Improving performance of soil covers over waste rock dump tops in dry climates

Abstract

Soil covers over mine waste rock dump tops in dry climates are intended to limit the ingress of oxygen and/or to limit the net percolation of rainfall infiltration into the underling potentially reactive waste rock excavated, in particular, on open pit mining. The climate in which the mine is located plays a key role. The aim of limiting the ingress of oxygen into the dump is to minimise the oxidation of any sulphidic waste rock, which is often found on mining an ore body beneath the groundwater table. The aim of limiting the net percolation of rainfall infiltration into an oxidising waste rock dump is to limit the transport of the oxidation products to the environment. In the absence of a water cover, limiting the ingress of oxygen into a dump is best achieved by using a near-saturated soil cover, while limiting net percolation is potentially achieved by applying a rainfall-shedding soil cover, or by applying a store-and-release soil cover. In practice, there are many impediments to achieving good performance from a soil cover over a waste rock dump. lt takes a number of years to construct a waste rock dump as open pit mining proceeds. The waste rock will emerge from the pit in a relatively dry state, and it will readily become, and will remain during construction of the dump, well oxygenated. Hence, there is ample opportunity for sulphidic waste rock in the dump to oxidise during the construction of the dump, so that limiting further oxygen ingress may be of limited value. During construction, a dump is also open to rainfall infiltration, allowing the wetting-up of the dump and the ultimate transport of any oxidation products that form. When the dump is eventually covered, the best that can be achieved is limiting further oxygen ingress and/or limiting the transport of oxidation products. However, this assumes that an effective cover can be constructed. This is made difficult by the ongoing settlement of the dump, by the potential absence of suitable cover materials, by the use of large-scale mine site equipment to construct the cover, by insufficient attention paid to cover design, construction and maintenance, and by climatic variability. The relatively poor performance of dry-climate soil covers over waste rock dumps is compared with the performance of dry-climate soil covers over landfills, which typically display good performance.