A New Stability Number for Vertical Cuts in Stiff Clays

Abstract

Excavation into a stiff cohesive material will result in outward movement of the new slope face due to lateral stress relief (LSR). Expansion of the slope face can create a discontinuity at the toe of the slope as well as a tension crack at the surface of the backslope. However, limit equilibrium methods of slope stability analysis only consider the equilibrium of a soil mass tending to move down slope under the influence of gravity without regard for the in-situ stress conditions or the development of shear zones that occur from LSR. The literature contains examples in which undrained failure theoretically applied and predicted a stable excavation, yet failure occurred during or immediately after construction. Finite element analyses performed for this study indicate a strong relationship between overall slope stability and the initial ko value. As ko increased, the horizontal movement at the slope face due to LSR significantly altered the backslope conditions and created shear and tension cracks in the soil mass. Studies performed herein indicated that the initial direction of crack propagation was strongly dependent on ko, with two principal failure planes developing in the soil mass at the toe of slope and another failure plane developing in the backslope along the crest. A vertical slope remained stable for ko = 1.0, as predicted by classical methods. However, LSR resulted in the formation of toe and tension cracks that lead to eventual slope failure for vertical slopes with initial ko = 2 and 3. Based on these studies, the impact of LSR has been incorporated into classical limit equilibrium methods utilizing a modified stability number.