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The Fehmarnbelt Tunnel is Denmark’s largest infrastructure project, designed to dramatically reduce journey times between Hamburg and Copenhagen. To do this it must travel 18km along the seabed between the German island of Fehmarn and the Danish island of Lolland. Just one problem. Boring the tunnel is not an option…

Subpar soil conditions in this stretch of the Baltic Sea present too many problems for conventional boring, in time, cost and safety. A long-standing plan to span the gap with a bridge was also ruled out in 2010 as raising a similarly daunting set of challenges.

Instead, the Danish project planners concluded that an immersed tunnel would be the safest and most cost-effective solution – but that would mean building the longest immersed tunnel in the world. (If you’re collecting records, it will also be the world’s longest underwater tunnel for road and the longest combined road and rail tunnel.)

A picture of the Femern Tunnel a tunnel between Germany and Denmark

Source image: Vinci Construction

Helping Denmark grow. Literally.

A colossal 19 million cubic metres of material will be dredged to create a trench in which the tunnel will sit. The resulting sand, stone and soil will be used to make Denmark a little bigger… It will be reincarnated as a new 300-hectare beach resort around the Danish side of the tunnel entrance.

The tunnel itself will be manufactured in – perhaps not surprisingly – the world’s largest tunnel factory, occupying a million square metres and containing six production lines to fashion the concrete elements.

The factory will turn out a completed element – 89 in total – around every nine weeks, and they’ll be cast in segments of 24 metres, with nine segments to an element. Each segment is 217 metres long and will weigh 73,500 tonnes.

Source image: World Highways

Towing and sealing

When ready, they’ll be kitted out with waterproof bulkheads and towed into place before being lowered to the seabed. Pumping water out of the bulkheads creates a partial vacuum, which draws the elements together and seals them. Once assembled, the lengthy task of fitting the railway tracks, road surfaces, ventilation, etc begin, with a target opening date of mid 2029. (The first phase of construction, including harbour and factory, began in the summer of 2020.)

The tunnel will be covered with a layer of stone, and planners expect the tides of the Baltic Sea to eventually coat it with sand, leaving it invisible.

Once finished it will replace the current 160km detour across a number of road linked islands, with 7-minute rail and 10-minute car journeys, as well as taking thousands of trucks off the road, and significantly reducing CO2 emissions. It’s therefore considered a major part of Europe’s green transport strategy.

And the cost? Around €7.4bn.

More than 400 contractors and subcontractors are involved in the project, and we’re proud to say several Seequent and Bentley customers are among them, including Max Bögl Stiftung & Co KG (Germany), Dredging International NV (Belgium) and COWI A/S (Denmark).

A Fehmarn Belt tunnel layout in Germany

Source image: Global Infrastructure

One more thing…

Hopefully the Fehmarnbelt Tunnel will always be free of leaks. But one of the world’s longest and most famous tunnels hasn’t been for a while. Which is it, and what’s happening to fix it?

The Delaware Aqueduct is 85 miles long, making it the world longest tunnel. It carries water to New York City, in fact providing half of the city’s supply. It was built during WW2, and was an astonishing construction. But time has not been kind. It’s been leaking since the late 1980s, sometimes more than 30 million US gallons a day. Which is why a two-and-a-half-mile bypass is being built around the most problematic section, 500ft beneath the Hudson. Later this year the aqueduct will need to be temporarily closed – a massive undertaking – while the final repairs are made, and the bypass put into operation.

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