Characteristics of spoil pile failures in a strip coal mine in Tertiary and Permian sediments

A study of the mechanisms of spoil pile slope failures was carried out at South Blackwa, ter Mine in Central Queensland, Australia by monitoring inclinometers, shear strips and piezometers installed in spoil piles cast opposite the highwalls, prior to subsequent stripping of overburden. It was found that the failure was of a two-wedge type as commonly observed but the failure surfaces could run through the floor materials, causing upthrusts at the toes of the spoil piles. Failure through the soft-rock floor was possible due to the slaking behaviour of the materials, resulting in a marked drop of shear strength when saturated. Back- analysis of the failure process, using shear strength parameters from laboratory tests on undisturbed cores, was in good agreement with field observations.