Abstract: The traditional failure mechanisms with constant shapes usually overestimate the basal stability of excavations in clay. A new mechanism is established based on the upper bound theorem. The least upper bound solutions and the corresponding failure mechanisms are obtained through the optimizations with geometry parameters. The non-homogeneous and anisotropic undrained shear strengths are considered. The new mechanism is verified through cases studies. The results show that the accuracy of the new mechanism is better than that of the traditional failure mechanism, and the proposed mechanism is close to that obtained from the multi-block upper bound method.