Exceptional travel distance for a Pilbara failure

Authors: Ross, DG

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DOI https://doi.org/10.36487/ACG_repo/2335_35

Cite As:
Ross, DG 2023, 'Exceptional travel distance for a Pilbara failure', in PM Dight (ed.), SSIM 2023: Third International Slope Stability in Mining Conference, Australian Centre for Geomechanics, Perth, pp. 535-550, https://doi.org/10.36487/ACG_repo/2335_35

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Abstract:
A 140,000 m3 slope failure occurred in a BHP Western Australia Iron Ore mine in the Pilbara region. This occurred in Cenozoic sediments and the Archean West Angelas Member of the Marra Mamba Iron Formation. The 80 m high failure occurred on a slope of 50° measured from pit crest to slope toe and resulted in a deposit runout of 133 m. This exceeded the 95th percentile Whittall (2015) Fahrböschung model travel distance prediction of 124 m. Individual particles travelled to 152 m. This runout behaviour was significantly beyond the author’s 12-year experience in the Pilbara iron ore environment. As the scale of iron ore mining in similar geological settings in the Pilbara is considerable, the intention is to share this runout behaviour for the benefit of the surface geomechanics community and particularly those operating in this environment. This case history outlines elements of the failure geometry, geology and hydrogeology. It then reconciles the runout against the Whittall (2015) Fahrböschung Volume model and the Optimised Mobility Index model. Whether the runout models are suitable for this environment and whether Pilbara Cenozoic and West Angelas materials are more mobile than the Whittall (2015) Fahrböschung dataset are discussed. Other observations are included, such as how runout estimates based on the moving surface area might change rapidly where failures expand rapidly. Lastly, the lack of groundwater data from within the slope and potential limitations, even if it had been present, is also discussed.

Keywords: runout, Fahrböschung, Pilbara

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