Building the World’s Longest Underwater Tunnel

tunnel in denmark faroe

In the Atlantic Ocean, two hundred miles north east of Scotland, sit the Faroe Islands. Centuries ago, the first Gaelic and Norse settlers had to travel a long way to their new home.  

But even once people had arrived and built their villages, life in the area remained very isolated. That’s because for generations sailing was the only way to get between the islands. But after centuries, life in the Faroe Islands has changed, thanks to the development of a new underwater infrastructure.  

The early 2000s saw a wave of underwater tunnels built with funding from Denmark to connect the islands. Now, 87% of the population is connected continuously, and journeys across the archipelago have fallen from a day to an hour. 

Tunnels that cross rivers and seas can completely transform life within a country, forging new economic links and improving mobility. But beyond the boundaries of individual countries like the Faroe Islands, tunnels can spark international opportunities. 

Stepping up a gear from the Faroe Islands project, Denmark is currently investing in a new underwater development that aims to create a link with a whole other country, Germany – creating the world’s longest underwater tunnel in the process.  

With its international route, ambitious design and complex delivery, the Fehmarn Belt Fixed Link will depend on the latest innovations from the construction industry – but the benefits across Europe could be huge.

The First Underwater Tunnel

At the start of the nineteenth century, London was an extremely busy shipping port. Docks had been constructed on both sides of the Thames at Wapping and Rotherhithe, once the river became too busy for the floating wharfs originally used to offload goods. It became clear that a tunnel connecting the northern and southern docks would make it much easier and quicker to transport incoming goods further on their journeys.

However, an underwater tunnel had never been constructed before – and brand-new engineering technology would be needed. Fortunately, just a few years earlier Marc Kingdom Brunel had invented a tunneling shield, that split construction into “cells” where workers could safely excavate sections of earth. With this innovation in place, work on the Thames Tunnel could begin in 1826, on the south bank of the river at Rotherhithe.

Construction, however, was difficult and dangerous. Workers faced fires, leaks of toxic gases and repeatedly flooding. But with an improved tunneling shield, Marc Kingdom Brunel and his son persevered. Nearly twenty years after work started, the Thames Tunnel finally opened to the public in March 1843. Brunel proved that underwater tunnels were possible – and the tunnel is still in use today, as part of the (ironically named) London Overground.

Technology for Today’s Tunnels 

Tunnel construction projects today can benefit from a number of technologies that weren’t available in the early nineteenth century, that might have significantly helped Brunel and the Thames Tunnel team: 

  • 3D modelling: Brunel’s design team had to make do with paper plans, drawn painstakingly by hand and passed between workers on the site. But today, engineers can used advanced 3D modelling to design and simulate new structures. With tools like virtual reality, stakeholders – including members of the public – can see the designs ahead of time 
  • Advanced scheduling: Sophisticated engineering projects take intricate planning, to ensure that schedules are followed and materials are available as needed. Digital planning tools can help project teams work out precisely when different labourers and materials will be needed, to avoid the kind of delays seen on the Thames Tunnel 
  • On-site (and off-site) collaboration tools: Ensuring that everyone is working from the same plans becomes even more important on advanced engineering projects, to avoid mistakes and miscalculations. Tunnel projects today are likely to involve multiple teams working away from the site. Digital collaboration platforms can ensure that every collaborator has the latest information to hand, to keep projects on track 

Ten Minutes from Germany to Denmark: An Underwater Tunnel Under the Baltic Sea 

Germany and Denmark are close neighbours. But between the German island of Fehmarn and the Danish island of Lolland lies 18-km of the Baltic Sea, currently linked only by a ferry line. That means that travelling between Hamburg and Copenhagen takes over 4 and a half hours by rail, and an extra hour by car.  

Now, a project is starting that will link the two countries closer together. Construction is underway on an immersed underwater tunnel to carry both road and rail traffic between Denmark and Germany.  

The ambitious submerged design was chosen over a bridge to protect traffic from stormy weather conditions, as well as to reduce its environmental impact.  

Construction will be undertaken by lowering precast concrete sections into the water, which will sit in a trench on top of a foundation of sand or gravel, and be covered by a protective layer of backfill. At its deepest point, the tunnel will sit 35 metres below the water.  

