Iterative geotechnical pit slope design in a structurally complex setting: a case study from Tom Price, Western Australia

Authors: Lucas, DS; de Graaf, PJH Download Paper

DOI https://doi.org/10.36487/ACG_rep/1308_33_Lucas

Cite As:
Lucas, DS & de Graaf, PJH 2013, 'Iterative geotechnical pit slope design in a structurally complex setting: a case study from Tom Price, Western Australia', in PM Dight (ed.), Slope Stability 2013: Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 513-526, https://doi.org/10.36487/ACG_rep/1308_33_Lucas



Abstract:
Rio Tinto Iron Ore’s (RTIO) Tom Price open cut mine in the Pilbara region of Western Australia commenced in 1966 and had a cumulative production to mid 2012 estimated at over 800 Mt. The South East Prongs (SEP) pit at Tom Price hosts one of the mine’s prime sources of high grade and low impurity hematite ore. An integrated mine planning and geotechnical design approach was required to evaluate and optimise late stage mining design options within large scale structural geological (adverse bedding strength anisotropy and fault) and hydrogeological controls. The SEP pit is structurally complex. The orebody is hosted within a doubly-plunging syncline bounded by the low-permeability shale, with predominant east-west striking faults confining the mineralisation to within the central part of the syncline. Multiple deformation events have resulted in significant folding and additional faulting. The complex structural geology, with bedding dipping into the pit void, and numerous faults, present a challenging geotechnical environment to design and implement robust mine design to maximise late stage mining ore recovery. By mid 2010, the SEP pit had been mined to between 600 and 640 RL. Mining ceased after accelerated movement was detected in the north wall. Additional geotechnical investigations were undertaken in late 2010 and 2011. This augmented information obtained from previous investigations. Stratigraphic units and faults were defined by mapping and geophysical logging, but areas of uncertainty remained, particularly in some fault locations, which could not be improved by further drilling. The planned final 375 m high pit shell is to extend 30 to 70 metres deeper than the current mining levels, and about 100 m below the regional water table. A modified pit design was required to fit within a constrained region of the pit, to minimise the likelihood of fault controlled instability impacting the main access ramp on the west wall, and to account for dewatering and depressurisation requirements. The project geotechnical team, consisting of RTIO geotechnical personnel and their geotechnical consultants (Mining One) worked closely with RTIO’s mine design team to develop workable risk assessed options which enabled an optimum design to be adopted, with a detailed hazard and risk management plan for implementation during mining. The final design was optimised to allow access by a large equipment fleet for more efficient mining, while deferring key geotechnical risks to a later stage of mining to provide greater reliability of ore supply for the short-term mining schedule and opportunity to improve design reliability as mining proceeds.

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