Rainfall Infiltration Into and Seepage From Rock Dumps — A Review

Authors: Williams, DJ; Rohde, TK Download Paper

DOI https://doi.org/10.36487/ACG_repo/802_7

Cite As:
Williams, DJ & Rohde, TK 2008, 'Rainfall Infiltration Into and Seepage From Rock Dumps — A Review', in AB Fourie (ed.), Rock Dumps 2008: Proceedings of the First International Seminar on the Management of Rock Dumps, Stockpiles and Heap Leach Pads, Australian Centre for Geomechanics, Perth, pp. 79-89, https://doi.org/10.36487/ACG_repo/802_7



Abstract:
Mine rock typically emerges from an open pit relatively dry, and is conventionally stored in surface dumps where it progressively becomes wet-up by rainfall infiltration over time. Slow infiltration into the rock dump following light rainfall is likely to be largely restricted to the traffic-compacted surficial layer of the dump from which much of it is likely to be evaporated. Rapid infiltration into the rock dump following prolonged heavy rainfall is likely to be dominated by flow along distinct preferred seepage pathways, leading to deep drainage and the emergence of some of this as seepage from the base of the dump. Rainfall infiltration into a bare, loosely-dumped rock dump is commonly assumed to be 30 to 60% of average annual rainfall, depending on the climate, and the physical and chemical characteristics of the rock. However, there is a lack of field data to confirm this. A 15 m high trial rock dump covering 0.71 ha was constructed at a mine in New South Wales, Australia. It was instrumented with two lysimeters at the surface of the dump and 24 lysimeters at the base of the dump to monitor infiltration through the top and base seepage beneath the top surface and the side slopes of the dump. The paper presents the results of 16 months of monitoring of the trial rock dump lysimeters. They show that total infiltration into the top of the trial dump was about 50% of the total 16- months of well below average rainfall, with the majority of the infiltration going into storage within the dump and only a minor amount reaching the base of the dump via preferred seepage pathways. Runoff from the top of the trial dump is not allowed so that the remaining 50% of the total rainfall was lost to evaporation from surface ponding. The trigger rainfall required for base seepage was found to reduce exponentially from about 30 mm initially to 2.5 mm, and the response time from about 10 days to 2.5 days as the dump wets-up. A review of the literature and other sources is also presented that provides available data worldwide on rainfall infiltration into and seepage from rock dumps located in a range of climates.

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