INADEQUATE TEMPORARY works design and construction errors led to the fatal collapse of Singapore's deepest ever cut and cover tunnel, the public inquiry into the disaster heard at the end of March.
A 110m section of tunnel being built for Singapore Mass Rapid Transit's new Circle Line, adjacent to the six lane Nicoll Highway, collapsed on 20 April last year (GE May 2004).
Four workers died when steel struts supporting the excavation's diaphragm walls failed, causing the tunnel to cave in. Part of the highway was also destroyed.
The disaster was triggered by deformation of a connection between the horizontal struts and waling beams running horizontally along the diaphragm walls, according to part of the summary of evidence submitted to the Committee of Inquiry in Singapore on 31 March.
The general causes of the collapse were agreed 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 details are being disputed by the main contractor and the LTA (see boxes).
The collapse occurred east of the planned Nicoll Highway station in a section of cut and cover tunnel adjoining a large diameter reinforced concrete shaft.
This had been built in preparation for the launch and extraction of a tunnel boring machine, which was to drive a section of tunnel under the nearby Kallang River.
The collapsed cut and cover section curved through an area of reclaimed land, comprising 6m of fi ll over 27m of soft marine clay and 5m of estuarine clay. It was unusually deep, with a base slab at 33m below ground level. This was to accommodate LTA's plans to build a future road tunnel above the rail tunnel.
Construction involved the installation of reinforced concrete diaphragm walls and two deep level jetgrout slabs before excavation began.
The lower slab, 33m below ground, was to form the permanent base of the excavation. It also acted as a major structural component, resisting earth pressure behind the diaphragm walls after excavation had been completed.
The upper slab, or 'sacrificial jet grout prop', 6m above, was installed to provide temporary support to the diaphragm walls. It was to be broken out and replaced with steel struts as excavation advanced.
In all, 10 levels of struts were to be installed between the base slab and the surface during construction.
These connected with horizontal walers running along the diaphragm walls in the curved section (see box).
Excavation was in its final stages - at 30m, just 3m above formation level - when collapse began. With level nine struts in place and working from the launch shaft end, the contractor began breaking out the sacrifi cial jet grout prop to allow excavation to continue.
But before the final level of struts could be installed, the level nine struts became overloaded and their connections with the waler beams yielded. This allowed the walls to deform, in turn causing overloading of struts above. This triggered progressive collapse of the walls.
Failure was rapid. Only an hour elapsed between failure of the first strut-waler connection and total collapse of the excavation.
NLC admitted to the inquiry that failure of the temporary works resulted from under-design and inappropriate detailing of the strutwaler connections.
NLC's design engineers misinterpreted building code BS5950, NLC's counsel Philip Jeyaretnam said. This resulted in the adoption of smaller than required steel sections, reducing the amount of redundancy in the design.
But this design deficiency was made drastically worse by omission of load spreading splays at the ends of the struts during construction.
The contractor's failure to include these crucial structural components in the strut-waler connection was not picked up during routine works supervision.
Consequently, the entire axial load of each strut was directed into the waling beam through a single point of contact.
Forces of 4,000kN-4,600kN were being channelled through a detail designed with a capacity of 2,551kN. Laboratory testing and fi nite element analysis of identical strut-waler connections after the collapse revealed an actual ultimate capacity of 4,030kN-4,260kN.
The resultant overloading of the connections led to buckling of the waler web in several locations before the collapse occurred.
NLC replaced waler web plate stiffeners with a C-channel stiffening detail - commonly used in Nishimatsu's home country Japan.
All parties have agreed the sudden failure of this detail on level nine strut-waler connections caused the collapse.
But there is intense disagreement between the parties over why this detail should have failed so suddenly and dramatically.
Head of the Committee of Inquiry Judge Richard Magnus was expected to reach a decision on who was to blame for the collapse at the end of April, soon after GE went to press.
The Land Transport Authority's expert witness, Benaim director of geotechnical engineering Richard Davies, will be speaking about the Nicoll Highway collapse and inquiry at New Civil Engineer's Megatunnels conference on 18 May. For more details visit www. megatunnels. com or tel +44 (0)20 7505 6944.