Tunnel Construction
Singapore: This island state has begun a major project to upgrade its sewerage infrastructure to meet the economic and environmental demands of the 21st century.
The project, known as the Deep Tunnel Sewerage System or DTSS, consists of two large tunnels with a total length of 80 km and 170 km of smaller link sewers. The large tunnels are 50 meters below the surface and have diameters up to 6.5 m. The tunnels carry the sewerage to two new centralized water reclamation plants. Treated effluent is then discharged into the Straits of Singapore via deep sea pipelines.
A portion of this tunnel system, covered by contract T04, was awarded to Samsung Engineering and Construction. It specified construction of 7.3 km of tunnel by tunnel boring machine and 800 m of tunnel by NATM. The main exit for the NATM tunnel is Shaft F. It was excavated about 40 meters from an elevated mass-transit railway. Because this was within a railway protection zone, building codes required that the construction had to be monitored in real time.
Before construction work began, three of the columns supporting the elevated railway were instrumented with EL tiltmeters to detect construction-related movement. Two tiltmeters were installed on each column, one to detect movement in longitudinal direction of the railway and the other in the transverse direction. The tiltmeters were located high up on the columns to prevent them being disturbed by the public. The columns are shown in the photograph at right.
The soil at Shaft F consists of layers of soft peat above granite, which had to be excavated by blasting. The excavation walls were formed by secant pile walls, shored temporarily by struts and walers at six elevations. Twenty-one spot-weldable VW strain gauges were installed on the struts and walers to monitor changes in stress throughout the construction activity.
The strain gauges and tilt sensors were connected to a CR10X data logger which was equipped with a wireless GSM modem. A computer at the site office, some 3 km away, calls up the data logger and retrieves the data. MultiMon software running on the office computer then processes and displays the data, all in near-real time. The two screen shots at right (columns and engineering drawing) show how MultiMon overlays a representation of the site with color-coded data boxes. If readings approach alarm levels, the data boxes turn to yellow or red. Monitoring was still in progress at the time of writing and will continue for three years.
CEP, Slope Indicator's representative and exclusive distributor in Singapore, supplied and installed the instrumentation for this project. We would like to offer special thanks for help in writing this story to Mr Chris Smart of Samsung Corporation, Survey Manager for the DTSS T04 project.
