Multiple-block plane shear slope failure

Abstract

Field examination of highwall slope failures occurring in some coal strip mines in Alberta in association with bentonite has led to the concept of a multiple-block plane shear failure mode in which a series of individual block failures regresses back into the slope for distances up to five times the slope height. These failures cause a rapid deterioration of the bench from which the dragline operates, reduce productivity, and necessitate bench maintenance to provide a safe, stable bench prior to the next pass of the dragline.A probabilistic limiting equilibrium analysis was developed to predict the positions of successive failure blocks and its predictions were compared with slope geometry and laboratory strength data from a producing coal strip mine in which a failure was active.Two slope failures reported in the literature (Folkestone Warren landslips and an Oxford Clay brick pit, both in England) were reanalysed using the multiple-block plane shear failure mode, and it is suggested that this mode of failure offers an alternative explanation for the slope kinematics observed.Finally, the most effective means of reducing the probability of failure or controlling the extent of slope failure retrogression is by dewatering. Key words: slopes, failure, coal mining, bentonite, clay bands, plane shear, case history, probabilistic analysis.