Building Europe’s next ‘Channel Tunnel’

A new undersea tunnel is being constructed to link Germany and Denmark – and it’s hoped to be as transformative as the one between Britain and France.

We take it for granted now, but the opening of the Channel Tunnel was the engineering of a miracle. In an instant, the centres of London and Paris grew closer, as travel times between the two fell to under three hours. Since the link first opened in 1994, an estimated 450 million railway passengers have taken advantage of the connection, transforming business and tourism alike.

And now 30 years on, ground has broken on a tunnel that arguably lays claim to being Europe’s next Channel Tunnel.

The Fehmarn Belt Fixed Link, as the 18km (11 mile) tunnel is known, is a planned road and rail connection that will create a new link between Germany and Denmark beneath the Baltic Sea. By joining the German island of Fehmarn with the Danish island of Lolland, it will cut what is now a 45-minute ferry journey to a ten-minute drive, or a seven-minute rail crossing.

More importantly though is how it will draw major cities closer together. It will reduce train travel times from Hamburg to Copenhagen from a circuitous four-and-a-half-hour journey via the Jutland peninsula and the Danish island of Funen, to just two and a half hours. And it will convert longer-distance journeys between Stockholm and the rest of the continent into viable rail connections, reducing the need to fly. Trains are expected to travel through the tunnel at 200km/h (124mph).

What makes the project particularly interesting is that, unlike the Channel Tunnel, the Fehmarn link won’t be bored beneath the seabed, but will sit on top of it, as an immersed tunnel that will be buried inside a trench dug on the seafloor.

Once completed, it will become the longest immersed tunnel ever built, surpassing the current record holder, the 13.5km Marmaray Tunnel in Istanbul, which crosses the Bosphorus strait. So, the obvious question to ask is: How do you go about building an underwater tunnel that’s so long? Read on to find out.

Like many megaprojects, the path to the first shovels actually hitting the ground has not been an easy one.

“I think it was in 1988 the first time I went to Germany with a feasibility study saying this is a good idea,” says Henrik Sylvan, head of the Department of Technology, Management and Economics Management Science at the Technical University of Denmark.

“We went down to Mainz [in Germany], and they couldn’t understand the feasibility study, and we got a lecture in cost-benefit analysis,” says Sylvan. “They thought there must be a mistake in the calculation or that this can’t be right because it’s so expensive. It’s so costly.”

Money was not the only concern, as according to Sylvan there were culture clashes on other aspects of the project too.

“The [environmental] appraisal in itself is like 10,000 pages,” says an incredulous Sylvan, who explains that in his experience, similar Danish assessments usually weigh in at a breezy thousand pages or so.

“In Germany, every single individual can bring a decision to court. This is a never-ending story,” says Sylvan. “You think you have taken the decision finally, but then it can be for some reason doubted.”

The debate over feasibility continued all the way until 2008, when the Danish government gave up on building the tunnel as a 50-50 joint venture with Germany, and instead took the lead on the plans, with the budget for the project set at around €10bn (around £8bn).

What also helped the project move forward is that it caught the attention of the European Union, which incorporated it into its continent-spanning Trans-European Transport Network strategy. The hope in Brussels is that the Fehmarn tunnel will complement the 55km Brenner Base Tunnel, which will link Austria and Italy under the Alps, to form the Scandinavian-Mediterranean freight corridor, and will help Europe meet its environmental goals by taking freight off of the road.

Despite the delays, construction did finally begin in March 2021, and as things currently stand contractors are hard at work digging the tunnel portals on both sides of the strait and dredging the trench beneath the sea – which will be eventually require excavating around 20 million tonnes of earth, to create a trench 12-15m deep and around 40m wide.

It is expected that the construction process will be unavoidably long. Before the tunnel can be constructed, the developers also need to build a factory on Lolland to produce the concrete segments that will make up the tunnel.

“The factory is being built, the first out of five production halls have been raised, the cladding of the walls and the roof are starting next week,” says Kaslund, “But it will take this year and to some extent also next year before the factory is totally finished. But it should be ready to start producing elements by the end of this year or beginning of next year.”

The first question to ask when building a fixed link is whether to go for a tunnel or a bridge. During Denmark’s last major sea-crossing project at the turn of the millennium, the iconic Oresund link between Copenhagen and the Swedish city of Malmo, the decision was made to split the difference by building a bridge that dives underground on an artificial island and sends traffic into a tunnel, so that shipping traffic can still pass. Similarly, in 1991 when Denmark constructed the ‘Great Belt’ that links the Jutland mainland to Copenhagen, the government opted for a bridge.

But with the Fehmarn link, the major factor appears to be environmental considerations.

“There were two competing projects, one planning for an immersed tunnel, and one planning for a bridge,” says Jens Ole Kaslund, the project’s technical director. “Price-wise it was more or less equal, but from an environmental point of view the immersed tunnel has a big advantage that when you’re done, it doesn’t affect the environment any more because it’s just under the seabed.”

