Railway tunnel under the Great Belt
Four giant tunnel boring machines were custom-built for the execution in difficult seabed conditions of the eight kilometre tunnel under the Great Belt.
One of Denmark's largest civil works
The eight kilometre train tunnel under the Great Belt is one of Denmark's largest civil works.
The tunnelling work was carried out from November 1988 to August 1996 by MT Group, an international consortium headed by Monberg & Thorsen, which today is part of MT Højgaard.
We bored two parallel tunnels between Sprogø and Zealand with 31 transverse tunnels at 250 metre intervals, which serve as escape routes as well as containing important installations.
Custom-built tunnel boring machines
Four tunnel boring machines were custom-built for the project – each machine with a boring head of 1,000 tons and rotating cutter heads with a diameter of 8.75 metres.
From Sprogø and Zealand respectively, we bored the two tunnels which were joined up under the seabed midway in the Great Belt. At its deepest point, the tunnel is more than 75 metres below the sea surface.
At the tunnel ends, we used the Cut & Cover method, which involves excavating a large opening in which the tunnel sections are cast. The buildings at the tunnel entrances house monitoring and control facilities.
Difficult seabed conditions and new techniques
One of the challenges in executing the project was the difficult seabed conditions. In the upper layers, we bored through moraine clay with granite boulders ten times harder than quality concrete. There were also sand pockets with a water pressure of up to 4.5 atmospheres.
Boring in similar subsoil conditions had never been done before, so we developed new techniques during the project. For example, we coated the concrete reinforcement elements with epoxy to ensure maximum service life and minimal maintenance.