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No.11 Nicoll Highway collapse

The Nicoll Highway collapse in Singapore was possibly the greatest civil engineering disaster of the last decade.

Inadequate temporary works and design and construction errors all led to the fatal collapse of Singapore’s deepest ever cut and cover tunnel in April 2004.

The collapse hit a 110m section of tunnel being constructed for the city’s new Circle Line, adjacent to the six lane Nicoll Highway. Four workers died when steel struts supporting the excavation’s diaphragm walls failed, causing the tunnel to cave in.

Failure lay with a connection between horizontal struts and waling beams. The official report into the disaster concluded that critical design and construction errors led to the failure of the strut and beam earth retaining wall system.

These errors were:

  • Use of an inappropriate soil simulation model which overestimated the soil strength at the site and underestimated the forces on the retaining walls in the excavation
  • An error in the design of the strut-waler support system with the connections being underdesigned
  • Omission during construction of props which would have spread load from struts to walers

“The net effect of these errors resulted in the strut-waler system being about 50% weaker than it should have been,” the report stated.

To date just one individual has been prosecuted for the collapse, even though the general causes of the collapse were agreed as long ago as 2005 by client the Land Transport Authority (LTA), main contractor Nishimatsu-Lum Chang joint venture (NLC), NLC’s designer Maunsell Asia, NLC project engineer Paul Broome, base slab subcontractor L&M, strutting subcontractor Kori, diaphragm walling subcontractor Bachy Soletanche, project insurer Aviva, LTA project directors Ng Seng Yoong and Sripathy, and Singapore’s Ministry of Manpower.

But regardless of legal fallout, construction lessons have been learned. Four years on, in 2009, NCE revisited the scene of the disaster to find works back underway - but with considerably beefed up temporary works.

Braced excavation, calling for heavy-duty temporary works, has been a standard feature of Singapore transport construction projects. But since the Nicoll Highway collapse extra robustness has been added.

Historically MRT stations have been built by conventional “bottom up” construction, with propping provided by steel struts supported at mid-span by king posts and bearing on to waling beams installed across the face of the diaphragm walls. Struts now are installed every 3m depth.

Many stations are even being constructed “top down”, with the permanent reinforced-concrete roof slab performing as a giant strut. This enables the elimination of struts, the installation of which is time-consuming and poses safety risks.

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