Sydney harbour crossing awarded

The AUD 2.81bn contract is for twin 15.5km tunnels and associated civil works, part of Stage 2 of the Sydney Metro project. Some 57 cross passages will be required, spaced approximately every 240m along the tunnel. Also required is a temporary access shaft and tunnel dive structures at Chatswood and Marrickville, a crossover cavern (for trains to cross between tracks) and six new underground stations.

Excavation works are expected to commence within weeks and the contract should be completed in mid-2021.

CPB Contractors and John Holland previously worked with Dragados to build 15km of twin tunnels for the Sydney Metro Stage 1, an AUD 1.15bn contract.

Transport for NSW conducted geotechnical work deep under Sydney Harbour to help determine the best way to deliver the new metro railway tunnels, taking rock and soil samples from more than 50 boreholes.

These geotechnical works settled the decision to use TBMs to tunnel through a combination of sandstone, clay and sediments between North Sydney and the new metro station at Barangaroo.

Five TBMs will be used to excavate the 15.5km twin railway tunnels from Chatswood to Sydenham, including four double-shield, hard rock, gripper type TBMs and a slurry machine. The TBMs will have a 7m diameter cutterhead and as they excavate each 1.7m section of tunnel, a vacuum lifting device will install six concrete segments on the walls to form a ring. Once constructed the internal tunnel diameter will be 6m.

Additional geotechnical investigations will be undertaken as part of detailed design.

The geology along the entire project alignment, which includes the tunnels and the station civil works, is 83 per cent sandstone, 10 per cent shale along with sedimentary deposits. The five TBMs will excavate through rock which is made up of 93 per cent sandstone.

Tender is underway to select a TBM supplier. The first of the five tunnel boring machines will be in the ground before the end of 2018.

Detailed design is underway on the segmental lining for the excavated tunnels to determine the optimum concrete mix and waterproofing requirements.

A precast facility will be set up at Marrickville to produce about 99,200 concrete segments needed for the project.