Waihi Underground Mine Report - August 2002

(Click on the bold headings below to view the report in sections)
EXECUTIVE SUMMARY (63.4KB) 1
1 INTRODUCTION (61.4KB) 10
1.1 Brief history of mining at Martha Hill, Waihi 10
1.2 Historical subsidences and collapses 10
1.3 Study objectives 13
1.4 Limitations of the study 14
1.5 Acknowledgements 15
2 VERIFICATION OF DATA AND CALCULATION OF STOPE BLOCK VOLUMES (73.6KB) 16
2.1 Historical data review 16
2.2 Filled and unfilled stopes and stope volume 16
3 ROCK MASS CHARACTERISTICS (97.8KB) 20
3.1 Classification of materials 20
3.2 Material properties 21
3.3 Rock mass classification 22
3.4 Defect orientations 23
3.5 In situ stresses 24
4 STOPE STABILITY ASSESSMENT (72.1KB) 25
4.1 Modified Mathews methods 25
4.1.1 The Stability Graph method (Hoek et al. 1995, Potvin et al. 1989) 25
4.1.2 Mining Rock Mass Rating (MRMR) stability assessment (Laubscher 1993) 26
4.2 Migration of voids 26
4.3 Crown pillar stability 26
4.4 Hanging wall collapse 27
5 BACK ANALYSIS OF PREVIOUS FAILURES (90.3KB) 27
5.1 Mathews method 27
5.1.1 Stability graph method 27
5.1.2 MRMR method 28
5.1.3 Comments 28
5.2 Factors effecting void Migration 29
5.2.1 Bulking Factors in Rock 29
5.2.2 Void migration 30
5.3 Crown pillar stability 32
5.4 Failure of welded ignimbrite layer 32
5.5 Hanging wall collapse 33
5.6 Conclusions on back analysis and failure mechanisms 33
6 ANALYSIS OF ROYAL, EMPIRE AND EDWARD STOPES (95.3KB) 35
6.1 Procedure for modelling stopes in three dimensions and estimating stope volume 38
6.2 Back – analysis of the Royal Lode collapses 39
6.2.1 The 1961 and 1999 subsidence craters 39
6.2.2 The 2001 subsidence crater 40
7 SHAFTS (46.6KB) 42
7.1 Subsidence hazard from abandoned shafts 42
7.2 Remedial action 44
8 VIBRATIONS ASSESSMENT (40.1KB) 44
9 SETTLEMENT AND GROUNDWATER MONITORING (51.2KB) 45
9.1 Groundwater levels 46
9.2 Surface settlement 47
10 PROBABILISTIC HAZARD AND RISK ASSESSMENT (109.6KB) 49
10.1 Probabilistic hazard zones for stopes 50
10.2 Risk assessment 52
10.2.1 Probability of a collapse at Waihi 52
10.2.2 Risk assessment review by Tony Taig 54
10.2.3 Properties and people at risk 54
10.2.4 Comparison with the Individual and Societal Risk criteria 58
10.2.5 Collapse of Shafts 61
10.2.6 Combining probabilities 61
11 MITIGATION MEASURES (57.4KB) 62
11.1 Royal and Empire lodes 62
11.2 Edward South Lode 62
11.3 Mitigation Measures and Costs 63
12 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS (60.2KB) 65
12.1 Summary and Conclusions 65
12.2 Recommendations for Further work 67
13 REFERENCES (47.5KB) 68
14 GLOSSARY OF TERMS USED (46.2KB) 72
15 APPENDIX 1 GNS CLIENT REPORT 2002-21 (5.3MB) 75
16 APPENDIX 2 REVIEW COMMENTS FROM TONY TAIG (103.6KB) 76