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Ten Minutes to Denmark: Building the World’s Longest Immersed Tunnel  

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 immersed 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. 

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. 

Read more about plans for the world’s first floating tunnel in Norway and its ancestor in Victorian England. 

Lau Luthjohan Jensen

Lau Luthjohan Jensen

Lau Luthjohan Jensen, Account Executive, Autodesk

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