tunnel boring machine
A tunnel boring machine or TBM. Image – Canva
  • Four huge tunnel boring machines have completed 20 months of digging
  • This marks a major milestone for Melbourne's Metro Tunnel Project
  • The massive machines will now be retired, their 'skin' forming a lining for the tunnels

Four huge tunnel boring machine (TBMs) – Milli, Alice, Joan and Meg –  have reached the end of what has been a 20-month long project digging underneath some of Melbourne’s most populated areas.

The four worked together to create two nine kilometre-long, 6.3metre high rail tunnels for Melbourne’s Metro Tunnel Project.

The outcome is a remarkable feat of engineering with some tunnelling 12 metres below Melbourne’s Yarra River, while other sections were dug just 1.5 metres below the still active City Loop train line.

Meg was the last TBM to arrive at what will be the Town Hall Station, located 23 metres below Swanston and Collins streets. Millie, Joan and Alice staggered their arrival over the past month.

A concept image of what will be Melbourne’s Town Hall Station. Image – Metro Tunnel

The four machines have been a part of a huge undertaking, tunnelling an average of 90 metres a week,  digging as deep as 40 metres below ground level and boring through basalt rock, gravel, silt and clay.

TBM Alice scored the record rate of 195 metres in a single week.

The TBSs have not only been digging but also installing waterproof concrete lining throughout the tunnel network.

The TBMs have successfully finished the network of tunnels between the planned five stations. However, roadheaders – which are also huge digging machines – are still at work in the final sections of tunnelling alongside the Town Hall Station platforms.

Victoria’s Minister for Transport Infrastructure, Jacinta Allan, is pleased with the milestone.

“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 with a huge amount of work to be done to finish the stations and get the tunnel ready to deliver more trains, more often.”

Jacinta Allan, Minister for Transport Infrastructure

“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 is supporting thousands of jobs.”

What is next for the now-retired TBMs?

They will be dismantled underground and their parts retrieved. The ‘skin’ of each machine will actually be left behind to form a part of a permanent lining. Other parts will be assessed for reuse.

The focus now shifts to the next part of the project. Construction is underway on 26 passages – shorter tunnels that will connect the main tunnels. These are an important safety feature in the case of incidents underground.

An array of systems are still to be installed in the tunnels. These include train controls, security, communications, mechanical systems, electricity.

The tunnel system is said to transform transport for Melburnians, reducing transit times from suburbs to the city by up to 50 minutes a day.

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