Residual shear strength mobilized in first-time slope failures

This paper presents a review of long-term stability of stiff clay and clay shale slopes, and detailed reanalyses of 99 case histories of slope failures in 36 soft clays to stiff clays and clay shales. We analyzed 107 sections using the observed actual slip surface. In a first-time slope failure in clay or shale, part or all of the slip surface is unsheared prior to the occurrence of the landslide. Most stiff clays and clay shales contain stratigraphic discontinuities such as bedding planes and laminations. The fully softened shear strength is shown to be the lower bound for mobilized shear strength in first-time slope failures in homogeneous soft to stiff clays and on the slip surfaces cutting across bedding planes and laminations. For many of the first-time slope failures it appears that part of the slip surface is at the residual condition. For excavated slopes, the residual condition could be present before the final slope is formed, or it may develop in response to excavation by progressive deformation along nearly horizontal surfaces including bedding planes or laminations. In addition to the permeability dependent rise in porewater pressure, and softening, delayed first-time failure of slopes in stiff clays and clay shales is caused by propagation of the residual condition into the slope, on horizontal or subhorizontal surfaces including stratigraphic discontinuities. The residual condition is present on the entire surface of reactivated landslides.