doi:10.3850/978-981-08-6396-8_P110

ABSTRACT

Singapore’s Downtown Line (DTL) Contract C908 comprises the design and construction of Telok Ayer Station and 300m long Cut and Cover Tunnels. This paper discusses the construction of the tunnels where they cross over the existing East West MRT Line (EWL) stacked tunnels at the busy road junction of Cross St and Robinson Rd. With a clearance of only 700mm between the existing and proposed tunnel structures and strict limitations being placed on the allowable movement of the existing railway, the design and method of construction was carefully thought through and periodically reviewed. 2D and 3D design analyses were performed to assess the effect of the works on the EWL. Robust 45m deep diaphragm wall headwalls were anchored into the Fort Canning Boulder Bed rock. Diaphragm wall side retaining walls were installed between the headwalls and across the top of the EWL with the trenching depth being physically controlled. Working under an overhead viaduct, a low headroom machine completely replaced the bouldery fill, which would have obstructed pipe jacking, with a homogeneous bentonite-cement mix. This matrix also provided a base level strut against the side walls. After excavating to below base slab level on each side of EWL, interlocked steel pipes were jacked across the top of EWL from one headwall to the other, forming a blanket directly above EWL to manage upward heaving forces and groundwater loss. Vertical steel reaction frames, jacks and ground level beams were provided as a contingency to actively control any EWL tunnel heave. During the whole three year construction period, the EWL was monitored and real time readings immediately uploaded onto a project instrumentation web site. Electro-level sensors, vibration meters, reflective prisms and automatic total stations inside the EWL tunnels measured any movement so that jacks could be engaged to directly counter any movement.

Keywords: Tunnel construction, Pipe jacking, Existing MRT tunnel.

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