Failure mechanism of spreads in sensitive clays

Abstract

Through detailed case studies from the literature it is suggested that a sensitive clay spread is formed by propagation of a failure surface in an intact slope and dislocation of the soil mass in horsts and grabens. It is proposed that the initiation and propagation of the failure surface can be explained by progressive failure mechanism. According to this failure mechanism, failure is initiated near the toe of the slope and the strain-softening stress-strain behaviour of sensitive clays is used to redistribute shear stress along the quasi-horizontal shear zone. The failure propagates inside the deposit reducing the horizontal stress. Active strength of the soil may be mobilised, explaining the dislocation of the soil mass above the shear zone in horsts and grabens. A numerical procedure is used to back calculate the 1994 spread at Sainte-Monique, Québec, Canada, involving slightly over-consolidated sensitive clay. The initiation and extent of the failure surface observed on site are explained by a soil having large brittleness during shear and large-deformation shear strength close to the remoulded shear strength of the soil.