The tunnel buried beneath the Baltic
The Fehmarnbelt Fixed Link tunnel will be buried forty metres below the surface of the Baltic Sea.
Welcome back to The Off Site, brought to you by Aphex!
As usual, we’re deep-diving into a monumental construction project — this time, the 18-kilometre long Fehmarnbelt Fixed Link tunnel that will be buried forty metres below the surface of the Baltic Sea.
The tunnel will connect the German island of Fehmarn to Denmark’s Lolland island, cutting travel between the two islands from a 35 minute ferry trip to seven minutes by train, or ten minutes by road.
It’s a tale of big dreams, porpoises, and some seriously heavy Danish engineering — come with us as we break it down!
Beating the bottleneck
The Fehmarn Belt is the waterway between the Danish island of Lolland and the German island of Fehmarn. It connects the Bay of Kiel to the Bay of Mecklenburg, and is part of a major Baltic Sea shipping route.
The Fehmarnbelt tunnel will largely replace the existing ferry service, a major existing bottleneck in the journey between Scandinavia and continental Europe.
On a good day, it takes about five hours to get between Denmark’s capital city, and Germany’s busy port city of Hamburg. With the Fehmarnbelt Fixed Link in place, that five-hour travel time is set to almost halve.
The tunnel will have a two-lane motorway and a double-track, high speed electric train-line. Each section is separated into its own tunnel section to maximise safety, meaning the overall tunnel structure is divided into four interior sections.
Buried, not bedrock
An 18 kilometre-long tunnel is impressive enough on its own. But the Fehmarnbelt Fixed Link is a record-breaking endeavour — when completed, it will be three times longer than the current longest undersea immersed tunnel, San Francisco’s Transbay Tube.
Longer undersea tunnels exist, like Norway’s undersea network, but they’re all blasted directly through bedrock.
Unlike these tunnels, the Fehmarnbelt Fixed Link will be made up of 89 concrete elements sunk into a trench, connected together and buried. At its deepest point, the tunnel will be forty metres below water. We’ll get into the details later on.
Solutions to the ferry bottleneck have been imagined for decades. As early as the 1960s, infrastructure like smaller bridges, roads and rail connections was developed with the goal of eventually tackling the Fehmarn crossing.
In the 90s, the Fixed Link was imagined as an enormous railway and road bridge. But in 2010, it was decided that a tunnel was more cost-effective, less environmentally invasive, and more resilient to the climate conditions of the Baltic Sea.
Conservation advocates still aren’t happy, especially with the amount of disruption to the habitats of porpoises and other marine life during the construction phase, but these concerns have mostly been overridden.
Denmark’s largest infrastructure investment
The Fehmarnbelt Fixed Link is Denmark’s largest ever infrastructure project, and they are wholly responsible for financing it. Currently, it’s projected to cost €7.4 billion. The EU is contributing €1.1B to the project, and the rest is ‘user-financed’ — as in, road and rail tolls will finance the repayment of loans. According to Danish state-owned infrastructure company Sund & Bælt, those loans are expected to be repaid by the year 2057.
Femern A/S, under Sund & Bælt, are in charge of the project. The main technical advisors and consultants are a three-way joint venture between Danish company Rambøll, British multinational Arup and Dutch TEC.
Danish-led FLC and Dutch FBC are the two contractor consortia, tasked with managing more than a dozen specialised contractors. The vast project involves a huge range of labour, from engineering, dredging and construction, to in-tunnel electric components, worker housing, and transport.
How to build an undersea tunnel
So, what actually goes into constructing an 18 kilometre-long undersea tunnel?
The Fixed Link will be built by submerging prefabricated tunnel components (89 in total) in a trench dredged from the Fehmarn Belt seafloor. Then, it will be packed in with gravel and covered in earth.
Dredging of the 12-meter deep, 100-meter wide, gravel-lined trench began in Denmark in 2020. But ground was broken far earlier — in 2011, with the preparation of a purpose-built factory sites in Lolland’s Rødbyhavn, and working harbours at both ends of the project.
According to Femern S&B, “Customised dredgers such as the Hopper Dredger, Vox Amalia, the Wire Grab Dredger Manta and Magnor, the world’s largest Backhoe Dredger, were used for the operation.”
These Transformer-like vehicles removed almost 15 million cubic metres of soil, stone and sand from the seafloor, and the subsea material they removed will be used to create recreation and nature spaces on Lolland. That dredging work was completed in mid-2024.
Work at sea also involved the construction of two permanent new coastlines out of 2 million tonnes of Norwegian granite.
The world’s largest tunnel factory
The Rødbyhavn site has been billed as the world’s largest tunnel factory.
Inside, the tunnel segments are crafted with minute detail that belies their massive size. Iron rebar is cut and welded onsite, making the tunnel section frames that are then methodically cast in concrete. To prevent excessive cracking, swelling or shrinkage of the concrete, the concrete segments are cured for 30-60 hours in a climate-controlled section of the factory.
After this, they’re closed at either end with a waterproof bulkhead and stored in a drydock. When the time comes, the drydocks will be flooded and, despite their enormous weight, the tunnel sections will float, allowing them to be towed out to sea by tug boats and submerged at the specific point of the trench site.
Each of the 79 standard elements clocks in at 73,000 tonnes: made of nine segments each, they are 217 metres long, 42 metres wide and 10 metres high. The other 10 elements are shorter but wider.
This casting work began in 2023, and, as of February 2025, two completed elements have left the Rødbyhavn factory.
An accelerating timeline
The process for the Fixed Link officially began in 2008, when Denmark and Germany signed a long-awaited State Treaty on the issue. In 2011, the idea of building a tunnel instead of a bridge was officially signed off on.
After decades of dreaming, the tunnel is intended to fully open in 2029.
With two layers of seals, secured with heavy-duty gaskets, and encased in hundreds of thousands of tonnes of concrete, the Fehmarnbelt Fixed Link tunnel is engineered to last 120 years.
On the pod
This week, Jason and Carlos welcome special guest Kelvin Kumangai to break down the reasons every engineer should get their hands dirty in construction.
The team also chat non-software construction innovations, and the EU’s warning on contractor underbidding tensions.
Tune in to catch the conversation!