Fehmarnbelt Tunnel: Building the World’s Largest Immersed Tube Tunnel

By Emily Newton

Once the Fehmarnbelt tunnel construction process is complete, there will be a direct route from the Nordic countries to Central Europe. It will reduce transportation bottlenecks and decrease travel time, freeing road capacities and lowering carbon dioxide emissions.

The Timeline for the Fehmarnbelt Tunnel

The Fehmarnbelt tunnel is an immersed road and rail tunnel that will connect the German island of Fehmarn to the Danish island of Lolland. Upon completion, it will reduce travel time, cutting the trip from a 45-minute ferry crossing to a 10-minute drive or a seven-minute train ride. It will also almost halve the journey time for trains running between Copenhagen and Hamburg.

Building the world’s largest immersed tube tunnel is no easy feat. The Fehmarnbelt tunnel construction process involves dredging an 18-kilometer trench from Denmark to Germany, where professionals will interlock separate tunnel elements.

The Lolland-based factory producing the parts finished construction in 2023. Femern Link Contractors — the state-owned Danish firm in charge of design and construction — expects to install lightning, communication and ventilation systems once it begins assembling the first segments in 2025. If everything goes to plan, the tunnel will open sometime in 2029.

However, a number of technical challenges have arisen. For instance, conventional dredging equipment can only reach so far, and the tunnel sits 40 meters below the Baltic Sea’s surface at its deepest point. Dredging is essential in waterway construction because it facilitates a durable, stable foundation, so no alternative is feasible. The crew needed specialized equipment ranging from purpose-built grab dredge pontoons to trailing suction hopper dredgers.

High-Capacity Production Lines

The project facility in Rødbyhavn on Lolland has been dubbed North Europe’s largest construction site. It spans around 500 hectares and includes a work harbor, a manufacturing plant and accommodations for workers. Henrik Vincentsen — Femern’s chief executive — has emphasized the enormity of the facility and believes the project is breaking records.

The work harbors on Fehmarn and Lolland will facilitate the transport of building materials on both ends. Since the factory’s completion, additional conveyor belts have been added to ensure the rapid site-wide transport of essential materials.

All six production lines are now at capacity, helping produce the 89 prefabricated tunnel sections needed to form the 18-kilometer tunnel. The 79 standard units are 217 meters long and 42 meters high, each weighing 73,000 tonnes. The 10 “special” variants are each 39 meters long, 47 meters wide and weigh 21,000 tonnes. They must cure for three weeks before professionals can install the steel bulkheads and ballast tanks to facilitate immersion.

Once manufacturing finishes, the Femern crew will use a tugboat to drag the prefabricated sections to their destination in the Fehmarn Belt. Each section has a ballast tank, enabling it to float despite its massive weight.

Some have already been towed to the lower basin. However, tensions are rising as deadlines grow closer. Although the factory has reached full production capacity, it is racing against the clock. Even if every part was ready and waiting, immersing each one is tedious, taking around 50 hours on average.

Upcoming Work

The first cast was completed in May 2024 and placed in a dry dock before being moved to the lower basin. With less than half of 2025 left, the contractor is still preparing for immersion. Phase six of the eight-phase project — immersing the tunnel sections — is scheduled to begin in 2025, so it is still on schedule. However, time is of the essence.

As of February 2025, seven standard tunnel elements and two special elements have been cast, which amounts to just 10% of the total precast units needed. The first few have been moved from the dry dock to the large basin in front of the factory, where they will remain until Femern adds the final touches. In the near future, more will follow.

Femern has roughly 3.5 years left to cast, fit and immerse 80 more prefabricated units while installing lighting, communication, signage and ventilation equipment. Then, it must conduct necessary safety tests and evaluate contingency procedures before opening the Fehmarnbelt tunnel to traffic.

Although it still follows the Fehmarnbelt tunnel’s construction timeline, it may push back the tunnel’s opening to get more time for final touches and tests. There is no harm in delaying to ensure integrity and safety. However, many eyes are watching this multibillion-dollar project, so being behind schedule and going over budget is not ideal.

Innovative Approaches

Femern has found ways to save time during the Fehmarnbelt tunnel’s construction. Typically, underwater welding is vital for undersea construction. However, becoming an underwater welder usually takes years of training and dedicated experience. To overcome this, the project leverages prefabricated parts, remote monitoring, special brackets and pressure differentials.

Crew members on a specially designed vessel will conduct the placement and immersion processes. They can achieve millimeter-level precision thanks to internet-enabled sensors embedded within the units and on the sea floor. Each section has a steel beam that fits into the corresponding bracket on the previously placed element, locking it in place.

When they pump out the seawater trapped between the new and existing sections, they create a pressure difference, further pressing both together. The outer rubber gasket makes the seal watertight, ensuring safety and durability.

Innovative dredging, material transportation and fitting techniques have streamlined this project. As a result, Femern has prepared the site, constructed work harbors, dredged an 18-kilometer trench, built a 500-hectare facility and produced several tunnel sections on schedule. Even if it falls behind during phase six, it may be able to make up for lost time.

Making Progress

With the Fehmarnbelt tunnel construction project officially underway, estimating the actual completion date is easier. So far, Femern has met every deadline, which is promising. However, even with the factory having reached full production capacity, the immersion process is lagging. Still, the initial 2029 completion date may not be out of reach if the crew moves quickly.

Emily Newton is a construction and industrial journalist.

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