Slope stability considerations of a major excavation in rock: the Mt Newman case study

This thesis is concerned with the slope stability considerations of the North Wall at the Mt N e w m a n open pit mine in Western Australia. This is the largest open pit iron ore mine in the world and the design of the North Wall presented an immense challenge. The results of detailed investigations and analyses into the geological and geotechnical factors which influence large-scale pit wall stability and subsequent design are presented in this thesis. Based on conventional mapping and drilling the geology has been studied including a detailed consideration of structural geology. Moreover, engineering geological aspects were considered comprehensively. Particular attention was given to the implications of geology, with three-dimensional consideration of geological structure, in relation to the geometry of the North Wall. These detailed considerations are synthesised into a clear picture of how the geological structure and engineering geological characteristics will influence slope stability considerations which are most significant for decisions in respect of mine design. These include potential failure mechanisms as well as factor of safety for bench-scale and overall stability. Innovative techniques have been used successfully to assist in providing a clear definition of the geological and geotechnical picture at Mt Newman. These include cross hole seismic tomographic imaging to define geological structure on a large scale, detailed three-dimensional computer graphics modelling in order to determine the shape and plunge of potentially unstable structures, and collection of all borehole geophysical data including gamma sonic and neutron logs with assay data in order to verify lithological interpretations. This investigation has also been concerned with a critical appraisal of previous geotechnical work at Mt Newman and, in particular, the shear strength test results. On the basis of this appraisal a new comprehensive testing programme was developed in order to define the shear strength of the rock masses and especially of the sheared and disturbed Jeerinah Shales and Fault Shales. The shear strength testing programme was developed on the basis of modern concepts concerning the behaviour of rock masses. Reliability of data was considered before finalising shear strength parameters for different types of rock mass. Failures which have occurred at Mt Newman were studied and appropriate slope stability analyses conducted. From these studies back analysed values of shear strength parameters were obtained and compared to corresponding values from laboratory measurements. Stability analyses for the North Wall were made using several limit equilibrium models considering slip surfaces of circular and non-circular shape. Before considering the potential for major overall failures in this way, potential failure mechanisms were considered on the basis of structural geology and engineering geological parameters. More importantly, sections were chosen for analysis in the following different ways: (a) conventional north-south sections, (b) sections normal to the pit wall, and (c) sections in the maximum plunge direction. Consideration was given to shear strength anisotropy wherever appropriate. Stability analyses were also made considering potential bench-scale instability using planar failure and wedge failure models. Having assessed the relative importance of various factors and the interaction of geological and geotechnical engineering parameters, consideration was given to the economies of several design options for the North Wall. An assessment of the alternatives indicated that two options were significantly better than others from the point of view of stability and economics. These two options have therefore been combined together to formulate the extraction sequence for the North Wall in the short term. In the longer term, the decision process involved in making the final choice of pit wall design will be based primarily, although not exclusively, on the results of an actual field trial involving the North Wall excavation. (Note that no failures in the Jeerinah Formation have, as yet, occurred.) A cautious approach to the mine design and extraction sequence has been recommended based on an appreciation of the fact that high shear strengths for the Jeerinah Formation measured in laboratory tests have yet to be proven in the field. The strategy associated with this cautious approach is the incorporation of a toe buttress of variable height. Decisions to finally remove the toe buttress would be taken in due course based on further observations of the performance of the North Wall excavation coupled with engineering judgement. The whole process of analysis and synthesis described in this thesis, which is primarily relevant to the Mt Newman mining operations, has significant implications for future open pit mining elsewhere in Australia and the world