Considerations for the geotechnical stability of coal mine spoil piles containing co-disposed tailings

Billions of tons of mineral wastes are generated annually by the Australian open-cut coal-mining industry. These wastes include the waste rock which is excavated in order to access the underlying coal seams, and the impurities which are removed when processing the unrefined raw coal. These processing wastes include a gravel-dominated coarse reject stream and a slurry-like tailings reject stream. Typically, the rock waste is disposed of in massive fill structures commonly termed waste dumps or spoil piles, which may be hundreds of meters in height. Coarse rejects from coal processing are trucked to and disposed of in the spoil piles, whilst the fine tailings rejects are traditionally pumped to and disposed of as an aqueous slurry in large tailings dams. These tailings dams are a major source of safety, environmental and financial risk for mine operators and as such, are subject to increasing financial, social and legislative pressures, coinciding with a decrease in the availability of land on which they can be constructed. An emerging waste disposal strategy in the Australian coal mining industry is spoil-pile codisposal. This entails partly dewatering the tailings rejects, mixing them with the coarse rejects, then disposing of the mixed rejects (herein termed mixed plant rejects or MPR) with the waste rock in the spoil pile. Spoil-pile co-disposal eliminates the requirements to establish and operate a tailings dam and therefore eliminates the associated risks. However, the introduction of potentially low-strength MPR mixtures with relatively high moisture contents poses an increased likelihood of spoil pile slope instability. To date, implementation of spoil-pile co-disposal in the Australian coal mining industry has been largely empirical and based on a “trial and error” approach for waste dump design. Engineering based stability assessments are seldom performed as, at present, the fundamental behaviour or MPR within a spoil pile is not well understood. This is largely attributable to a limited understanding of how various hydrogeological processes facilitate excess pore pressure and varying degrees of saturation during spoil pile construction.