Recent large-diameter tunnel construction in Singapore using sprayed concrete linnings

Abstract

Sprayed Concrete Lining (SCL) or the New Austrian Tunnelling Method (NATM) has had limited exposure in Singapore. Previously this has been restricted to excavations of less than 6 metres inside diameter in the boulder clay and decomposed granite. For the North East Line Project (NELP) a section of SCL tunnel was constructed as part of the tunnelling works on Contract 710. This paper describes the design, planning, and construction issues of twin 8.4-metre inside diameter crossover tunnels spaced less than one diameter apart constructed in the Jurong Formation. In addition, the construction of a SCL tunnel in the Old Alluvium, is also described. This tunnel is an advance tunnel for the future Deep Tunnel Sewage Scheme (DTSS) and is being constructed as part of the NELP Contract 705 Works. Details of the Jurong Formation and Old Alluvium are given together with their geotechnical characteristics, which played a major role in the selection of temporary support methods used and the behaviour of the ground during tunnelling. The planning and design issues concentrate on aspects to ensure overall integrity and stability of both SCL projects. The success of various construction techniques utilised to construct the tunnels safely is discussed and a summary of the ground movements is presented.