An engineering geological investigation into pit slope stability at Macraes Gold Mine, Macraes Flat, Otago, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Macraes Gold Mine at Macraes Flat is located approximately 60km north-west of Dunedin, and is currently the largest producer of gold in New Zealand. Open-pit mining currently takes place in the three large pits of Round Hill, Southern and Innes Mills along the Hyde-Macraes Shear Zone (HMSZ) which is the source of gold mineralisation. This study is an engineering geological investigation into pit slope stability at Macraes. Pit slope stability is an integral part of open-pit mining since slopes should be as steep as possible to minimise waste material which needs to be removed, yet shallow enough to minimise potential hazards to personnel and equipment below pit slopes. Joints at Macraes are characterised by high friction angles (45-55°) as a result of roughness/waviness along the joint surface at low normal stresses. Geotechnical testing of fault gouge, and back analysis of fault controlled failures, shows much lower shear strengths for faults (c=0 kPa, Ø =6-14°) than those previously assumed for the mine (c.f. c=10-14 kPa, Ø=15-17°), with the small discrepancies (≈8°) between laboratory test results (5°) and back analysis results (13°) being attributed to both the removal of coarser material from laboratory samples and surface roughness/waviness, which would otherwise increase the friction. X-ray diffraction analysis shows a dominance of interlayered swelling chlorite/smectite in clay fraction of fault gouge which will heave when water is present increasing the instability of pit slopes. Intact rock strength testing on schist from the mine shows much lower strengths (4.8 - 61.2 MPa) for the material than previously determined at other project locations in Otago (c.f. 29 - 86MPa). Pelitic schist at the mine appears to be stronger than psammitic schist which was unexpected and contrasts with previous testing in Otago. The higher strengths of pelitic schist may reflect annealing processes associated with mineralisation of the HMSZ, but has not been fully investigated. Two structural domains can be recognised at the mine which are controlled by the Hanging Wall Shear (HWS) of the HMSZ. The rock mass above the HWS is dominantly psammitic and is referred to as the Hanging Wall Zone Domain (HWZD), while the rock mass below the HWS is dominantly pelitic and referred to as the Ore Zone Domain (OZD). Three joint sets and five fault sets are recognised in the HWZD, while in the OZD the same three joint sets as for the HWZD are recognised, but only four of the fault sets are present. A standard recording sheet for pit slope failures is developed to assist engineering geological investigations by recording the relevant information in a consistent format. Such a sheet forms the basis for a pit slope failure database, ensures consistency in recording and provides records of failure for assessments of failure development with time. Failures at Macraes are classified according to the geometry of the failure surface and the types of discontinuities controlling failure as: high angle planar, low angle block, toppling, joint-joint wedge, joint-fault wedge, and fault-fault wedge. High groundwater pressures, and lower shear strength material than previously assumed, are interpreted as major factors in driving pit slope failures at Macraes, and horizontal drainage is recommended as the most effective method in preventing future pit slope failures. Two large pit slope failures, RH27 (volume = 900m³) & RH28 (volume = 450 000m³), located on the north wall of Round Hill Pit occurring prior to this project, are mapped in detail at scales of 1:500 and 1:800 with failure mechanisms interpreted. RH27 is a complex wedge failure formed between the Slip 27 fault and a westerly dipping joint set. RH28 is a large wedge failure formed between two faults plunging at low angles (≈3°) out of the pit slope and driven by high water pressures along a fault set parallel to the pit slope. The stability of RH27 and RH28 is determined, and future development and implications to mining operations assessed. Thirteen types of failure are predicted to occur at Macraes based upon the integration of hydrological influences, strength properties, rock mass structure, and calibration with those failures observed at Macraes to date. Kinematic models for these thirteen failure models have been constructed to assist in recognition of these predicted failure modes in the field. Prediction of pit slope failures should be based on sound monitoring and management of recognised active failures, and should include: visual inspections and photography, movement monitoring, hydrologic monitoring and warning monitoring, so that the hazards and adverse effects associated with unexpected slope failure movements are minimised.