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Metro Tunnel

Tunnel Boring Machines

Tunnel Boring Machines (TBMs) are large machines that tunnel through ground, progressively installing concrete linings to support the excavated tunnel. The excavated material is transported through the machine to the surface for removal by trucks.

TBMs have been used to build Sydney Metro Northwest's rail tunnels and London's Crossrail tunnels.

How do they work?

TBM cross section

TBMs have two main elements:

  • The first part uses a rotating cutter head that excavates through the ground and disposes of the excavated rock and soil via a conveyor belt or pipes that run through and behind the TBM.
  • The second element involves erecting pre-cast concrete segments from within the TBM, which provides a final lining to support the ground and minimise groundwater seepage into the tunnel.

At the start of the process, a large shaft is dug from ground level down to where the TBM needs to start tunnelling. The TBM is lowered into the shaft in sections and assembled before it begins excavating towards a station or tunnel entrance. It is then 'pulled' through an excavated station box to begin excavating the next section of tunnel, or extracted at a station or retrieval shaft and taken back to the launch site to start digging another section. The eventual sequence of the Metro Tunnel TBMs will be determined by the appointed contractors.

TBM launch and retrieval shafts

A number of TBM launch and retrieval shafts will be needed to support tunnelling activities.

The TBM launch site for the western section of the project will be likely located at Arden station and a TBM retrieval site is planned for the western tunnel entrance area.

For the eastern section of the project, the location of the proposed new Domain station on St Kilda Road has been identified as a potential work sites to support tunnelling activities.

A range of activities need to take place around the TBM launch shafts to support tunnelling operations. These include office and site facilities, the processing and management of excavated material prior to disposal, as well as storage of the concrete segments used to line the tunnels. There will also be cranes installed to lower the TBM components into the shaft. A concrete plant within the Arden station site will produce the concrete segments, which will be transported by road to the eastern section of the project.

The TBMs to be used on the Metro Tunnel can work up to 40 metres underground and will likely operate on a 24/7 basis.

Tunnelling under the Yarra River

The Metro Tunnel would also be constructed under the Yarra River, which is up to four metres deep in this location. The top of the twin tunnels, each around seven metres in diameter, will be around seven metres below the riverbed, east of the Princes Bridge.

To minimise impacts on the Yarra River and surrounding areas, tunnelling using TBMs has been selected as the preferred method of construction for this section of the project. Other options considered include an 'immersed tube' and a cofferdam construction method.

Did you know?

TBMs cost around $20-$30 million each and can move between 300 and 360 metres a month, depending on ground conditions.