A case study of two tunnels driven in the Singapore ‘Boulder Bed’ and in grouted coral sands

Abstract

Abstract As part of the construction of the Singapore Mass Rapid Transit, two tunnels had to be driven from a shaft to Raffles Place Station. Each drive was approximately 170 m long and was mostly in the Singapore ‘Boulder Bed’. This formation underlies much of the Central Business District of Singapore, and is thought to be a colluvial deposit comprising sandstone boulders in a stiff clay matrix The first 60 m of the upper tunnel had a mixed face with an old coral reef above axis and the Boulder Bed below it. The corals and sands of the reef were highly permeable and had to be injected with chemical grout before tunnelling commenced. The Raffles Place area is in the commercial heart of Singapore and the tunnels passed close to two high-rise buildings. One of these buildings had an underground car park within 5 m of the upper tunnel. Shield drives in the Boulder Bed had recorded high surface settlements, and there was concern about the underground car park which was known to be sensitive to settlement. It was proposed to drive the tunnels using the New Austrian Tunnelling Method, and one of the justifications for choosing the technique was that the resulting soil movements could be minimized. In order to check this claim surface settlement arrays, inclinometers and extensometers were installed to monitor movements caused by the tunnelling. Records of ground movements measured are presented, and compared with the movements recorded above shield driven tunnels in the same deposit. Methods of grouting and testing used in the coral and sands are also reported.