ABSTRACT

Twin bored tunnels, each 6.4 m in diameter with an aggregate length of some 3.5 km, were constructed using a full-face tunnel boring machine under compressed air between Somerset and Newton Stations for the Singapore Mass Rapid Transit System. The tunnels were bored beneath Orchard Road and Scotts Road close to some of Singapore’s most prestigious and substantial structures. This paper examines certain problems encountered during the tunnelling works, in particular the settlements generated by the work and their effect on neighbouring structures and utilities. The settlements were principally due to consolidation of deep deposits of soft compressible clay encountered by the tunnels. The paper shows how the magnitude of the compressed air pressure had a major effect on piezometric water levels in the clay and hence consolidation settlements. The methods adopted to minimise these settlements are described. The paper also discusses the factors involved in determining an adequate provision of compressed air supply during tunnelling for the Singapore Mass Rapid Transit System.