Finite element analysis of basal heave stability for braced excavations in clays

Abstract

The basal heave stability of braced excavations in clay soils is one of the key considerations in both design and during construction. The finite element method with shear strength reduction (SSR) is increasingly being used for estimating the safety factor for geotechnical engineering problems. In this paper, results of basal heave stability for deep narrow braced excavations in terms of factor of safety (FS) and stability number (Ns) using two-dimensional finite element method with SSR are presented. Total stress finite element simulations are carried out to parametrically analyse the effects of system stiffness, undrained shear strength and excavation geometry on the base stability of braced excavations in different clay soils. For the cases considered, the study shows that the system stiffness has minimal influence on the factor of safety against basal heave. The stability number Ns calculated from finite element analysis is independent of the undrained shear strength of clays. Also, the Ns value decreases almost linearly as the ratio of the depth to the width of the excavation increases. The failure zone from finite element analysis is more extensive than that proposed by Terzaghi. Comparison of the factor of safety between the calculations from conventional methods and from finite element analysis indicate that the modified Terzaghi’s method that considers the wall embedment depth are in close agreement with the results based on the plane strain finite element analyses.