Large diameter tunnels for high voltage power circuits

Nearly 30km of large diameter tunnels ranging in diameter from 3.5m to 6m have been designed to carry high voltage power cables in Singapore for PowerGrid. Of these, approximately 6km are currently under construction. The 200 million Singapore dollar project began when PowerGrid commissioned Mott MacDonald, in association with Development Resources, to undertake a study into the feasibility of constructing large diameter tunnels to accommodate HV power cables.

The initial scope of the study addressed the feasibility and implications of installing high voltage cables in tunnels, plus the design of two tunnels between Senoko power station and Gambas Avenue in the north of Singapore and between the sub-stations at Ayer Rajah in the west and Paya Lebar in the east. The study also developed route options and planning procedures for constructing an island-wide network of cable tunnels.

The Senoko to Gambas Avenue tunnel comprises 1.7km of twin 3.5m diameter tunnels accommodating ten power circuits. This contract includes the construction of four deep shafts. The Ayer Rajah to Paya Lebar tunnel totals 17km in length and measures 6m in diameter, also housing ten power circuits. The design includes ten shafts, with a maximum depth of 55m for general construction purposes plus cable entry and the permanent ventilation of the tunnels.

The feasibility of a third tunnel between Seraya and Labrador Sub-station, along the west coast of the island was also included in the study. The 8.5km route (4km of undersea tunnel and 4.5km of land tunnel) examined new bored and immersed tunnel solutions and the refurbishment of the existing undersea cable tunnel. Preliminary engineering was undertaken and tender documentation prepared to enable design-and-build contracts to be sought.

Fundamental to the design are the cooling requirements for the power circuits, the overall ventilation of the tunnels and space for installation of the circuits, safe working, maintenance and jointing. The tunnels and control buildings are equipped with fire and life safety facilities designed and specified by Mott MacDonald specialists.

Our commission on the project included the management of the overall tender process and went on to involve construction supervision. Completion of the Gambas Avenue and Labrador tunnels is scheduled for 2005.