The process of building the longest road tunnel – the world

Fehmarnbelt tunnel may not be as famous as the Manche strait tunnel connecting England and France 30 years ago, but this project is still equally impressive, according to CNN. Connecting Denmark and Germany, Fehmarnbelt will include two -lane highways in both directions, along with two tram lines. All are neatly located in the sinking pipes below the waves in one of the most crowded waterways in the world.

With 18 km, the length of Fehmarnbelt is much lower than the Manche strait tunnel (50 km). However, this is the road and railway tunnel as well as the longest submersible pipe tunnel in the world. Due to being a submerged pipe tunnel, instead of digging through solid soils such as the Manche Strait tunnel, Fehmarnbelt was built by using prefabricated concrete segments placed on a dug groove at the seabed, connected to each other and then buried.

In February 2025, the first prefabricated concrete tunnels left the factory in Rødbyhavn on the Denmark head of the tunnel, completing the first part of the journey from the mainland to the sea expected to be completed in 2029. The project worth 7.7 billion USD has a huge scale in all aspects. Each section of the tunnel is massive. These concrete structures are 217 m long, 42 m wide and 9 m deep, weighing 73,000 tons, equivalent to 10 Eiffel towers. In total, 79 standard pipes and 10 special pipes (39 m long, used to install the electrical equipment of the tunnel) will be placed from one end to the other, from Rødbyhavn on Lolland Island to Puttgarden on Fehmarn Island of Germany, forming a complete tunnel at a depth of 40 m under the Baltic Sea.

In addition, the factory built for the project is the largest tunnel production facility in the world, combined with the port to work on an area of ​​220 hectares, equivalent to more than 300 football fields. The three spacious construction halls contain 6 production lines (5 for the standard tube and one for a special pipe) is working continuously all day for every 9 weeks to complete a 217 m long tube. Casting around the thick reinforced steel frame, each concrete tube consists of 9 parts of 24 m long. After that, the tube was pushed to the low -lying tank in the area in front of the factory to check the quality and install the device.

“We try to install many devices as much as possible. It is the necessary equipment in the finishing tunnel,” Denise Juchem, spokesman for Femern A/S, Denmark Company in charge of the project, said. “This will save time, reduce the challenge of transportation for work in the submerged pipe tunnel.

The steel baffle tanks are fixed at each end of the pipe so that they are waterproof. Such pipe sealing helps them emerge despite the huge weight. After emerging, the tube is pulled into the lower basin to prepare for the final journey to the tunnel groove. The construction house has not closed the time to submerge the first tube but the preparation is taking place. “This is a very complicated and dependent process on the weather. Currently, we are checking the ships specially built for the project,” Juchem shared.

Engineers will use two floating bridges “Ivy 1” and “Ivy 2” to make sure each tube is lowered in position. The floating bridge will be attached to the beginning of the pipe before a fleet of pulling them put them in the position above the 12 m deep groove, 100 m wide and filled with gravel, completed last year. The submerged process will be monitored from the control room on the buoy bridge. The long steel wire winch of more than 200 km will be used to lower the pipe section while the tank is filled with water to help support the structure to the seabed. The underwater camera and its own production device will place the pipe in a position with an accuracy of up to 12 mm. Giant rubber cushions and closed pads are made to survive 120 years will ensure waterproof tunnel.

The construction of the two tunnels in Rødbyhavn and Puttgarden is progressing. Both positions are very close to the port, reducing the journey from 45 minutes by ferry to 10 minutes by car and 7 minutes by train. When the tunnel is open, a tourist from Hamburg to Copenhagen only takes 2.5 hours instead of nearly 5 hours today.