Travel between countries in Europe is like traveling between American states. Yet, some connections are not so simple.
Hence, the Fehmarnbelt tunnel, which will connect Germany to Denmark. Upon completion, the 18-km (11.2 mi.) long tunnel will be the world's longest immersed tunnel connecting the northern German island of Fehmarn and the south Danish island of Lolland. The tunnel will be a game changer in terms of traveling within the European Union, and its complex construction is worthy of study.
(Video courtesy of Femern A/S)
If You Build It, They Will Come
Currently, it takes approximately 60 minutes to cross between the two areas and over the Baltic Sea. When the tunnel is complete, the trip will be just seven minutes by train and 10 minutes by car.
The tunnel will be heavily trafficked if the owner's projections are correct. They anticipate the ADT will be more than 12,000. In addition, they expect 111 trains to travel through the tunnel daily. It will have five tubes — two for a two-lane motorway in each direction; two electrified rail tracks; and one for a service passageway.
Speed is just one of the benefits the tunnel will provide.
"This project aims to address current transportation bottlenecks, improve logistics for freight transport, and provide a faster, more reliable route for both passenger and cargo traffic," said Denise Juchem, spokesperson of the Danish project owner Femern A/S. "The tunnel also will contribute to the European Union's goal of creating a seamless and sustainable transport network across Europe."
Immersing Tunnel
Many technical and construction challenges come with building the world's longest-immersed tunnel. Hence the lengthy construction schedule. Work began in 2020 on the Danish side and in 2021 on the German side, and the project is expected to be completed by 2029.
The first stages focused on extensive preparations. The team built a large work harbor on the Danish side, which serves as a way station for the sand, cement, steel and gravel that will be used to cast tunnel elements.
Hundreds of workers from the contracted international construction consortium Femern Link Contractors (FLC) are building the elements in a huge tunnel factory located behind the work harbor close to the future tunnel entrance. In the factory, which has three production halls and six production lines, numerous teams are casting the tunnel's elements. Work is taking place 24/7. Each of the 79 standard elements is 217 m (712 ft.) long and weighs 73,000 tons. The team projects that it will take nine weeks to produce each project element.
Working in the water has been a significant challenge for the Dutch construction consortium Fehmarnbelt Contractors (FBC). Dredging the tunnel trench, which began in 2021 and was completed in 2024, involved excavating 15 million cu m (19.6 million cu. yds.) of seabed.
"Large excavation vessels, including Magnor, the largest backhoe dredger in the world, was used as part of the process," said Gerhard Cordes, project director of Femern A/S. "The dredged material has been used to construct new coastal areas on the Danish island of Lolland."
The tunnel elements will not be placed directly on the seabed but on a gravel bed.
"The gravel layer in the tunnel trench is laid out before immersion and serves as an accurate foundation and the precision work is carried out by a purpose-built pontoon," said Cordes.
Moving the element from the factory to the sea is a major undertaking. Before immersion, the elements are sealed at both ends with a temporary bulkhead to keep water out. Immersion pontoons consist of two vessels supporting the element as they are floated from the work harbor to where they are to be placed.
Cordes explained the process of moving the element to its location in the water.
These pontoons hold the element in place, similar to a clamp, by gripping the element at the front and back. When the element is in the correct position, it is immersed on steel cables and placed close to the installed elements by positioning the immersion pontoons.
A guiding system ensures the exact position relative to the preceding element, alignment is ensured by adjustable supports towards the already placed element, and the first contact between the elements is established with mechanical devices.
The seals' main compression and, thus, the element joint's closure occurs without technical assistance but purely using water pressure. As soon as the water is pumped out of the cavity between the two bulkheads, atmospheric pressure is established in the chamber and the water pressure from the opposite end of the element compresses the element joint.
The challenges do not end there. Laying the tunnel elements in the water correctly requires extreme precision. The team uses a positioning system specially developed for this project. The system makes it possible to immerse with an accuracy of a few millimeters.
"The elements are immersed on steel cables and joined to the elements already installed by positioning the immersion pontoons," said Cordes. "A locking system [pin and catch] secures the exact position relative to the preceding element and the alignment is ensured by adjustable supports."
To add to the complex process, immersion is weather-dependent, and the weather at the Fehmarnbelt can be rough.
The owner of the project estimates immersing each tunnel element will take approximately 40 hours. This includes transportation of the element from the work harbor in Rødbyhavn.
To ensure that everything goes smoothly, detailed simulations are carried out before immersion. The crews involved in the immersion also will use specialized equipment, such as underwater cameras and sensors, enabling real-time monitoring.
Other Benefits, Costs
Besides reducing travel time, the technological wonder is billed as providing green benefits.
The Fehmarnbelt tunnel promotes sustainable transport by providing a faster and more efficient route for road and rail traffic. Switching freight transport from trucks to environmentally friendly electric trains also will be more attractive. This will help reduce CO2 emissions by decreasing travel distances and times.
According to Juchem, the project includes extensive environmental measures to protect the surrounding marine and terrestrial ecosystems, ensuring minimal impact during construction and operation.
The project's projected cost is 7.4 billion euros ($8 billion U.S. dollars). Denmark is responsible for financing the Fehmarnbelt tunnel, but the link is user-financed. Revenue from traffic through the Fehmarnbelt tunnel will be used to repay the loans raised to finance the construction.
When the Fehmarnbelt tunnel is completed, the public will enjoy multiple benefits, such as reduced travel times, improved connectivity between Denmark and Germany, and enhanced economic opportunities. The project will create new recreational areas and natural habitats, improving the quality of life for local communities. CEG
