Queensland Chapter

Collapse of a Deep Excavation in Asia – Lessons Learnt

Dr Preba Nadarajah

On 20th April 2004 a 30m deep excavation adjacent to Nicoll Highway in Singapore collapsed, killing four people. The excavation was carried out in a reclaimed land using a temporary diaphragm wall retaining system. The strutting system used consisted of 10 levels of steel struts and two levels jet grout slabs. The collapse occurred in a ‘brittle’ manner after removing the upper sacrificial grout layer. Observations on site showed this was associated with ‘sway’ failure of the waling beams at a strut-waler connection which had a detail using ‘C channels’ to stiffen the flanges of the wailing beams.

The Committee of Inquiry into the collapse concluded that the failure of the connections between the steel struts and the steel walers at the lowest level initiated the collapse of the excavation. The committee also identified other fundamental design and construction related shortcomings.

In this presentation lessons learned from the collapse and the follow up actions taken will be discussed.

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