Slide 1: Ameliorating the strainburst risk in a mechanised deep level gold mine: the South Deep experience
Ameliorating the strainburst risk in a mechanised deep level gold mine: the South Deep experience PETER ANDREWS Co- Author Fhatuwani Sengani Vice President and Group Head of Geotechnical Graduate Rock Engineer South Deep Mine
Slide 2: Forward looking statements
Forward looking statements Certain statements in this document constitute “forward looking statements” within the meaning of Section 27A of the US Securities Act of 1933 and Section 21E of the US Securities Exchange Act of 1934. In particular, the forward looking statements in this document include
Slide 3: Outline
Outline Contents ● Mine Location, ● Geology ● Destress Mining Method ̵ Adaptations over time, ● Areas prone to Strainburst Risk, ● Strainburst Risk reduction, ̵ Face pre-conditioning ̵ Design, data collection, and quality assurance ̵ Yield Pillars ̵ Design, data collection, and quality assurance ● O
Slide 4: South Deep Mine Location
South Deep Mine Location ● Located ~70km south-west of Johannesburg in the Witwatersrand basin, ● Deep level mine 2600m to 3400m below surface (Currently active 2600m-3000m) ● 37.3Moz Reserve, ● Mine life on reserve to 2095 (78 years) • Horizontal capital development requirements • Total – 102.2 km
Slide 5: Local Geology
Geology Local Geology ● South Deep is located in the Far West Rand Goldfield on the north- western rim of the Witwatersrand Basin. ● Lithology above the mining area consists of the Pretoria Group sandstones, overlying the Malmani Dolomite, and the Ventorsdorp lavas of which the Ventorsdorp Contact R
Slide 6: Geology Cross-section
Geology Cross-section • The thickness of the Elsburg reefs range from 5m thick in the west to 120m at the east. • Depending on the grade distribution within the separate units, corridors can target different reef packages Ameliorating the strainburst risk in a mechanised deep level gold mine: the So
Slide 7: Mining Method: Destress Concept
Mining Method Destress Concept ● The mining method at South Deep uses the destress concept which relies on creating a zone of lower stress above and below the destress slot to allow LH stoping to occur in a reduced stress window. ● The window is actually smaller and the stress reduction is less than
Slide 8: Low Profile Destress (LPS)
Mining Method Low Profile Destress (LPS) ● There have been several destress layouts over time. ● The original corridor mining span was 240m wide, with 4 corridors, ● Low profile development with dimensions of 2.2mH x 5.0mW ● Crush pillars were 2mW x 10m long and were confined with backfill bags ● Su
Slide 9: Low Profile Destress (LPS)
Mining Method Low Profile Destress (LPS) ● There was a 17m vertical distance between destress cuts with a 25m overlap ● Each successive cut has to be developed into a high stress abutment ● Once the upper destress cut and 50% of the lower cut was complete long hole stoping was to begin ● Stoping was
Slide 10: High Profile Destress (HPS) Yield pillars
Mining Method High Profile Destress (HPS) Yield pillars ● The new corridor mining span is 180m wide with 6 corridors, which has shown to reduce convergence when combined with the yield pillars ● High profile development with a profile of 5.5mH x 5.0mW ● Yield pillars are designed at 8mW x 20m long ●
Slide 11: Strainburst Risk Prone Areas
Strainburst Risk Strainburst Prone Areas Beck Eng 2016 ● Potential strainburst risks areas were highlighted to be at the high stress faces and also within yield pillars if yielding did not fully occur, ● These locations are based on observations, historical data and confirmed by numerical models, ●
Slide 12: Strainburst Risk Reduction Method: Face Preconditioning
Strainburst Risk Reduction Method: Face Preconditioning ● External consultants Detonation were used of the production andtofour provide face- guidance for pre-con perpendicular trial pre-conditioning holes were sequenced withand1 fivemillisecond ● Two trials (four delays face-perpendicular chronolog
Slide 13: Strainburst Risk Reduction Method: Face Preconditioning
Strainburst Risk Reduction Method: Face Preconditioning ● All five face-perpendicular Detonation of the production pre- and face- conditioning pre-conditioning perpendicular holes were drilledholes at a were minimum of sequenced at 1.5m more than 1 millisecond the as delays production drill lengths.
