Report No. : GEO Report No. 137

Report Title : A Study on Past Failures of Trench Excavations (2003), 63 p.

Author : J.S.M. Kwong

Abstract

The limited file search under this study has discovered that between 1986 and 2000, there had been fifteen "known" trench excavation induced failures involving a total of ten deaths and four injuries. In addition, in a single incident in 1966, six passers-by were killed and sixteen others were injured as a result of the collapse of a retaining wall caused by trench excavation.

This report includes a review of the sixteen cases of trench collapses and trench-induced failures of adjacent slopes and retaining walls. The study has revealed that the common causes of trench collapses and trench-induced slope failures are inadequate shoring and/or improper drainage provision to prevent water ingress into trenches. Such problems could be due to inadequate contract specifications for trench excavations and/or non-compliance of the works with the specified requirements. A review of the existing standards under this study has also identified the lack of a comprehensive guideline on trench excavations in the HKSAR, including temporary shoring support and precautionary measures to prevent any possible adverse effects on the stability of any adjacent slopes due to trench excavations. Site supervision by utility undertakers and auditing check by the authorities may also not be enough to ensure compliance with contract specifications and statutory requirements.

In view of these problems and deficiencies, recommendations are made in this report to enhance the safety of trench excavations, including (a) enhancement of standards, (b) education and promulgation of good practice and (c) increase in site supervision and auditing check.

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