The Fehmarnbelt fixed link is being constructed to represent the world’s longest underwater immersed rail and road tunnel. Poised for completion in 2029, construction began in 2020 on the Danish side and 2021 on the German side. The project is challenging traditional engineering practices and setting new records. The third and final part of a three-phase infrastructure development will link Danish island Lolland and German island Fehmarn across the Baltic Sea and include a two-track railroad and four-lane roadway. State-owned Danish company Sund and Bælt is owner and developer of this engineering advancement. The Danish planning company Femern A/S is charged with construction and delivery with the Ramboll-Arup-TEC Joint Venture partnership as the main advisor and consultant.

Engineering feats

Not only is the Fehmarnbelt tunnel a large and long tunnel, but it also represents Denmark’s largest infrastructure project as well as Northern Europe’s. It is five times as long as the Øresund Tunnel, which much of its design was gleaned from. It is also three times as long as the Transbay Tube. This is the previous record-holder for longest immersed tunnel.

As an immersed tunnel, its parts will be constructed elsewhere and put in place on site. This is described by the CEO of Sund and Bælt, Mikkel Hemmingsen, as similar to using Lego blocks. This is a more efficient and better method for the environment than more traditional boring methods. It will use industrial series production. He described the process as more of building an IPhone than traditional construction. While it might appear simple in construction, the pre-engineering was very detailed and complex.

Seventy-nine 217 meter (3008.5 feet) long pieces and ten shorter, but wider, pieces will comprise the tunnel. Each of the 79 segments weighs a whopping 73,500 metric tons (81,020 U.S. tons). Nine weeks are needed for one complete element. The construction is being completed by six optimized production teams. This has been a long prepared-for project. There were concrete tests being performed as much as fifteen years back to prepare for this tunnel.

A 200 hectare large harbor and special tunnel factory were needed to accommodate the material intake of sand, cement, steel, and gravel and construction of the massive precast units. The components will be assembled, closed with water-proof bulkheads and transported to a dry-dock to be flooded to allow them to float. An innovative ventilation system is required for finalizing the tunnel’s interior ballasts, road surface, and mechanical systems. After construction, there will be a 24-hour manned control center and computer-managed traffic control system for utmost safety. All travel corridors are completely isolated as well to minimize accidents or interruptions.

Impact of revolutionary travel

This tunnel is being created for three main reasons: to remove a bottleneck, reduce time required to travel, and strengthen connections between Scandinavia and Central Europe. A trip from Rødbyhavn to Puttgarden will be possible in under ten minutes by train or car. Currently, this journey requires a ferry and well over an hour or around six hours on land. A passenger train trip from Copenhagen to Hamburg will be halved: from four and a half to two and a half hours. Even passenger trains not taking the tunnel will benefit, as freight will no longer require these routes as often.

Sustainable travel is being realized with this tunnel’s creation as well. This will enhance electric freight transport train travel for all of Europe. This is a component of the green transition as carbon dioxide emissions will be reduced. An entire 160 km (nearly 100 miles) detour will be removed and more rail transport will be assumed. Sund and Bælt’s sustainability strategy is composed of four parts. First, forming a crucial element of Europe’s sustainable transport network with electric freight train travel and promotion of electric vehicles. Second, constructing the tunnel in a climate friendly by generating less carbon emissions during construction than defined, minimizing carbon footprint of building materials, and building the link as a long term investment. It is planned to remain operable for 120 years. Third, establishing a foothold in the green building and construction sector with green pilot projects, alternative fuel use and open knowledge sharing. Fourth, ensuring the tunnel’s operation is green with all green energy powering it, offsetting emissions with forestation and deploying energy-efficiency.

In fact, this construction recognizes social, environmental and economic sustainability. Due to its location, German, Danish and European environmental legislation is being followed. Where natural areas are interrupted, new natural landscapes will be constructed to double what was lost (i.e., eight-hectare replacement for Strandholm Lake will be replaced with a sixty-hectare new nature section). Stone reefs are being reconstructed with stone protections to work as habitats for nearby animal and plant life. Green corridors will allow insects, amphibians, orchids, and other plants and animals to flourish. Ægir is an environmental portal that houses results from sensors, surveys and other live data. It shows monitored information on the newly created nature areas and marine life and health through maps and models.

Sund and Bælt are also working with contractors and trade unions to ensure the health, safety and well-being of workers is prioritized. 1,300 workers are living in special accommodations with detailed requirements to verify livability. There are strict guidelines for pay, working hours and pensions to stay in compliance. Apprenticeships and internships for students and thousands of construction, maintenance and operations jobs are being created with this project. The Target Zero - A State of Mind campaign aims to create the safest construction site in Europe with regular safety inspections, training and oversight.

Infrastructure and construction

Map on their site showcasing construction sites: https://femern.com/the-construction/how-were-building-the-fehmarnbelt-tunnel/

Over its eight to nine year construction plan, there are eight phases this project will undergo. The first phase is site preparation. From channels and access roads to newly installed water holes and amphibian fences, both sides of this project required a lot of construction site creation. There are two major work harbors created on both sides of the tunnel. The Danish side began in July 2022 and Germany’s in July 2023. Major dredging took place in 2021 until expected 2024. Nineteen million cubic meters (670,978,668 cubic feet) of soil, sand, and stone will form new beaches and land areas after being removed. A tunnel village to house workers was completed in spring 2023 and the tunnel factory in August 2023. A portal is required on both sides of the tunnel to transition from standard roadway to beneath the surface. A light grid will allow a smooth transition from natural light to tunnel light for travelers. The Danish side was set to finish in 2024 and the German side in 2025. The first element of the tunnel is expected to be immersed sometime in 2025. This will transition into the first technology and equipment installation immediately following. Ventilation, communication, lighting, signage, and transport ways are included in this phase. The final phase consists of testing and operation. The opening of the tunnel is set for 2029, in just four years.

In fact, as of February 2025, the first two elements left the factory and moved into the flooded basin.