A regressive mode of highwall failure in coal strip mines

CIM Bulletin, Vol. 79, No. 892, 1986


A number of highwall failures have occurred in some of the Alberta coal strip mines of a type called by the authors "Multiple block plane shear slope failure"<'>. They cause rapid deterioration of the dragline bench, reduced productivity and maintenance costs. The failures are invariably associated with the presence of almost horizontal (± 5 degrees) bentonitic clay bands in the overburden, often immediately above the coal seam, and regress behind the slope as a series of tension cracks which isolate detached blocks. As many as 17 sets of cracks have been observed, at lateral distances in excess of five times the slope height from the toe. The theory of this mode of failure is presented and a limiting equilibrium computer program which will produce deterministic or probabilistic (Monte Carlo) solutions is briefly described. A case history of one such failure is documented. It is shown that the extent of the regressive failure is exceedingly sensitive to the water table elevation, leading to the conclusion that drainage, if practical, represents the best control strategy.
Keywords: Slopes, Plane shear failure, Regressive failure, Coal strip mines, Rock mechanics, Open-pit mining.