Assessment of Wall Deflections and Ground Settlements for Braced Excavations Subjected to Groundwater Drawdown: Numerical Simulations and Design Charts

Abstract

Due to the dense population and limited space in urban areas, the development of underground space is desirable, highlighting the importance of deep excavation. Deep excavations in soft clay deposits may cause excessive ground movements and thus result in potential damage to adjacent buildings and supporting utilities. This paper describes the general project and site conditions of four Mass Rapid Transit (MRT) stations of Downtown Line Stage 2 (DTL2): Cashew, Hillview, Bukit Panjang, and Beauty World. Then the paper summaries the field performance of braced excavations in Bukit Timah Granite (BTG) residual soils, based on the instrumented data from the four sites. Extensive finite element analyses were carried out to develop a series of base case design charts for assessing the wall deflections and ground settlements for braced excavations in BTG residual soils. The modification factors are also proposed, indicating the influences of groundwater drawdown, system stiffness, excavation width, excavation depth, and rock head level above the final elevation level on wall deflections and ground settlements, respectively. A comparison between the measured results from the sites and the predictions based on base cases validated the rationality and feasibility of the proposed modification factors, as well as the presented design charts.