Slide 14: Strainburst Risk Reduction Method: Face Preconditioning
Strainburst Risk Reduction Method: Face Preconditioning Ameliorating the strainburst risk in a mechanised deep level gold mine: the South Deep experience |Peter Andrews|10th October 2017 14
Slide 15: Method: Face Preconditioning
Strainburst Risk Reduction Method: Face Preconditioning ● During investigations between 4 hole the 4 hole and 5 hole trials the 11.2cm following attributes were investigated: ● Hole depths and diameter post firing, ̵ There was less hole length after blasting in the 5 hole pattern indicating better m
Slide 16: Method: Face Preconditioning
Strainburst Risk Reduction Method: Face Preconditioning ● Rockmass fracturing: ̵ More intense fracturing in the 5 hole pattern than the 4 hole pattern ● Hangingwall profiles: ̵ The data obtained from the hanging wall indicated a smoother hanging wall using the 5 hole pattern and intermediate to mino
Slide 17: Method: Face Preconditioning
Strainburst Risk Reduction Method: Face Preconditioning ● Face advance: ̵ The 5 hole pattern showed better face advance than the 4 hole pattern: ̵ The higher intensity of face fracturing resulted in easier scaling and barring down providing a higher advance rate ̵ 5 hole advance rate was generally 0
Slide 18: Quantification: BH Camera
Strainburst Risk Reduction Quantification: BH Camera ● After preconditioning holes were fired, several boreholes were drilled into the face to determine how well preconditioning worked. ● The amount of fractures per meter are counted and a borehole fracture log is created for each borehole viewed, ●
Slide 19: Method: Face Preconditioning
Strainburst Risk Reduction Method: Face Preconditioning The borehole fracture frequency data 4 hole pattern was used to determine the face burst risk per meter for each blast hole 4 hole pattern results ● 48.6% very low risk, ● 23.0% low risk, ● 23% Medium risk, ● 5.4% high risk 5 hole pattern resul
Slide 20: Methods: Ground Penetrating Radar (GPR)
Strainburst Risk Reduction Methods: Ground Penetrating Radar (GPR) ● A decision was made to reduce the number of people from the face during quantification of preconditioning effectiveness ● South Deep use the Reutech Sub- Surface Profiler (SSP) to investigate the effectiveness of preconditioning th
Slide 21: Quantification: Ground Penetrating Radar (GPR)
Strainburst Risk Reduction Quantification: Ground Penetrating Radar (GPR) ● GPR images from four pattern produced weak reflections within 1m to 3m ahead the face, which indicate that there were less changes in material properties and least number of discontinuities ● Beyond that, the rock mass ahead
Slide 22: Methods: Ground Penetrating Radar (GPR)
Strainburst Risk Reduction Methods: Ground Penetrating Radar (GPR) ● GPR images from the five face-perpendicular pre-conditioning practice produced a significant difference in the nature of fracturing ahead of pre- conditioned faces as compared to four face-perpendicular pre- conditioning practice ●
Slide 23: Methods: Ground Penetrating Radar (GPR)
Strainburst Risk Reduction Methods: Ground Penetrating Radar (GPR) ● More recent results of GPR Pre-con holes Zone of intense fracturing around holes Ameliorating the strainburst risk in a mechanised deep level gold mine: the South Deep experience |Peter Andrews|10th October 2017 23
Slide 24: Results: Face Preconditioning
Strainburst Risk Reduction Results: Face Preconditioning ● South Deep now only use the 5 hole pattern for pre-conditioning for the following reasons: ̵ It produces deep fracturing away from the face rather than in other areas and reduces risk of face bursting. ̵ This transfers stresses away from the
Slide 25: Rib Pillars with Borehole Camera
Strainburst Risk Reduction Rib Pillars with Borehole Camera ● Since site changed to HPS, an observational method based on numerical modelling results has been used to ensure pillars that will crush or yield. ● To get visual proof that pillars are yielding SD have been using a borehole camera for ~18
Slide 26: Methods: Borehole camera
Strainburst Risk Reduction Methods: Borehole camera • Snapshots taken from the borehole videos of regular HPS pillars to compare: Ameliorating the strainburst risk in a mechanised deep level gold mine: the South Deep experience |Peter Andrews|10th October 2017 26
Slide 27: Quantification: Borehole camera
Strainburst Risk Reduction Quantification: Borehole camera ● The amount of fractures per meter are counted and a borehole fracture log is created for each borehole viewed, ● Logs indicate other fractures, borehole breakout, hole closure, etc. ● The amount of fractures per meter are counted and the f
Slide 28: Method: Borehole camera
Strainburst Risk Reduction Method: Borehole camera Pillar Burst Risk Rating ● To date borehole camera Medium Strain Burst Risk High Strain Burst Risk (5 - 10 Fractures/metre) 0.4% <5 Fractures/metre) monitoring has been successful 8.1% Low Strain Burst Risk12.4% with pillars found to fracture (10 -
Slide 29: Quantification: Ground Penetrating Radar
Strainburst Risk Reduction Quantification: Ground Penetrating Radar ● Eight boreholes were drilled around the pillar with the purpose of analyzing fracture frequency ● Boreholes were at 1.5m above floor and indicated the pillar was fractured throughout North wall ● GPR scanner was then conducted in
Slide 30: Quantification: Ground Penetrating Radar
Strainburst Risk Reduction Quantification: Ground Penetrating Radar ● GPR scan of pillar shows at least 3m of intense fracturing into the pillar along the long axis of the pillar and 2m in from each end. ● It also indicates that the pillar core is not fractured which is different from the borehole c
Slide 31: Theory: GPR Pillar Section
Strainburst Risk Reduction Theory: GPR Pillar Section ● The scenario below is believed to be what is happening within the pillar. ● The solid sections at the top and base of the pillar have been seen before with a fractured core. Ameliorating the strainburst risk in a mechanised deep level gold mine
Slide 32: Ameliorating Strainburst Risk: Results
Ameliorating Strainburst Risk Results ● The design changes undertaken at South Deep from LPS with crush pillars to HPS with yielding pillars have been effective in providing a safer working environment by allowing effective mechanized support installation. ● With the use of borehole monitoring withi
Slide 33: Ameliorating Strainburst Risk: Conclusions
Ameliorating Strainburst Risk Conclusions ● Mining at deep levels or in areas of high stress will always expose people/equipment to areas of potential strainbursting. ● South Deep shown that through a combination of techniques that strainburst risk can be reduced. The techniques employed at South De
Slide 34: The End
The End Thank you to the following ● Gold Fields’ South Deep Mine management ● Fhatuwani Sengani. The majority of the work outlined today will be used in his Masters Project ● South Deep Rock Engineering department for data collection Ameliorating the strainburst risk in a mechanised deep level gold