A 4.3m diameter Robbins double shield TBM is scheduled for delivery early next month, ready to start construction in December on the first of twin 1.8km long cable tunnels in the Woodlands district of northern Singapore.

The S$77m design-and-construct contract runs from Senoko power station to Gambas and is being built by Japanese-Australian joint venture Obayashi/McConnell Dowell which won the award in April 2001.

Currently the joint venture is on schedule to complete construction by November of the 12m diameter, 50m deep TBM starter shaft at Gambas. The shaft is being sunk as an 800mm thick diaphragm wall to 40m with the remainder excavated by drill and blast. The rock sections are to be concrete lined with a waterproof membrane installed.

The contract also includes the construction of a similar sized TBM exit shaft at Senoko power station and the Woodlands shaft, which will house the tunnel ventilation and cooling equipment.

When the TBM goes to face in December it will bore through mostly granite with strengths

up to 300Mpa. The machine will operate in hard rock mode with the ability to thrust off the segmental lining that the machine will install, leaving a tunnel i.d. of 3.7m.

The segments for the lining are being manufactured in Malaysia with the moulds being delivered from Taiwan.

The two high voltage power cable tunnels run parallel with short cross passages at regular intervals. Tunnelling of the first bore is scheduled for completion in April 2002 when it will be handed over as a stage 1 completion to the client, Power Grid. The second drive is to start between June and September 2002.