But what’s most impressive is the length: at nearly 18-km long, the completed structure will become both the longest road and rail tunnel and the longest immersed tunnel in the world, surpassing the 13-km Marmaray Tunnel in the Bosporus, Turkey. Nonetheless, the link will be some way off the Macau Bridge in Hong Kong – which is the world’s longest sea crossing bridge-tunnel, at 55km. 

Trains will be able to travel up to 200km/h – so when the tunnel replaces the ferry link, it will cut the journey time between Copenhagen and Hamburg by two hours on rail and one hour by car. Crossing the border itself will take just a ten-minute drive. 

Having the Fehmarn Belt fixed link will support even closer integration between the two neighbours. But importantly, on the European scale, the tunnel will connect central Europe to the Nordics, creating widescale economic and cultural opportunities.  

The Challenges of Cross-Border Construction 

Scoping such an ambitious project – and particularly one that spans two countries – is a complex undertaking. Construction was originally scheduled to start back in 2012, but has actually only been getting underway in the last few months.  

One of the challenges of this international project has been agreeing exactly who the owner should be and dividing responsibility between the nations. Under the eventual agreement, Denmark will be the sole owner of the asset, taking on the full risk of its construction and responsibility for the upgrades needed to the connected rail and road routes on the Danish side. Nonetheless, the project will need to adhere to the safety standards of both countries. 

The construction will take significant preparation. Facilities will be needed to manufacture the tunnel’s 89 sections of precast concrete, as well as to house specialist equipment like the dredging tools. In fact, the first phase of delivery is building a custom-built plant for this purpose at Rødbyhavn. 

This facility will also provide accommodation for the large workforce needed on the project. Employees may themselves need to be upskilled; boosting technical and digital skills is a key part of infrastructure projects in other parts of Europe, such as the Hinkley Point nuclear reactor in the UK

Finally, the underwater tunnel presents a number of engineering and technical challenges. Its sheer scale means that 20 million metres of soil will need to be dredged in the construction of the tunnel. Engineers are also tackling the creation of a traction power system that’s compatible with both countries’ railway lines; in Germany the voltage is 15kV, compared to 25 kV in Denmark. 

From the design through to its eventual operation, creating this impressive piece of infrastructure will be no mean feat. 

Benefits for the Future 

The project to construct the Fehmarn Belt fixed link will require exceptional coordination, from the planning and preconstruction phases through to the delivery and operations.  

Digital tools are likely to play a key role in sharing information through the many stakeholders, even across the borders, and supporting the tunnel’s safe operation into the future. 

It’s a hugely ambitious project. But when the tunnel is finally completed in 2028, it will represent an exceptional achievement in engineering, construction and international cooperation. 

New Boundaries: Norway's Floating Tunnel 

Engineers in Norway are looking to push the boundaries of tunnel construction even further, with the world’s first floating underwater tunnel. The 680-mile journey from Kristiansand to Trondheim traverses deep, wide fjords surrounded by steep mountains. That means travellers at present take seven ferry trips – and can expect to spend around 21 hours on the road.

The nature of the terrain makes it difficult to build bridges or drill tunnels. So, an engineering team is considering a 4,000-foot-long floating tunnel – around three times the length of the Thames Tunnel. The structure would consist of two concrete tubes suspended by cables 100 feet below pontoons on the surface. Alternatives are still being considered. But if work goes ahead, the world may have a new engineering first in the next couple of decades.

Two hundred years separate the Thames Tunnel and the floating tunnel proposed today. But each case involves pioneers using innovative engineering concepts and cutting-edge technology to push the boundaries of construction.  

The work of the Brunels has stood the test of time. The tunneling shield innovations informed the technology that’s still used today and the Thames Tunnel itself is still used by Londoners to cross the water – even if it’s not on the gas-lit horse-drawn carriages originally planned. The Norwegian floating tunnel could represent another incredible world first – and let’s hope it can enjoy similar longevity.

Newsletter Subscription Banner

Special thanks to expert contributing author: Christian Balbontin Laudien, AE, Norway, Sweden & Africa, Autodesk Construction Solutions.

Lau Luthjohan Jensen

Lau Luthjohan Jensen, Account Executive, Autodesk