The majority of the T2D tunnels will be excavated using tunnel boring machines (TBMs). A short section at each end will be constructed from the surface using cut and cover methodology.

TBMs are very large and complex pieces of machinery. A TBM excavates tunnels and can bore through a variety of ground conditions. This allows communities and businesses to continue undisturbed above ground while construction happens below ground.

The TBMs we will use to construct the T2D tunnels will be:

  • around 100 metres long with a diameter of around 15 metres.
  • launched and retrieved from purpose-built sites called main laydown areas.

The TBMs are manufactured overseas and then delivered to Adelaide. This process will take approximately 18 months. It will start after the contract for main construction is awarded.

They will be delivered to Adelaide in modules. It will then take approximately six months to assemble and commission them before they can start tunnelling works.

We expect tunnelling works for the Southern Tunnels to start in 2026, and the Northern Tunnels in late 2028 to early 2029.

TBM operation

The TBMs used to excavate the T2D tunnels will operate continuously and progress about 8 to 10 metres per day.

A cutterhead at the front of a TBM rotates to excavate soil and rock (known as spoil) from the tunnel face using cutting teeth and discs. A series of conveyors move spoil from the front of the TBM to the back. The spoil eventually moves out of the tunnel.

A segment erector inside the TBM installs ten precast concrete tunnel lining segments to form a ring. These rings form the tunnel walls. When each ring is completed, hydraulic cylinders press against the tunnel ring to push the TBM forward and excavate more of the tunnel face.

The tunnel lining segments and other materials needed to work in the tunnels are transported into the tunnels by multi-service vehicles. They are 27 metres long and can be driven from both ends.

The rest of the TBM houses logistics facilities needed to operate the overall system. There will be up to 20 people working inside.