The performance of EPBS and the soil response to the tunnelling was indeed very encouraging. The measured vertical and lateral displacements of soil during tunnel excavation, as recorded by several inclinometers and settlement points installed along the tunnel route, were found to be significantly less and the zone of disturbance was confined to a limited corridor width of 10 m on either side of the tunnel alignment.
The measured soil response in terms of final ground settlement along the tunnel alignment is plotted in Figure 1. The average ground settlement was found to be about 53 mm which is considered very small when compared to the size (3.75 m outside diameter) of the tunnel excavation.
The project demonstrated that careful planning and appropriate selection of tunnelling technology can significantly mitigate construction risks associated with tunnel excavation in poor ground conditions.
A slurry shield was used in Singapore for the first time in 1982 to install medium size sewers (DN900 to DN2100) in water charged silty sand strata. Since then several dozen slurry shields have been used in Singapore to install sewers in similarly difficult grounds.
Soil response to slurry shield tunnelling was measured in the early nineties in a project where five slurry shields were used simultaneously to install 11 km of DN1200 to DN2100 sewers
A typical ground response to slurry shield excavation by a DN1500 shield through very soft marine clay strata is shown in Figure 2.
No significant ground settlement occurred along the sewer route and the average ground settlement was only 15 mm. The very low ground settlement was due to the fact that the slurry pressure at the tunnel was deliberately kept high to balance the soft marine clay.
It is not easy to balance the tunnel face in marine clay due to its fluidity. Its active pressure coefficient was estimated to be 0.9 and thus any slight reduction in slurry pressure will result in free flow of marine clay, causing the ground above to settle, while any increase in slurry pressure will result in upheaval as shown in the Figure 2. The average upheaval of the ground was only 8 mm.
Microtunnelling in Singapore
Hundreds of kilometres of sewers ranging from DN225 to DN900 have been installed by microtunnelling technology since its introduction in Singapore in 1982. Singapore’s exposure to the technology and accumulated know-how and expertise in the trenchless application has enabled many neighbouring countries including Malaysia, Brunei, Thailand China, Korea, Taiwan and Hong Kong and lately India to adopt and use the technology to install sewers in difficult and built-up urban areas.
The most commonly used microtunnelling system was the slurry shield system. However, Singapore has also extensively used the following microtunnelling systems during the initial period in the early eighties. Even today some of these machines are still being used.
• Earth Arrow method (two stage excavation process);
• Iron mole method (two stage excavation process);
• Horizonger method (single stage excavation process); and,
• Slurry shield method (single stage excavation process).
The Singapore experience has demonstrated that trenchless sewers can be laid accurately and at a competitive cost. It shows that the method achieves faster installation at a relatively low cost. Thus the trenchless sewers are expected to lower considerably the overall of construction costs of sewerage infrastructure development and significantly shorten the construction time in developing countries where thousands of kilometres of sewers are yet to be laid in the metro cities.
Microtunnelling in Cobble Stones in Sydney, Australia