Stability and Impacts of Unsupported Vertical Cuts in Stiff Clay

Natural stiff clay deposits often have an in-situ lateral earth pressure that is greater than the vertical pressure. An unsupported excavation into these materials results in outward movement of the slope face due to lateral stress release. This induces shear stress some distance away from the slope face and large increases in horizontal tensile strain occur. Classical solutions to the theoretical depth that an excavation can stand without lateral support do not consider the in-situ stress conditions or the development of shear zones that occur from lateral stress relief, which can significantly impact overall stability. Finite element analyses demonstrate the influence of the at-rest lateral pressure coefficient on the development and propagation of shear zones from excavation. Two distinct shear zones consisting of a horizontal and an inclined shear plane occur at the toe while a tension zone creates a vertical tension crack at the crest. Finite element analyses are used to evaluate stresses within the slope and shear propagation considers Mohr-Coulomb and tensile strength theory. Analyses indicate good agreement with case histories. Finite element analyses also indicate good agreement with classical solutions for normally consolidated soils, but these solutions indicate unsafe conditions for overconsolidated clays.