...shops also have complained in recent months of cracks in their walls and blamed the construction of the Circle Line for the damage. More ominously, three other failures have occurred on the project, requiring backfilling, redesign and excavation. One occurred in the circular shaft and the other two on the cut-and-cover line. None caused loss of life.

After the April 20 collapse, Nicoll Highway settled more than 3 m, while on the other side of the cut the settlement was on the order of 10 m. A crane, construction steel and other materials also sank into the 100 x 130-m-wide crater.

According to Rajan Krishnan, LTA’s director of projects, the tunnel was being dug on reclaimed land, sitting on gooey marine clay and just 100 m from the river. Krishnan told reporters that he would not regard it as a particularly difficult excavation to handle. He explained that the tunnel-boring method used elsewhere on the project could not be used at the accident site because tunnels criss-crossed at this point and there was no alternative to open excavation.

“The soil puts a lot of pressure on the walls when it is being removed,” says an engineer involved in the project who asked not to be named. “The inner support needs to be replaced by something equivalent. When a part of a wall or a strutting collapses, it results in a domino effect, causing everything else to fail.”

T.S. Low, LTA’s deputy chief executive, says that Nishimatsu has a good track record of performance. The contractor has assured the government that it used good materials for construction. “The steel materials that we use on site for the strutting system, some of the steels are new or brand new and some of the materials are rented or leased. And the rented and leased material was in good condition before it was reused,” says Paul Broome, Nishimatsu project coordinator.

The excavation work at all the other Circle Line sites is on hold until the design of the temporary works and the constructed temporary works are thoroughly checked and found to be in order.

The accident gave rise to a serious danger of waters from the nearby river flowing into the worksite. The first task was to block a damaged canal within the collapsed site. The soil and the slopes in the surrounding area of the site were protected by covering them with canvas sheets. Cracks in the ground around the site were filled with grout to prevent water seepage.

Nine electrolevel beams were installed to monitor movement of the structure. Four additional inclinometers were installed to monitor lateral deflections in the soil. Readings are being taken every few hours. Five EL beams and two additional inclinometers also have been installed at Golden Mile Tower, an office and shopping complex just 200 m from the accident site.

It also was necessary to protect the Merdeka Bridge next to the accident site. The approach slab before the abutment of the bridge had collapsed. Three days after the accident, a cut was made between the first and second spans of the bridge to separate the first span and mitigate the risk of the first span dragging down the entire bridge in the event of movement of the first span. Eight prism points to measure x-y-z movements and five tiltmeters were installed to monitor the bridge.