After rainfall, a deep open pit in Western Australia experienced a large-scale instability which hampered access to the pit and restricted mining activity. At the client’s request, SRK undertook a site visit and developed a work program to provide analytical and operational support for resolving the stability issues. SRK’s team, compromising Daniel Prado and Ian de Bruyn, performed 2D and 3D numerical stability analyses for back-analysis of the existing instability to understand the mechanism of failure and the critical elements affecting the performance of the slope and to calibrate a model to analyse options for further mining. A staged work methodology was fundamental to rapidly identify recommendations for risk mitigation and continuance of mining.
The first stage involved careful reassessment and understanding of the 3D structural model, followed by RS2 (Rocscience software) 2D finite element analyses to recreate the failure at strength reduction factor at or marginally below 1. The likely failure mechanism was confirmed by varying the strength parameters of the pit wall materials and fault structures that were thought to be contributing to the failure. Once the back-analysis was deemed representative of the expected conditions on site and the failure mechanism reproduced, further mining options were considered. In agreement with the client, construction of an intact rock buttress at the toe of the instability was identified as the most suitable solution for mitigation of risk and to allow for continuation of mining.
FLAC3D (Itasca software) finite difference stability analyses were completed to confirm the model inputs from the 2D back-analysis and to further assess the complex failure mechanism in 3D. Appropriate measures to allow safe mining down to the proposed final pit depth were evaluated, considering the location and dimensions of the intact rock buttress and the expected position of the phreatic surface. The recommendations provided by SRK in terms of slope angle, buttress width and extension were included in the client’s operational plan, and the mining of the pit was successfully achieved.
