Engineers Australia

Geotechnical characteristics of coal mine spoil

Stephen Fityus, Greg Hancock and Tony Wells


This paper presents data on the geotechnical characteristics of the overburden, or spoil, produced during open-cut mining operations in the Hunter Valley, specifically in relation to the stability of spoil piles and the hydraulic characteristics of spoil and post-mining landscapes. A value of 33.4° is reported for the peak shear strength of “typical” spoil. This value is shown to reduce slightly at large shear strains and further when spoil materials are sheared after breaking down on exposure to water. Data on the permeability of different spoil types is presented, showing that permeability decreases with mudrock content. The presence of coal in the spoil is shown to have a significant effect on its hydraulic properties. Due to its hydrophobic tendencies, coal tends to resist the infiltration of water. However, spoils containing coal also tend to resist the storage of water that does infiltrate, causing coal-rich materials to drain quickly. This is demonstrated by considering the water retention properties of washed coal. The spoils are shown to have significant quantities of total leachable salt, but that leaching by a significant volume of water is needed for its total removal. The results show that a significant amount of salt is released upon first leaching, but that the rate of leaching decreases significantly with continued rainfall percolation. The paper concludes with a brief discussion of the potential for salt leaching from mine-spoil landscapes and its consequences for water quality in post-mining environments.