Fehmarnbelt: World’s longest immersed tunnel

The Fehmarnbelt tunnel is an 18-kilometre long immersed tunnel located 40 meters beneath the Baltic Sea connecting the Danish island of Lolland with the German island of Fehmarn. It will be the world’s largest immersed tunnel and includes a two-track electrified railroad and a four-track motorway. It is the largest infrastructure project in Denmark to date, with a total budget of €7.4 billion.

CNN describes Fehmarnbelt as the tunnel that “will change the road and rail map of Europe.” By linking Scandinavia with Central Europe, the tunnel enables a faster journey across eastern Denmark to Germany by taking seven minutes by train and ten minutes by car.

Ramboll has been an advisor to the client, Femern A/S since 2009 within the Ramboll-Arup-TEC Joint Venture partnership. This includes delivering the designs and plans for the tunnel and a wide variety of services such as consultancy in competitive dialogues processes and bud evaluations, support of planning of approval processes on the German side delivering technical support, review design and conducting construction assurance.

Furthermore, Ramboll is providing consultancy on the sustainability strategy covering environment and social sustainability priorities within the construction and operation phases

This immersed tunnel solution challenges existing tunnel building standards and will set a world record.

Fehmarnbelt is five times as long as the Øresund Tunnel, and three times as long as the Transbay Tube in San Francisco, the longest immersed tunnel in the world today.

Constructing an 18-kilometre immersed tunnel requires meticulous design and planning. Our experts within the Ramboll-Arup-TEC Joint Venture have established solid foundations for the project including requirements for the basic geometry of the tunnel and its elements, the precise site location, and preparation of the technical specifications. the Ramboll-Arup-TEC Joint Venture assists the owner in reviewing the contractors’ detailed design for the tunnel.

Planning this massive project includes erecting a large-scale harbour to facilitate the delivery of casting materials. A tunnel factory with six production lines was also built to cast 79 standard elements that will make up the tunnel. Each element is 217 meters long and weighs 73,500 tons. Another 10 special elements were created which are shorter but wider.

After the tunnel elements are cast and assembled within millimetres of accuracy, they are pushed from the factory into a dry dock, which is then flooded. To ensure the elements can float, they are closed with waterproof bulkheads containing air at each end. The first elements will be immersed in 2025.

Once in position below the seabed, the interiors will be put in place such as the ballast, road surface as well as technical, and mechanical installations. This includes a pioneering longitudinal ventilation system and state-of-the-art safety and security features.

When completed, Fehmarnbelt will be as safe as a conventional land-based motorway or railway.

Road and rail traffic are separated into two separate tunnel tubes, one for each direction of travel to minimise the risk of traffic accidents and there will be emergency car lanes and exits in both directions along the entire length. Moreover, the design will safeguard against train derailment.

A computer-controlled traffic control system and a 24-hour manned control centre will also be installed. In addition, there will be traffic information on FM radio, signage for road users, and varying lighting to help maintain drivers’ concentration.

In addition to pushing innovative design solutions, Femern A/S has a strong focus on demonstrating social and environmental responsibility in the construction and operation phases.

The Ramboll-Arup-TEC Joint Venture is assisting Femern A/S with strategy development, implementation, analysis, and impact assessments within three strategic sustainability priorities:

1. Educating the next generation of the workforce through apprenticeships and training. Femern A/S has committed to offering at least 500 apprenticeships in total for all relevant tenders and contracts covering the project.
2. Minimising the project’s climate impact and becoming CO₂ neutral during operation.
3. Implementing one of Europe's safest construction sites by developing a strong safety culture for all working on the project. The labour demand in the construction phase alone is estimated to be up to 20,000 person-years of direct employment, with the strongest demand in 2024-2027.