magnitude 22, 303, 313, 320 scoping study 179 RIF (response influence factor) 409, 411–414, 467, 470, seam 79 472–477, 486 seepage 79, 114 risk 21, 22, 214, 301, 303, 305, 307, 309, 313–315, assessment accuracy 384 seismic event category 390 319–321, 328, 330, 332, 334, 335, 345 matrix 387, 388, 416, 424 seismic risk 301 risk-based design 377, 379, 380, 384, 386, 395, 399, 400, 417, self-drilling bolts 47 419 self-drilling friction rock stabiliser 47 risk, involuntary 383, 392 separation of joints 99, 284 risks averaged over the whole population 391 service life 288, 294, 345, 353, 363 risk, voluntary 383, 392 shear RMR (rock mass rating) 15, 83, 97, 99–101, 108, 110–112, modulus 252 182, 220–222, 296, 437, 441 modulus of grout 252 rock arch concept 204 stiffness of grout 245, 252, 255 rockbolt loads 243, 248, 254, 257 test 148 rock brittleness, effect of 311 wave velocity 304 rockburst 9, 15, 19, 21, 22, 29, 30, 32, 46, 49, 59, 100, shear rupture 21, 313 172–175, 199, 226, 232, 291, 299, 301–303, 306, 307, 309, shear strength of discontinuities 29, 93, 94, 419 311–315, 317, 318, 320–323, 325–328, 333–335, 338–341, shotcrete 448, 449 panel testing 160 rock mass pillars 69, 72 characterisation 77, 80, 85, 87, 97, 98, 106–111, 114, thickness 65 179, 196 simulated rockburst 174, 175 518 Index single strand 48 stress path 24 site effect 309–311, 313, 332 stress reduction factor 100, 226, 445 size bias 185 stress relaxation 17, 18, 22 17, 117–120, 122, 129, 221, 239, 400, 402 slabbing 15, 17–20, 226, 449 vertical 17 sliding 3, 8, 18, 33, 41, 45, 95, 166, 207, 210, 247, 270, 293, stress concentration 331, 332, 417 stress corrosion cracking 353 small-scale roughness 103 stress-induced buckling 203 societal safety risk 392, 424 strike 57, 83, 96, 97, 100, 274 soft loading 19 strong ground motion 302–305, 310, 313, 325 solid dynamic bolt 45 structural models 82, 84, 179, 199 source mechanism 22, 313 structure orientation from core 83 source radius 303, 304, 320 superplasticisers 62 spacing 5, 8, 15, 23, 56, 57, 81, 83, 97, 99–101, 110, 141, 158, support characteristic curve 202 183, 203–206, 208, 210–212, 215, 220, 222–224, 240–242, support damage scale 330 245, 249, 280, 282, 286, 296, 297, 323, 419, 420, 441, 446 support of discrete blocks 200 spatial coincidence 427–429 support pressure 141, 202–204, 220, 221, 238, 276 spiling 222, 294, 296, 297 surface exposure mapping 104 squeezing surface fixtures 49 index 271, 278, 279, 281, 282, 297 surface support 3, 4, 27, 49, 52, 55, 57, 59, 60, 68, 69, 73, level 271 74, 98, 141, 142, 154, 156, 162–164, 168–173, 175, 200, mechanisms 270, 284, 287 211–214, 216, 227– 229, 231, 244, 246, 265, 288, 291, 292, squeezing ground 6, 10, 11, 15, 29, 30, 46, 49, 144, 221, 223, 294, 298, 314–317, 323, 327, 330, 331, 339–341, 345, 420 232, 246, 260, 267, 269, 270–274, 277–279, 281–284, 286, survivability chart 341 288, 290–294, 297, 298, 315 suspension 200, 248 stability graph method 109, 227 S-wave 307 stages of mining project development 179 synthetic fibres 64, 66, 159 static loading 70, 166, 170, 171, 179, 301, 318 static stress drop 303–305, 320 T statistical analysis 119, 186, 465 target levels of data confidence 179, 180, 182 steel fibres 64, 65 televiewer 84 steel sensitivity 348 temporal coincidence 427 steel sets 221, 275 tensile strength 41, 63, 64, 73, 90, 157–159, 168, 172, 173, steel straps 58 182, 187, 247, 253, 260, 353, 359, 360, 364, 419, 448 stereographic projections 183 thin spray-on liners 68 stiff loading 19–21, 23, 33, 172 thread bar 34, 36, 37 strainburst 21, 22, 226, 311, 313, 319, 449 three-dimensional wedge analysis 208, 214 straps tilt tests 95 mesh 58, 59, 314 total core recovery 82 59, 292, 294, 314 Osro total dissolved solids 348 steel 58 total energy absorption 165–171, 316 W-strap 58 total strain 281 strength test toughness 63, 64, 158–160 Brazilian test 182, 401 trace lengths 96, 185, 186, 417 triaxial compression test 88 transportation risks 391 stress 88 horizontal stress 118, 120, 125 triaxial compressive strength stress measurement 117–119, 122, 123, 125, 126, 131, 134, truncation bias 185 136, 137 tunnel strain 271, 273 stress memory techniques 129, 131 two-dimensional wedge analysis 207, 210, 214 519 U Y ubiquitous joint 249 yielding UCS (uniaxial compressive strength) 5, 8, 85, 87–93, 99, 129, anchors 44 148, 182, 183, 186–188, 190, 191, 240, 242, 245, 250, bolts 43, 44, 166, 232, 339 252, 255, 277–279, 318, 322, 335, 337, 338, 401, 403–405, reinforcement 43–46, 166, 170, 171, 293, 335, 337 409–411, 437, 438, 461–463, 471, 472, 474, 476 yielding surface support 171 umbrella arch method 294 Yield Lok bolt 45 uncertainty, types of 137, 179 yield zone 241, 254, 255, 273 underground drop test 173 Young’s modulus 88, 89, 148, 242, 245, 250, 255, 260, 286 uniform or semi-uniform corrosion 353 unravelling 16 Z 5, 248 V zone of instability variability 80, 92, 121, 128, 136–138, 144, 153, 167, 175, 179–186, 188, 202, 216, 227, 249, 253, 333, 367, 377, 382, 383, 399, 400–402, 414 verification of code 264 vertical stresses 117, 119, 120, 129, 206 vibrating wire borehole stressmeter 137 violent failure 15, 21, 311, 319 visual monitoring of corroded support 370 volumetric joint count 105, 106 voluntary risk 383, 392 W wall beam building 200 wall strength 78 water-cement ratio 149, 152 water conductivity 353 weathering 78, 80, 95, 101, 114, 182, 188 welded wire mesh 56 weldmesh 56–59, 63, 156, 158–161, 168, 169, 253, 291, 364, 365, 419 wet mix 60–62, 64, 65 wheel of design 381 Wiggle Bolt 37 workforce exposure 301 working capacity 27, 144, 148, 153–157 World Stress Map 118, 119 W-strap 58

520