Engineering Geological Characterisation and Slope Stability Assessment of Whitehall Quarry, Waikato.
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Whitehall Quarry is located 4 km east of Karapiro, near Cambridge within the Waikato District. Current quarrying operations produce between 150,000 and 300,000 tonnes of aggregate for use in the surrounding region. This study is an investigation into the engineering geological model for the quarry and pit slope stability assessment. Pit slope stability is an integral aspect of quarrying and 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. This study also assesses the stability of complex wedge located within the north western corner of the quarry. Initial estimates approximate a wedge mass volume of 500,000 m3; failure was triggered during the late 80‟s due a stripping programme at the head of the mass. Field and laboratory investigations were carried out to identify and quantify engineering geological parameters. Photogrammetric and conventional scanline analytical techniques identified two domains within the quarry divided by the Main Quarry Shear Zone (MQSZ). Discontinuity orientations are the key differences between the two domains. Bedding planes appear to have slightly different orientations and each domain has very different joint sets identified. Point load, shear box, ring shear testing approximated intact rock strength, shear strength and fault gouge behaviour properties. The main geological units at the quarry are greywacke sandstone interbedded with argillaceous mudstone. Uniaxial compressive strength estimates approximated the intact rock strength for sandstone to be between 30 and 230 MPa depending on weathering grade, while the strength of mudstone was approximated at 5 MPa. Residual shear strength was carried out to estimate the approximate effective angle of internal friction for both a smooth UW-SW sandstone joint and a mudstone bedding plane, these being 37o and 34o respectively. Laboratory testing on fault gouge indicated an approximate angle of internal friction of 13o and the presence of kaolinte and montmorillonite was identified. Pit slope stability analysis utilised the Markland test for identification of potential failures within slopes. Wedge failures are the most common potential failure type, then planar failures along bedding planes. Potential failures are most likely to be less than 10 m3 due to the high fracture frequency and low persistence of common joint sets. However, larger failures in the order of 100 m3 are possible along fault and bedding planes where persistence is typically greater than 20 m. Kinematic analysis of the Northern Wedge Failure estimated a mean factor of safety of 0.97. Currently the wedge is assessed as marginally unstable. Electronic distance measurement over 11 months recorded an approximate mean wedge velocity of 19 mm/month. Sensitivity analysis identified pore water pressure as a key parameter. De-watering the wedge via a series of inclined drainage holes appears to be the best mitigation method. However, the mine and monitor approach is also acceptable but with anticipated risk to personnel and equipment. Monitoring instrumentation such as a wireline and crack-meters should be implemented.