Largest tunnelling effort in Australian history underway

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Five tunnel boring machines now active under Sydney.

Five 1,000 tonne tunnel boring machines are now working concurrently under Sydney in the largest tunnelling effort in Australian history. The machines, overseen by an integrated team, including Bechtel, are building tunnels below the city’s central business district and digging the first-ever rail tunnels under the iconic Sydney Harbour for Sydney Metro.

The final machine, a specialised mixed shield slurry tunnel boring machine (TBM), began digging in August. Together, the five machines will construct 31 kilometres of new tunnels for Sydney Metro. The metro is Australia’s biggest public transport project. Services started in Sydney’s north west (from Tallawong to Chatswood) in May this year, with a train every four minutes in the peak.

Work is well underway to build Sydney Metro City and Southwest, which includes delivering 15.5km twin tunnels from Chatswood to Sydenham under the Sydney CBD and Sydney Harbour.  As Sydney Metro’s delivery management partner, Bechtel is overseeing the delivery of the tunnelling contract which was awarded to John Holland CPB Ghella Joint Venture in 2017. In addition to building twin tunnels with five TBMs, the team is excavating six new stations within the heart of the city.

As the final TBM began to dig the historic crossing deep under Sydney Harbour, the project passed its tunnelling halfway point. “It’s been a huge month for the Sydney Metro team as they surpassed the halfway point in tunnelling for the City and Southwest and launched a giant borer, named Kathleen, under Sydney Harbour. Kathleen will dig at depths of up to 40 metres,” said Ged Silva, Bechtel’s regional manager for Asia Pacific.

Each machine is 150 metres long – longer than two Airbus A380s – and specially designed for Sydney’s sandstone geology. The borers operate as underground factories, digging and lining the tunnels as they go. Collectively, the machines will excavate 5.9 million tonnes of rock – enough to fill about 940 Olympic swimming pools. The excavated material from the new tunnels will be reused at another Bechtel project, the new Western Sydney Airport, where it will be used as a base for the new runway.

Almost 100,000 pre-cast tunnel segments will be used by the mechanical moles to form the rings that make up the tunnels. The shape of each ring determines the curvature in the rail line.

In 2024, Sydney will have 31 metro railway stations and a 66 kilometre standalone metro railway system, revolutionising the way people travel around Australia’s biggest city.