Authors: Williams, J; Rathbone, S; Campbell, L

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DOI https://doi.org/10.36487/ACG_repo/708_23

Cite As:
Williams, J, Rathbone, S & Campbell, L 2007, 'Depressurisation of Open Pit Walls - A Practical Example from the South East Prongs Pit, Tom Price, Western Australia', in Y Potvin (ed.), Slope Stability 2007: Proceedings of the 2007 International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 371-378, https://doi.org/10.36487/ACG_repo/708_23

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Abstract:
The South East Prongs open pit is currently being mined 59 vertical metre below the pre-mine water table, with a further 45 m of vertical advance to reach final design depth. In-pit dewatering induces a steep hydraulic gradient between the pit walls (Mount McRae Shale) and the ore body (mineralised Dales Gorge Member) as in-pit bores dewater the relatively permeable ore body more rapidly than the low permeability pit walls. This may potentially result in pit wall instability. To reduce the hydraulic gradient two-dimensional groundwater models have been used to assess several depressurisation drilling patterns, leading to adoption of 25 m hole spacing and 50-100 m horizontal depth. Horizontal drilling has resulted in rapid groundwater pressure reductions of up to 15 m being recorded within 48 hours of drilling. Elevated pore water pressure has not been assessed as a dominant factor in South East Prongs pit wall instability since intensive horizontal drilling began in 2005, suggesting that depressurisation efforts to date have been successful in reducing geotechnical risk.

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Depressurisation of Open Pit Walls — A Practical Example from the South East Prongs Pit,
Tom Price, Western Australia J. Williams, et al.
378 Slope Stability 2007, Perth, Australia




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