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
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
Coffey and Partners Pty Ltd (1991) South East Prongs pit dewatering study Tom Price: Internal RTIO report.
Coffey and Partners Pty Ltd (1994) South East Prongs West South Wall: Internal RTIO report.
Coffey and Partners Pty Ltd (1999) South East Prongs North Wall: Internal RTIO report.
Morris, R.C. (1985) Iron Ore: Australia, Commonwealth Scientific and Industrial Research Organisation, Division of
Mineralogy Research Review, 1983, pp. 36-39.
Pell Sullivan Meynink Pty Ltd (2005) Review of the geotechnical design - South East Prongs: Internal RTIO report.
Thorne, A.M. and Tyler, I.M. (1997) Mount Bruce, W.A (2nd edition): Geological Survey of Western Australia, 1:250
000 Geological Series Explanatory Notes.
Trendall, A.F. and Blockley, J.G. (1970) The iron formations of the Precambrian Hamersley Group, Western Australia,
with special reference to the associated crocidolite: Western Australia Geological Survey, Bulletin 119, pp. 174-
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