It was also determined that an immersed tunnel was a better option than boring under the sea like the Channel Tunnel.

“We have to have a motorway in each direction with three lanes – two main lanes and the emergency lane,” explains Kaslund. “If you have a bored tunnel, which is, of course, a cylinder, you have a lot of extra space above and beyond the road, which makes it more expensive, and requires more concrete.”

Essentially then, an immersed tunnel enables the developers to avoid excavating more earth than they need to – and the tunnel can be as minimally resource-intensive as possible.

In terms of design, the Fehmarn tunnel is closely modelled on the one in the Oresund strait, but Kaslund also takes inspiration from the recently completed link between Hong Kong, Macau and the Chinese mainland, which also uses an immersed design. But there is one crucial design innovation: unlike its predecessors, Fehmarn will contain a basement level beneath the road and rails to house the tunnel’s service layer. The hope is that by keeping the technical equipment separated, it will be easier to keep traffic flowing.

“You can park your car, go down to the basement, fix whatever you have to fix and take your car and continue again without disturbing traffic at all,” explains Kaslund. “That’s one of the huge changes we’ve made in this tunnel compared to the [previous] ones.”

And though the project has had a long gestation period, the wait has enabled the designers to innovate in a way they wouldn’t have been able to only a few years ago.

“For longer immersed tunnels, the ventilation can be a challenge, and you would normally have a ventilation island, but actually the client was really brave about this,” says Rachel Kenny, an associate director at Arup who has spent the last decade working on the tunnel. “They allowed us to plan for cleaner future emissions. We didn’t have to strictly follow the requirements in the standards, and together with a relatively flat alignment, it allowed us to eliminate the need for a ventilation island for this particular tunnel and ventilate it longitudinally.”

This means that instead of having to build an artificial island in the middle of the sea and poke out a chimney to keep the air below breathable, instead the designers can take advantage of a phenomenon called the ‘piston effect’, in which moving vehicles force air forward as they move.

“I think the Danes are very brave when it comes to construction projects. They’re not scared to innovate, or do things in a different way,” says Kenny.

To build the tunnel, the plan is to essentially slice up the tunnel tube into 89 200m-long pieces – or ‘elements’ – each of which will be pre-cast in concrete on land, in a purpose-built factory at Rødbyhavn on Lolland. Once a segment is completed, it will be sealed at both ends, floated out to sea and carefully sunk into position. Once the tunnel elements are in place, the barriers can be broken – and  the world’s longest immersed tunnel will have been completed.

But this isn’t to say that it will be easy. One particularly critical part of the design is the placement of the ten of the 89 elements that contain the special services basement, which has Kaslund feeling understandably trepidatious.

“There are a lot of interfaces that have to go together,” explains Kaslund. “There has to be a slope where you can put the deeper element and get all that in place. So, when we have placed the first special elements I’ll feel even more safe that we will operate successfully at the end.”

But before this happens, the first of 79 regular elements have been successfully completed and sunk to the bottom of the strait.

“When we are able to sail out the first element, to have the first 200m of the 18km-long tunnel in place, then I will be relatively relieved and say, ‘Well, that went well, we can fit the first one, we should also be able to fit the next 88’,” he laughs.

According to the current timeline, Fehmarn tunnel should be up and running in 2029, and the expectation is that it will be as transformative as the Channel Tunnel.

“They are going to change the whole timetabling pattern,” says Sylvan, who is particularly excited about the possibilities for new rail routes that are being explored by the German team.

“A new train from Paris Gare du Nord is going to come to Copenhagen and Stockholm. Six years ago, we closed down the last night train. But now [there are] new initiatives: night trains coming through Copenhagen, going to Berlin,  and going to the Alps during the ski seasons. Norway wants to join in [too] and investigate if we can have a train from Oslo.”

And Sylvan expects the new tunnel to facilitate much more traffic than either the existing hydrofoil ferry link or the passenger estimates that are officially on the project paperwork.

“The forecasts are so conservative,” says Sylvan. “The official forecast is for [around] one million passengers a year. That’s not many passengers a day. [But] I would say look at the Great Belt or the Oresund Link, where the number of trips now is like five times, six times, seven times when they only had the hydrofoil link before.”

Once completed, people can travel between Puttgarden – at the tip of Fehmarn (the German island that is the tunnel’s namesake) – and the Danish port of Rødby in just 10 minutes by car, or 7 by train.

When each element of the tunnel is ready to be transported, build will seal both ends of it with a bulkhead and the dry dock in which the construction is based is filled with water. They will then tow it out into the deep end of the dock. After which they lower the water level to sea level, tugboats will tow the element into position on the Fehmarnbelt.

Constructors fill the ballast tanks with water and then lower the element onto the seabed. Once manoeuvred into place, they pull the elements together, and then pump out the water inside. This creates a partial vacuum, which ensures a watertight connection.

This new infrastructure will produce twofold benefits, enhancing road connections between the Mediterranean region and Scandinavia while enabling faster road and rail transport of freight between Scandinavia and central Europe.

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