Performance of braced excavation in residual soil with groundwater drawdown

In densely built-up Singapore, relatively stiff secant-bored piles and diaphragm walls are commonly used in cut-and-cover works to minimize the impact of ground movement on the adjacent structures and utilities. For excavations in stiff residual soil deposits, the associated wall deflections and ground settlements are generally smaller than for excavations in soft soil deposits. However, if the residual soil permeability is high and the underlying rock is highly fissured or fractured, substantial groundwater drawdown and associated seepage-induced settlement may occur. In this study, the excavation performance of four sites in residual soil deposits with maximum excavation depths between 20 and 24 m is presented. The maximum wall deflections were found to be relatively small compared to the significantly larger maximum ground settlements, owing to the extensive lowering of the groundwater table. In this paper, details of the subsurface conditions, excavation support system, field instrumentation, and observed excavation responses are presented, with particular focus on the large groundwater drawdown and associated ground settlement. Specific issues encountered during the excavation, as well as the effectiveness of various groundwater control measures, are discussed. The case studies will provide useful references and insights for future projects involving braced excavations in residual soil.