Centrifuge Model Tests on Double Propped Wall Excavation in Soft Clay

Abstract

In this study centrifuge model tests of a vertical excavation in normally consolidated soft clay were carried out using a newly developed test system, in which the construction sequence of a double propped wall for an open excavation can be properly simulated in flight. Settlements of the ground surface behind the wall, earth pressures on the wall, strains along the wall and pore water pressures in the ground were measured during the test. Effects of propping and embedment of the retaining wall into the bottom sand layer on the behaviour of the wall and ground were carefully studied. It was found that only 1 m embedment into the bottom sand could increase significantly the stability of excavation. Under the condition with the embedment into the bottom sand layer, deformation became a main concern instead of stability against failure. The deformation after a certain excavation depth was mainly determined by the mechanical properties of the sand, i.e., stiffness or stress-strain relationship. Propping can prevent marked increase in the settlement; however, it is very difficult to recover the settlement and deformation once it occurs by increasing the strutting force (i.e., preloading). Hysteresis and non-linear behaviour of the soil is considered to be one of the main reasons of the irrecoverable deformation.