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
(https://papers.acg.uwa.edu.au/p/708_23_Williams/)
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.