Quantifying Rockfall and Rockburst Risk in Underground Mines William Joughin

Objectives Realistically quantify the rockfall and rockburst risk under a given set of conditions from a safety and economic perspective – Enable the comparison of support systems, mining layouts, safety strategies – Assess the effects of quality of support installation and support strength variability

Design philosophies Deterministic (Capacity/Demand) – Factor of safety 1.5 – 2.0 for high consequence Variability not considered – Limit states design 95% fall out height Civil engineering – reinforced concrete code – Ultimate limit state and Serviceability limit state – Partial safety factors for load and material properties based on 5 percentile (1-95%) Variability partially, but not all cases are considered Probabilistic – Monte Carlo simulation to determine Probability of Failure All cases considered, load and material variability considered What is an acceptable probability of failure???? – Risk Evaluation!! Incorporates all of the above Accepted risk criteria based upon safety and economics

Risk Evaluation Process Keyblock Stope Collapses Rockburst Seismically induced rockfall Injury to personnel Damage to equipment Expected fatalities Evaluate against accepted injury risk level Loss of production Excavation damage Pillar failures Expected economic loss Human Resources Public Relations Unforeseen rock mass behaviour Rockfalls of varying size Industrial Action Stakeholder resistance Loss of reserves Evaluate loss of revenue against cost of improved risk control Evaluate effect on NPV Accepted level of risk – based on above? Fault tree to determine the potential for rockfalls Event tree to determine the risksAccepted risk levels

Overview Accepted level of risk? Rockfall analysis (keyblock analysis –Jblock) Injury analysis Economic Evaluation Rockburst analysis (empirical analysis)

Accepted level of risk Zero tolerance, zero fatalities, zero injuries, zero harm How do you get to zero, when people are exposed to hazards? How do you design for zero risk? Need to set realistic, measureable targets. Injuries and Fatalities

Accepted level of risk International Benchmarking against all industries – Annual Probability of injury / fatality Ongoing Improvement Expected frequency of injuries/fatalities Expected frequency of incidents with one or fatalities Individual injury/fatality risk DIFR / FIFR 2013 Benchmark – all accidents Injuries and Fatalities

Accepted level of risk Injuries and Fatalities – International benchmarking F-N Graph

Accepted level of risk Injuries and Fatalities F-N Graph International Criteria

Accepted level of risk Injuries and Fatalities

Accepted level of risk Injuries and Fatalities

Accepted level of risk Cost HighLow High Low Rockfall/Rockburst Hazard Economic evaluation

Accepted level of risk Cost HighLow High Low Expected cost =  (Cost of expected incidents) Rockfall/Rockburst Hazard Economic evaluation

Accepted level of risk Rockfall/Rockburst Hazard Cost HighLow High Low Expected cost =  (cost of expected incidents) Economic evaluations for given scenarios Economic evaluation

Accepted level of risk Compare different scenarios Annual costs – Cost of risk control + – Expected loss Discounted cashflow – Capital outlay (at start of implementation) – Annual costs – Reserve loss (at end of life of mine) – Net Present Cost (NPC) Economic evaluation

Rockfall analysis - JBlock Not just probability of failure of a single event Frequency of large and small rockfalls (size counts) Represent as many failure modes as possible Represent a realistic rock mass with variable discontinuity characteristics Represent realistic support patterns with installation and strength variability

Rockfall analysis - JBlock Simulation area Represents area mined Primary keyblock Secondary keyblock Unformed block Joint traces on oriented stope hangingwall Represents a ground control district / geotechnical domain 3D keyblocks

Rockfall analysis - JBlock Joint dip & dip direction Joint shear strength Joint length and spacing Joint strength Faults (additional joint set) Other features – Parting planes – Ramp domes - bushveld thrusts – Circular domes – eg pillow lavas, cross-bedding (separate entities) – Stress Fractures? – Blast fractures? Simulation area Primary keyblock Secondary keyblock Unformed block Joint characteristics (variability)

Rockfall analysis - JBlock Usually > 10,000 keyblocks or reasonable simulation area Simulation area required to generate blocks is recorded to enable normalisation of results to area mined per annum Represents a ground control district / geotechnical domain The exact same block set can be re-used for support comparisons Simulation area Primary keyblock Secondary keyblock Unformed block Set of blocks for stability analysis Individual blocks are generated!!!!

Rockfall analysis - JBlock Stope dip (0  - 90  )and dip direction Outline Area of interest Stope area

Rockfall analysis - JBlock Types of support – Point support (props, tendons) – Line support (headboards) – Area support (packs) – Membrane support (shotcrete, mesh, TSL backfill?) Very simple strength estimates Variability – Spacing & out of line) – Strength (either installation or unit strength) Support pattern

Rockfall analysis - JBlock Test each block individually (no unravelling effect) Failure (rockfall) – In between support – Failure of support – Block rotation Unravelling is not modelled!!!! Keyblock stability

Rockfall frequency - JBlock analysis Set of failed keyblocks (rockfalls) Simulation area to be normalised per annum Keyblock characteristics – Area – Volume – Height – Failure mode – Face length affected Output

Rockfall analysis - Data calibration Lonmin & Impala data 3 years Represents 1000 crews 810 Rockfalls 0.5m 3 to 4000m 3

Rockfall analysis - Data calibration Small rockfalls? – more rockfall injuries than rockfalls Length, width and height Errors in database Unknown variables – support, mechanism, ground control district etc Face or Back area – time dependency? Falls after blast, removed by barring?

