TUNNELING UNDER TRAINS

The Singapore deep tunnel sewerage system consists of a 90 km long tunnel network that will convey wastewater by gravity flow to two new secondary treatment plants being planned for the island nation. The sewer tunnel must pass under a pair of soft ground tunnels for the 20 km long Northeast Line mass transit extension project. Because a deeper excavation of the sewer tunnel would increase operational costs substantially, the Singapore Ministry of the Environment wanted to place the sewer tunnel as close to the mass transit tunnels as possible. The rail line will be operational by the time sewer tunnel construction is underway, and the 5.8 m diameter rail tunnels will be extremely sensitive to ground movements. So the Ministry decided to construct an underground chamber for the sewer beneath the transit tunnel alignment in advance of the rest of the sewer tunnel. This soft ground chamber will enable the sewer tunnel boring machine (TBM) to later pass 2.5 m below the finished rail tunnels.