Bulking and Subsequent Self-Weight and Saturation Settlements, and Geotechnical Stability, of Deep Coal Mine Spoil Piles

Australia's open cut coal mines are extending to ever increasing depths, with correspondingly deeper associated spoil piles. As the depth of spoil increases, the net bulking of the spoil becomes critical to the design, sizing, geotechnical stability assessment, and cost estimation, of both in-pit and out-of-pit deep spoil piles. In turn, this impacts the economics of deep open pits and associated deep spoil piles. Erroneous estimates of the settlement and geotechnical stability of deep spoil piles can lead to the need for costly out-of-pit spoil piles later in the mine life, and the geotechnical instability of spoil piles can hinder and even threaten further mining.

ACARP Project C19022 entitled "Bulking and Subsequent Self-Weight and Saturation Settlements, and Geotechnical Stability, of Deep Coal Mine Spoil Piles" focussed on the geotechnical stability of deep coal mine spoil piles of varying spoil types. This was achieved through applying soil mechanics principles to investigate the load-settlement and degradation behaviour of a range of open cut coal mine spoil types tested both at their as-sampled moisture content and in a water bath, to assess the settlement and geotechnical stability of deep spoil piles.

The objectives of Stage 1 were to test in the laboratory the mechanical behaviour of a representative range of coal mine spoil types, to develop a predictive model for the settlement and geotechnical stability of deep spoil piles of mixed composition, and to demonstrate the application of the research results. The objectives of the Stage 2 extension were to extend the previous compression and strength testing to a larger-scale, and to further develop, validate and calibrate the spoil settlement and net bulking predictive model.

The laboratory testing included physical and basic chemical characterisation testing, and geotechnical parameter testing. Among the characterisation testing was moisture content, total suction, Atterberg limits, specific gravity, electrical conductivity and pH, and Emerson crumb testing. Particle size distribution analysis was also carried out, including SplitDesktop analysis, dry sieving of air-dried and oven-dried specimens, wash sieving both without and with dispersant, and hydrometer testing of the fine fraction, both without and with dispersant. The geotechnical parameter testing included Standard compaction, direct shear strength testing of dry and wet specimens, compression (oedometer) testing of dry and wet specimens, and degradation testing on exposure to the weather.

The project identified and quantified three components of spoil settlement as self-weight settlement of initially dry spoil, "collapse" settlement on wetting-up of the spoil, and degradation-induced spoil settlement on exposure. The outcomes of the Project included understanding and quantification of the initial bulking and subsequent self-weight, collapse and degradation-induced settlements, and geotechnical stability, of a representative range of coal mine spoil types, the development of spoil testing and analysis protocols, and the development of a spreadsheet-based model for predicting the settlement and net bulking of deep spoil piles of mixed composition.

The Project ran for 4 years from April 2010. The total cost of the Project was $ 1,341,500, including funding from ACARP of $427,000.