Numerical upper and lower bound limit analysis for braced excavations

Abstract

This thesis reviews limit theorems and their applications for stability problems in geotechnical engineering. Rigorous numerical solutions of limit analyses can be obtained through finite element discretization of the soil mass and formulation of the limit theorems within a linear programming framework. The current research uses a formulation proposed by Sloan et al. (1988) and extended in a recent Ph.D. thesis by Ukritchon (1998) to include soil-structure interactions. The thesis details the input and output required for numerical limit analysis and presents an example application for the stability of a broad excavation for the MUNI Metro Turnback project in San Francisco. This well documented case study involves a 13 M deep excavation within a deep deposit of May Mud that was supported by an SPTC wall with three levels of cross-lot bracing. The numerical limit analyses calculate factors of safety, FS = 1.03 - 1.36, against basal instability. The factor of safety used in the original design (FS = 1.2) is contained in this range. The results illustrate that numerical limit analysis offers a practical alternative to limit equilibrium methods in evaluating the stability of braced excavations.