During tunnel construction, the nature of the rock largely determines how the tunnel is designed and how the work is carried out. The tunnel boring machine Åsa was specially adapted to cope with the conditions in the Hallandsås ridge. The machine both broke the rock and constructed a tight concrete tunnel. Around 20 people worked per shift on the tunnel boring machine. Åsa was put into operation in 2005, and the tunnels through the Hallandsås ridge could open just over 10 years later.

This type of tunnel boring machine mainly consists of a shield and a drilling head. The shield protects the workers from potentially falling rock. The machine progresses by pushing the rotating drilling head against the rock and breaking it loose. The excavated rock is then transported out of the tunnel via a conveyor belt. Groundwater leaking in is pumped out to a water treatment plant outside the tunnel.

To prevent water from flowing into the finished tunnel, it is lined internally with concrete, a method called lining. The tunnel is built by the machine drilling forward and then stopping to construct a watertight concrete ring. A ring consists of eight parts – called segments. They are placed in position under the shield by a robot. The machine progresses with the help of jacks that brace against the completed tunnel ring.

Ahead of the borer Åsa, the Hallandsås ridge was continuously surveyed by drilling three holes into the rock. The holes were about 54 metres long. This was done to determine the quality of the rock in front of the machine and the prevailing water conditions. If the rock in front of the machine contained a lot of water, it needed to be pre-treated. The treatment involved sealing the water-bearing fissures in the rock in front of the machine with cement.

The borer Åsa required regular maintenance. Workshops were located on the work site where various types of repair work were carried out. Major maintenance work was carried out at least every 1000 metres. In addition to this, the cutters on the drilling head wore out and needed to be replaced continuously. On average, one cutter was replaced per constructed ring. This was done from inside the drilling machine.