Base stability of deep excavation in anisotropic soft clay

Base stability of a deep excavation in clay is affected not only by soil properties such as soil strength and strength anisotropy, but also by several construction factors, such as the stiffness and depth of the excavation support wall and the depth of excavation. However, not all these influencing factors can be dealt with by the limit equilibrium methods currently used to analyze the base stability of a deep excavation in clay. The method proposed in this paper takes into account the influence of strength anisotropy and nonhomogeneity of soft clay, and the depth of the wall. The method is derived from the upper bound theory of bearing capacity and an assumed failure mechanism and is applied to study the base stability of deep excavations supported with internal braces and a concrete diaphragm wall in soft clay. To describe the strength anisotropy of soft clay, a total stress anisotropic strength criterion is adopted. The suitability of this method for analyzing the base stability of deep excavations is verified by comparison with numerical study results of deep excavations in Boston and a base failure case in Taipei. Limitations of this method, such as the minimum depth of excavation wall required and the lower bound ratio between the depth of the excavation wall and the width of excavation, are discussed also.