And finally… How the world’s longest immersed tunnel will be built

Fehmarnbelt TunnelConnecting Rødbyhavn in Denmark with Puttgarden in Germany, the 18km Fehmarnbelt Tunnel project will be the world’s longest immersed tunnel and the world’s longest road and rail tunnel under water.

In contrast to a bored tunnel, an immersed tunnel is made up of hollow concrete elements, cast on land and assembled section by section to form the tunnel.

A trench for the tunnel must first be dug in the seabed in order to build the Fehmarnbelt link. This trench will be up to 60 metres wide, 16 metres deep and 18 kilometres long. In total, some 19 million cubic metres of stone and sand will be excavated from the seabed. This will be used to establish approximately three square kilometres of new natural areas on Lolland and, to a lesser extent, on Fehmarn.

When the trench in the seabed is ready, the work of putting the tunnel elements in place can go ahead. Each tunnel element weighs 73,000 tonnes, however it can float in the water because it is hollow and sealed with bulkheads. Large tugboats will tow the elements out into the Fehmarnbelt, where they will be lowered down onto the seabed with a high degree of precision and then assembled.

When the tunnel tube is in place, the technical installations can be completed. These include tracks for the electric trains, communications systems, lighting, ventilation, transformers and pumps. As an evolutionary change to the Øresund tunnel, the Fehmarnbelt tunnel will have 10 special elements with an additional lower floor.

This is what a special element could look like
This is what a special element could look like

A special element will be located approximately every two kilometres of tunnel and will have equipment for operation and maintenance. This means that the Fehmarnbelt tunnel will be cheaper and easier to maintain, and will require significantly less concrete since the standard elements can be made smaller.

Construction of the Danish railway landworks between Ringsted and Rødby is already underway.

Once construction is complete, it will be possible to travel by train between Copenhagen and Hamburg in just 2½ hours compared to 4½ hours today. Motorists will save almost one hour each way on their journey across the Fehmarnbelt.