Ground movement predictions for braced excavations in undrained clay

Abstract

The writers introduce a design approach for braced excavations based directly on the data of carefully chosen soil tests, conceived within the framework of plasticity theory, but allowing for strain hardening. Mobilized shear stresses beneath and around braced excavations are found by a stability calculation based on a proposed plastic deformation mechanism. Strains required to mobilize these stresses are deduced from a direct simple shear test on a representative sample taken from a selected location in the plastic zone of influence. These strains are entered into a simple plastic deformation mechanism to predict boundary displacements. Hence, the proposed Mobilizable Strength Design (MSD) method can satisfy both safety and serviceability in a single step of calculation, without the need for finite element analyses. In this method, design parameters can be chosen rationally with regard to the initial state of soil, the stiffness following the appropriate stress path, and the level of acceptable deformations under working conditions. Examples demonstrating the success of the MSD method are given for a variety wall and soil conditions. Comparisons are made both with previously published field studies and with comprehensive nonlinear finite element analyses.