Abstract
The object of this work is to investigate the performance of excavations, supported by anchored diaphragm walls, with particular reference to London Clay, Both the factors that affect excavation performance and past design and analysis are considered in a literature review. For the purpose of the study, the deformation behaviour of London Clay is interpreted in terms of cross-anisotropic elasticity. A plane finite element computer program, developed during the course of the work, is briefly described. It is capable of simulating the sequence of excavation and of allowing relative movement on the soil/wall interface. Displacements, stresses, soil pressures on the back of the wall, structural forces on the wall, and excess pore pressures due to undrained loading may be computed. The end of excavation performance of an instrumented excavation in London Clay is back analysed. The undrained stiffness of the day is found to increase significantly with depth from a low value at the ground surface. Reasonable agreement is obtained between the analytical results and field observations. A parametric study is carried out on an excavation in an almost incompressible elastic soil, chosen to be reasonably representative of London Clay. While the presence of a cantilever diaphragm wall reduces elastic displacements in the vicinity of the excavation, it is found that prestressed tiebacks are required to achieve a considerable reduction. The influence of the tiebacks is very dependent on the level of prestress, but relatively insensitive to either inclination or distance to the anchor zone. The study also shows that taking soil anisotropy into account changes the magnitude, though not the pattern of displacement. Assuming the soil to behave in a time-independent elastic manner, the results of undrained and drained analyses are compared, A moderate increase in excavation movements is predicted due to drainage.