The Airport link tunnel opened this week. This road network between Bowen Hills, Kedron and Toombul has come at a cost of $4.8 billion dollars. But I wonder if teachers, children and the general public know how these tunnels are excavated?
Enter the Tunnel Boring Machine or TBM for short. These mechanical marvels are at the heart of tunnel digging technology today. The TBM first came into the public spotlight through the construction of the channel tunnel. The Channel Tunnel is a 50.5km under sea rail tunnel that links England and France that opened in 1994. Although this is not the longest transport tunnel, that honour currently belongs to the Seikan Rail Tunnel in Japan at 53.85 km.
It is the TBM that has made this travel revolution possible. The TBM is a complex machine to say the least. At the business end is a rotating cutter head. The cutter head uses several steel cutting disks to create compressive stress fractures on the rock face being drilled. This fractured rock falls away and is scooped in through the cutter head and transported out through the machine on conveyers. The TBM provides itself with forward pressure by bracing itself against the walls of the tunnel it has created. The large hydraulic feet not only provide forward pressure to the cutter head, but also allow the machine to be steered through the ground.
Behind all the cutting and rock removal the machine also lines the tunnel with pre-formed concrete rings. These rings, made from pre-formed concrete pieces provide support to the tunnel walls and result in a smooth safe tunnel ready for the construction of a road or railway. It’s this ability to cut the rock, remove the rubble and line the walls all at he same time that makes these machines so successful. The other added bonus of using a TBM is the limited vibration and noise that these machines create compared with the traditional drill and blast tunnelling methods. This makes the TBM ideal for tunnelling under urban areas such as the suburbs of Brisbane.
The two TBM’s used on the Airport link project “Rocksy” and “Sandy” are 12.5 metres wide with the largest “Rocksy” being 195 metres long, the largest machine of its type ever used in Australia. Each machine was built specifically for the Airport Link project in Germany at a cost of $45 million each, and took three months to assemble at the airport link site. Rocksy even needed to start tunnelling so there was room to build that back half of the machine inside the assembly box at Toombul.
Due to the size of these machines and their specialised construction it was decided that they would be entombed in the ground at the end of the tunnel construction. So next time you dash off to the airport or drive through the link to the city you will drive over the TBM’s which have been buried 16m under the roadway.
The Airport link is the longest tunnel complex in Australia. But even if you add the 4.8km of the Clem 7 to the 6.7km of Airport Link, it is still not half the length of the Laerdel Tunnel in Norway. At 24.5 km in length the Laerdel tunnel is the longest road tunnel in the world. The Laerdel road tunnel is still less than half the length of the longest rail tunnel the Seikan in Japan.
We now await the commencement of the TBM working on Legacy Way, connecting the Western Freeway to the Inner City Bypass and the Clem7 and Airport Link tunnels.