A solution to the braced excavation collapse in Singapore

Abstract

At about 3:30pm on April 20, 2004, a 30m deep excavation adjacent to Nicoll Highway in Singapore collapsed, resulting in four casualties and a delay of part of a US$4.14 billion subway project. This thesis examines the flaws in the original design of the bracing system, which have been cited as causes of the failure. The Author then proposes a revised design for the braced excavation system. The Plaxis finite element program was used to simulate the excavation process and compute forces on the major structural elements in the original design. Some pertinent background information on this program is provided throughout the thesis in order to better understand the significance of certain errors in the input data of the original model that ultimately led to the incorrect assumptions and calculations of the original design. A new model using this same program was regenerated with a corrected set of input assumptions, thereby leading to reasonable estimates of structural forces. These results were then used to propose a revised design of the excavation support system and compare this design to the original used in the excavation project. There are several lessons that could be learned from this structural failure, one being the need to acknowledge the limitations built in advanced analysis software systems, and another being the importance of ascertaining that the user understands every feature of the product. A cost estimation of the proposed design is given and compared to the original design in order to evaluate the viability of the proposed design in the construction bid. Finally, some important conclusions are drawn from this study that should be applied to future large-scale construction projects where public safety and welfare is at stake.