The formation of planes of weakness in the highwall at Goonyella Mine, Queensland, Australia

Abstract

Large highwail failures at Goonyella Mine, Queensland occur by the sliding of blocks of overburden along thin, soft, clayey bands near the base of the highwall. These bands, called black clay planes, are conformable with the lithological sequence but have a distinctive outcrop pattern, being softer and blacker than the enclosing sediments. Chips from the black clay planes disperse on immersion in water whereas the enclosing materials do not. Despite these differences, the black clay planes are shown to have almost identical chemical and mineralogical properties to the enclosing mudstones, indicating a common sedimentary origin. Bedding-plane shear is suggested as a mechanism for the formation of the black clay planes from typical mudstones. By breaking particle/particle bonds, the shearing has resulted in low strength and allowed the dispersion of the clay. The darker colour of the black clay planes is mainly due to their higher in-situ water content. The suggested mode of formation of the black clay planes is supported by laboratory modelling of the shearing process.