Rockfall analysis - Data calibration

Injury Analysis Injury Event Tree

Injury Analysis Time Exposure Worker category Areas where people spend time Stope face Gullies Dev end Access tunnels Stope driller 6.0 h 0.5 h 1.5 h Stope team 4.5 h 2.0 h 1.5 h Miner 3.0 h 2.0 h 3.0 h Dev driller 0.0 h 6.5 h 1.5 h Shift boss 1.5 h 2.0 h E = (hours/day x days at work per annum)/(365 x 24 hours)

Injury Analysis Task Max no of Persons Exposure (Hours/day) Exposure (Hours/ annum) Protected (hours/day) Unprotected (hours/day) Slot Development Bogging Shotcrete Bolting Drilling Slot Production Drilling Bogging Face Prep Charging Western Decline Inspection Exposure analysis example

Injury Analysis Spatial Coincidence Probability of coincidence C = Rockfall Area/ Exposure Area Small rockfalls – Low probability – High annual frequency Large rockfalls – High probability – Low frequency Individual injury frequency Ind= E x  (C i ) for each rockfall Total Expected injuries Inj = E x N x  (C i ) for each rockfall Expected incidents (multiple injuries) Binomial distribution Fatal injuries Factor (accident data) Number of people exposed (N)

F-N Graph output

Economic Evaluation Equipment Damage Event Tree

Economic Evaluation Stope Damage Event Tree

Dilution cost Re-supporting cost Area to be re-supported Rockfall Economic Evaluation - Small Rockfalls Production loss (Cleaning up and re-supporting)

Reserve loss (NPV) Production loss (during re- establishing) Area of Sweepings Lost Rockfall Economic Evaluation - Small Rockfalls Pillar Sweepings loss

Economic Evaluation - Small Rockfalls

Economic Evaluation - All Rockfalls

Rockburst analysis Cumulative Frequency – Magnitude Distribution

Rockburst Analysis Calibration for forward modelling Seismic dataElastic Modelling

Rockburst Analysis Calibration for forward modelling a value Modelled Energy

Rockburst Analysis Calibration for forward modelling Constant b value

Rockburst Analysis Time of day analysis

Rockburst Analysis Potential for Rockburst Damage ?

PPV Scaling law StopeDistance Source Stope Seismic Data Source Modelling Location accuracy??

Rockburst Analysis Heal, Potvin & Hudyma, Australian & Canadian mines 83 case histories, 254 damage locations Excavation vulnerability potential (EVP)

Excavation Vulnerability Potential Stress to strength ratio (E1), Support capability (E2), Excavation span (E3), and Geological factor (E4)

Rockburst Analysis E1 (Stress to Strength ratio)

Rockburst Analysis Rockburst sites and Blast experiments Yielding and containment E2 (Support capability) E

Rockburst Analysis E3 = excavation span (m) Geological factors (E4): – Seismically active major structure: 0.5 – Unfavourable rock mass / no major structure:1.0 – Massive rock mass / no major structure:1.5 E3 (excavation span) & E4 (Geological factor)

Rockburst Analysis Rockburst Damage Scale Rockburst DamageApproximate Area R1No damage, minor loose0 R2Minor damage, less than 1 tonne displaced 0.5 m 2 R31 – 10 tonnes displaced5 m 2 R410 – 100 tonnes displaced20 m 2 R5100+ tonnes displaced150 m 2 Rockburst Damage Scale

Rockburst Analysis Rockburst Damage Potential

Rockburst Analysis Injuries and Damage R2 R3 R4

Rockburst Analysis Injuries and Damage R5

Rockburst Analysis Damage comparison example

Rockburst Analysis Rockburst injury analysis example

Conclusions A risk evaluation model has been developed to quantify rockfall and rockburst risk It enables quantification of safety (injury and fatality) and economic risk It enables the comparison between different support systems or safety strategies Rockfall risk is quantified using statistical keyblock anlaysis Rockfall risk is quantified using statistical and empirical methods Data is required for calibration

Acknowledgements Mine Health and Safety Council (MHSC) The management of Lonmin and Impala Platinum are thanked for providing rockfall data for this research South Deep & Telfer (Rockburst Risk) Lawrence Rwodzi – economic analysis Roger Stewart – Risk and economic evaluation software Essie Esterhuizen – JBlock upgrades Jody Thompson, Tony Jager, Dave Roberts, Johan Wesseloo, Dick Stacey, Luis-Fernando Contreras, Graham Howell, and Oscar Steffen.