The majority of the T2D tunnels will be excavated using tunnel boring machines (TBMs), with a short section at each end 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 100m in length, with a diameter of around 15m.
  • launched and retrieved from purpose-built sites called main laydown areas.

The TBMs will be manufactured overseas and then delivered to Adelaide. This process will take approximately 18 months and will start after the contract for main construction is awarded, around mid-2024.

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

Tunnelling works for the Southern Tunnels are expected to start in 2026, and the Northern Tunnels in late 2028-early 2029.

TBM operation

The TBMs used to excavate the T2D tunnels will operate continuously, progressing about 8-10 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 and eventually out of the tunnel itself.

A segment erector inside the TBM installs precast concrete tunnel lining segments in a ring to form the tunnel walls. When each ring of ten segments has been completed, hydraulic cylinders press against the tunnel ring to push the TBM forward to 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 (MSVs), which are 27m long and can be driven from both ends.

The rest of the TBM houses logistics facilities needed to operate the overall system, with up to 20 people working inside.