Case Histories of Failure of Deep Excavation. Examination of Where Things Went Wrong: Nicoll Highway Collapse, Singapore

During the construction of Contract C824 of the Circle Line in Singapore, on 20th April 2004 an 80 m long section of excavation, 30 m deep, totally collapsed. The resulting crater was as deep as 15 metres and was more than 100 m in diameter. Six lanes of the adjacent Nicoll Highway subsided by as much as 13 m. Four construction workers were killed. Fortunately no vehicle was involved. A Committee of Inquiry was established and held hearings from August 2004 until March 2005. The findings of the Committee were published in May 2005. These identified the causes of the failure and made recommendations concerning safe practices for deep excavations in the future. The paper includes an account of the events leading up to the failure, the identification of the causative factors, and the reasons for the total collapse. There were many factors which caused the initial failure and the subsequent overall collapse. Although the trigger for the failure was found to be inadequate detailing of the connections between the steel struts and the steel waling beams, many contributory factors led to the whole structural system being unable to cope with the failure and the systematic failures in the management system. In addition to prosecution, the Authorities in Singapore took cognizance of the lessons learnt and took immediate follow up actions. These included immediate checking of the design of all similar deep earth retaining structures. Interim Guidelines were introduced which have since been followed up with revised standards such as independent checking of temporary works design, independent contractors for instrumentation and monitoring, and upgrading the factors of safety for deep temporary excavations to be the same as those for permanent works. The paper concludes with observations of what has happened in the subsequent seven years. Whereas a number of controls on procedure have been tightened, similar mistakes in detailing and lack of comprehension of the computer analyses have been observed and failure of a similarly deep strutted excavation occurred only three years later, but not in Singapore