How the immersed tunnel is built

Since the 1930s, concrete immersed tunnels have been constructed for undersea road and rail links throughout the world. The Øresund tunnel between Denmark and Sweden is the longest of its kind. It has won an international reputation for construction methods and is now the model for the Fehmarnbelt project.

The central principle of an immersed tunnel is that it is constructed from elements that are prefabricated in dry docks. As with Øresund, elements for the Fehmarnbelt tunnel will be produced in closed halls where production can proceed independent of weather conditions.

Each element is then floated out to sea, lowered into an excavated trench on the seabed and connected to the previous section. Finally, it is covered over and protected by stones and gravel with the seabed gaining the same level as it had before work commenced.

The design for the Fehmarnbelt tunnel includes 79 standard elements, which are each 217 m long, along with 10 shorter, special elements. The standard sections will be cast at the production site at Rødbyhavn close to the tunnel alignment. It is not yet decided where the special elements are to be produced.

The Fehmarnbelt tunnel will be some 18 km long in total. Currently the world's longest immersed tunnel for road and rail is the Øresund tunnel, which is approximately 4 km.

Take a look at the animation that shows the manufacturing, transport and immersion of elements for the Fehmarnbelt tunnel.