Engineers Australia

Laboratory geotechnical characterisation of scalped coal mine spoil

D.J. Williams and A.K. Kho


The sedimentary overburden materials in the Australian coalfields vary from essentially uncemented rocks to cemented sandstones, including variations between these extremes. The uncemented spoil materials break down on excavation to extract coal and bulk up to a very loose density on end-dumping by haul truck in spoil piles. These loose materials then undergo three forms of settlement: due to their self-weight, “collapse” settlement on wetting by rainfall, and settlement due to degradation on exposure to weather, resulting in a substantial increase in density. Due to their lack of cementation, these spoil materials degrade rapidly on exposure to the weather, leading to significant settlement, followed by some reversal on re-agglomeration and swell. Collapse and degradation-induced settlements, both being associated with exposure to water, occur simultaneously on wetting. Wetting also causes a substantial reduction in the shear strength of the materials. The cemented spoil materials undergo more limited breakdown on excavation, dumping and wetting. Cemented spoil materials bulk up to a more limited degree on excavation, and settle less after dumping, than uncemented spoil materials. The paper characterises uncemented and cemented overburden materials excavated on open pit coal mining, and quantifies the laboratory compaction, shear strength, compressibility and degradation of scalped samples.