Numerical analysis of a multipropped excavation in stiff clay

Abstract

This paper presents the procedures and the results of numerical analyses of a multipropped deep excavation at Lion Yard, Cambridge, using the nonlinear Brick model. The computed results are compared with the comprehensive case record. The observed small deflections and bending moments of the wall, low prop forces, and relatively small ground movements during the main excavation have been taken into account. Shear strains which developed around the site during the main excavation were generally less than 0.3%. Significant reduction of lateral stress in the ground during wall installation and the highly nonlinear stress-strain characteristics of the Gault Clay are the chief reasons for the observed unusual behaviour. The Gault Clay exhibits first yield at a threshold shear strain of about 0.001%, beyond which the stiffness deteriorates significantly from an initial very high value. This high stiffness at very small strains may be due to cementation bonding between soil particles, as a result of the presence of calcium carbonate. Simple drainage assumptions for the stiff fissured clay on both sides of the diaphragm wall appear to be inadequate for design analyses.Key words: numerical analysis, multipropped excavation, Gault Clay, nonlinear brick model, small strain stiffness.