RECOVERY ROOM Golden Mile Complex, damaged by tunnel collapse, looms over excavators removing site debris. (Photo and image below courtesy of LTA)

Reopening of a Singapore highway closed when a tunnel excavation collapsed last April is expected by year’s end. But the facts found by a court of inquiry looking into the collapse are undermining the owner’s confidence in the design and construction team while shining an unflattering light upon the owner’s own project management.

The April 20 collapse of the Land Transport Authority’s Circle Line mass-rail-transport (MRT) tunnel excavation killed four workers and substantially damaged 100 meters of highway nearby (ENR 5/17 p. 12). A seven-stage recovery began in May with debris removal and the placement of foam concrete to displace the water that had seeped into the excavation. Stage 4 backfilling of the collapse site now is in progress by the joint venture of Tokyo-based Nishimatsu Construction Co. and Lum Chang Holdings (NLC), Singapore, the contractors responsible for the original excavation. Restoration and reconstruction by the same team will begin after the highway is reopened to traffic late this year.

But LTA–the Singapore government’s transportation and construction arm–intends to examine and approve every stage of excavation in future rail projects rather than rely on the assessment of the professional engineers.

"On the supervision of construction works, LTA has introduced an additional level of checks at each stage of excavation," says a senior LTA official, speaking on condition of anonymity. "Previously, contractors only had to seek necessary approval, such as from the professional engineer. Now, they would also have to seek approval from LTA before proceeding with the next level." Designs submitted by the contractor also will require additional checks and endorsements before acceptance by the Building Construction Authority, he says.

All parties see the design of temporary works as key to the problem. "The primary responsibility for the design, construction and supervision of the temporary works fell on NLC," says K. Shanmugam, LTA’s lawyer. "LTA’s acceptance of, inter alia, the design, does not relieve NLC of any of its responsibilities."

And there had been other problems. A year ago, a wall at another location deflected 400 mm into the excavation. In September, a professor at the Nanyang Technological University told LTA that the contractor was using the simulation software incorrectly, LTA informed the court. But NLC’s professional engineer, Paul Broome, told LTA it could not dictate the design of temporary works, Shanmugam told the court.

"The lack of an independent check for temporary works in complex engineering projects is a glaring omission of the system," said Philip Jeyaretnam, NLC’s counsel.

NLC and Maunsell Consulting Co., the Australia-based designer of the original and some temporary works, are performing the recovery. But LTA refuses to say whether they will be retained for further construction. LTA has not yet worked out the extent of project delay and cost increases caused by the accident.

Maunsell says the accident could have been avoided if NLC had recognized the amount of stress on the supporting walls of the tunnel. NLC did not take inclinometer readings that would have shown the deflection of the supporting structures from their normal position, the designer claims.