NUMERICAL-ANALYSIS OF A MULTIPROPPED EXCAVATION IN STIFF CLAY

This paper presents the procedures and the results of numerical analys es of a multipropped deep excavation at Lion Yard, Cambridge, using th e nonlinear Brick model. The computed results are compared with the co mprehensive case record. The observed small deflections and bending mo ments of the wall, low prop forces, and relatively small ground moveme nts during the main excavation have been taken into account. Shear str ains which developed around the site during the main excavation were g enerally less than 0.3%. Significant reduction of lateral stress in th e ground during wall installation and the highly nonlinear stress-stra in characteristics of the Gault Clay are the chief reasons for the obs erved unusual behaviour. The Gault Clay exhibits first yield at a thre shold shear strain of about 0.001%, beyond which the stiffness deterio rates significantly from an initial very high value. This high stiffne ss at very small strains may be due to cementation bonding between soi l 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.