Four tunnel boring machines (TBMs) have completed 20 months of digging to create two 9km-long, 6.3m-high rail tunnels underneath Melbourne, as part of the Metro Tunnel Project. 

The tunnels are lined with around 250,000 tonnes of concrete, completed by tunnelling 1.5m underneath the City Loop as trains continued to run, as well as 12m under the Yarra River bed and around 7m below the Burnley Tunnel.

TBM Meg arrived at the Town Hall Station site 25m under Swanston and Collins Streets, following Millie, Joan and Alice, which have broken through at the station throughout May 2021. 

TBMs Joan and Meg dug their final 670m under Swanston Street from the State Library Station site over March, April and May 2021. 

TBMs Millie and Alice began tunnelling in December 2020 to complete the final 1.8km leg, located under St Kilda Road and the Yarra River.

It has been a major job for the four machines, which have tunnelled on six separate legs between Kensington and South Yarra, installing more than 55,000 curved concrete segments – creating a waterproof tunnel lining.

“This is a huge milestone for the amazing team at the Metro Tunnel. This vital project is a year ahead of schedule but there’s still a huge amount of work to be done to finish the stations and get the tunnel ready to deliver more trains, more often,” Minister for Transport Infrastructure, Jacinta Allan, said. 

The TBMs have dug as deep as 40 metres below ground (underneath the northern end of Swanston Street) and tunnelled through basalt rock, gravel, silt and clay.  

After starting work in August 2019, the TBMs tunnelled an average of 90m a week, with TBM Alice recording the best rate of 195m in a single week.

The machines have removed 600,000 m3 of rock and soil, or approximately one third of the total 1.8 million m3 to be excavated for the entire project – enough to fill the MCG 1.2 times.

Whilst the TBMs have finished the rail tunnels between the project’s five stations, roadheaders – huge digging machines excavating the CBD station caverns – are still digging the final section of tunnels alongside the platforms at Town Hall Station.

The four TBMs are being dismantled underground and retrieved, with the ‘skin’ of each TBM shield to remain in the tunnels to form part of the permanent lining. 

The remaining components will be assessed for potential reuse on other tunnelling projects.

The next phase of construction is underway on the 26 cross passages – the short tunnels that connect the main tunnels, an important safety feature for passengers should an incident occur. 

The tunnel entrances at South Yarra and Kensington are now completed, allowing the entrance structures to the tunnels to start being connected. 

A variety of systems are needed, including signalling, train control, security, communications, power and mechanical systems, as well as associated control systems. 

Each of these systems must be able to function and interact safely with each other, and the existing network and testing needs to ensure the new tunnels and stations can operate safely and reliably. 

The Metro Tunnel will transform Melbourne’s train network, deliver more trains to and from the suburbs and slash travel times by up to 50 minutes a day.  

“Projects like the Metro Tunnel will connect key locations including the Parkville medical and education precinct, St Kilda Road and the Arden Precinct to the rail network for the first time and are supporting thousands of jobs,” Ms Allan said. 

For more information about the Metro Tunnel Project and TBMs, click here

Image credit: Metro Tunnel Project

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