Tunnelling: Denmark's Fehmarnbelt tunnel begins to take shape

Contractors have cast the first tunnel element for the record breaking Fehmarnbelt road and rail crossing between Denmark and Germany.

Construction of the €7.1bn (£6.1bn) Fehmarnbelt tunnel that will connect Rødbyhavn on the Danish island of Lolland to Puttgarden in northern Germany is well underway.

In the stretch of water between Lolland and Fehmarn, known as the Fehmarnbelt, a massive dredging operation is underway. It will create the 19km long trench in the seabed where the tunnel elements will be placed.

When the 18km undersea tunnel is completed in 2029, it will have record breaking dimensions. It will run at a maximum depth of more than 30m below sea level and will lay claim to be the world’s longest immersed tube tunnel and the world’s longest combined road and rail tunnel.

The project has involved the construction of what is believed to be the world’s largest tunnel element factory where the precast concrete tunnel sections will be manufactured. The factory at Rødbyhavn covers approximately 1M.m2 and is east of the Danish tunnel portal site.

The factory will have six production lines across three buildings for the standard and special elements that will make up the tunnel. Once an element is produced, it will be pushed out of the factory into a dry dock where it will be fitted with waterproof bulkheads at both ends, enabling it to float. The dry dock will then be filled with water and the element will be towed out to sea by tug boats and sunk into place in the pre-dug trench on the seabed. After this, the trench will be backfilled to lock the tunnel element into place. A protective rock layer will be spread on top of the fill to bring it level with the seabed.

The enormous project will be facilitated by a large-scale work harbour that has been constructed next to the factory. The harbour will take delivery of the vast quantities of sand, cement, steel and aggregate required for the tunnel elements.

On the German side at Puttgarden, a smaller harbour will also be built to transport raw materials during the construction of the tunnel portal at that end of the tunnel.

Femern Link Contractors (FLC) – a consortium featuring Vinci, Aarsleff, Wayss & Freytag, Max Bögl, CFE, Solétanche Bachy, Bam Infra, Bam International and Dredging International – is designing and constructing the tunnel and the factory. The project is being overseen by Femern A/S, a specially created subsidiary of the Danish state-owned transport management firm Sund & Bælt.

Inserts from L-R: Tunnel entrance at Danish end, and a dredger digging the trench for the immersed tube

The strategically important project to connect Denmark to Germany has been in the making since feasibility studies for a fixed link were carried out in the 1990s. The original plan was to build the Fehmarnbelt link as a bridge, explains Femern A/S head of media relations Jens Villemoes.

“That was actually plan A, all the way up until 2011,” says Villemoes. “A bridge and an immersed tunnel turned out to be more or less equal cost-wise. But an immersed tunnel had several advantages.”

One advantage was that a tunnel will enable rail and road traffic to travel under the Fehmarnbelt’s busy shipping lane, while a bridge would have been a permanent obstacle to large vessels, says Villemoes. A bridge would also have interfered with a popular bird migration route, as the Fehmarnbelt is the shortest way for birds to cross from Scandinavia to central Europe.

Crosswinds also made a bridge running across the Fehmarnbelt from north to south impractical, notes Villemoes. A north-south bridge would be exposed to strong east-west winds blowing across the structure. This would mean that large and light vehicles like empty lorries, would be restricted from using it in stormy weather.

“Finally, it would have been the longest suspended bridge in the world, so there would be unknown technical challenges,” Villemoes adds.

A bored tunnel option was also considered, but was deemed too expensive. It would have required three of the biggest tunnel boring machines (TBMs) ever built to create tubes big enough to accommodate the railway and roads, says Villemoes.

But the main issue was that the geology of Denmark makes it “a terrible place” to build a bored undersea tunnel, Villemoes notes.

Femern A/S senior design manager Henrik Schjøler Dahl explains: “The Femern fixed link is a combined road and rail tunnel which will require extremely large boring diameters. The geology at the alignment is on the Danish side dominated by moraine clay till containing significant amounts of boulders. This soil type has previously proven to be difficult for TBM works.

“On the German side the dominant soil type is a Paleogene clay, which is extremely plastic. The combination of soil types and very large TBM diameters was evaluated to pose a significant risk for the execution of the drives, and the method was therefore abandoned for this project.”

The complex geology suggests there could be pockets or cracks in the ground through which water could seep and flood the tunnel.

Villemoes adds: “Plus, because of the gradient you would have to bore much deeper.

“We are building just below the seafloor, but for a bored tunnel you would have to go much lower, and because of the gradient for freight trains, the tunnel would need to start much further inland, which would increase our footprint. That would be problematic especially on the German side.”

The final design was an immersed tube tunnel because it was “the safest and the most environmentally friendly option”, says Villemoes. It was also known technology as the method had been employed on the Øresund link, the multi-element fixed link crossing between Sweden and Denmark that includes a 3.7km long immersed tube tunnel.

This year has been critical for the project with the first of three giant factory buildings and four out of five standard production lines operational at the time of going to press. The tunnel’s 79 standard precast elements will be produced in two factory buildings and 10 special precast elements will be produced in a third, smaller building.

The two main factory buildings are divided into specific areas for different activities. There is a prefabrication area for reinforcement cages, a casting hall, a curing hall, a fit out area and a dock with shallow and deep basin areas.

When the factory complex enters full operation mode in mid-2024, one standard element will be produced approximately every two weeks during a continuous production process over three and a half years. These elements will then be pushed out of the halls and into the dry dock.

One standard element is made up of nine segments approximately 24m long and 42m wide. These segments will then be fitted together to create 9m high elements which are 217m long, 42m wide and weighing 73,500t.

Following the completion of one of the standard element factory halls FLC has been able to cast the first segment for the first element. It has so far fabricated three reinforcement cages for the first tunnel element’s segments and will continue to fabricate more cages as the casting of the next segment gets underway. Casting each segment will involve a continuous concrete pour lasting around 36 hours.

FLC has already started to produce the rebar cages for the second tunnel element and expects to have two to three elements cast by early next year.

Since the tunnel is designed to operate for at least 120 years, a high degree of uniformity and quality is required in the concrete to prevent cracking and ensure that it is watertight. As such, the casting and curing of the concrete segments is taking place inside climate-controlled halls, explains Femern A/S project manager Caroline Mia Krag.

“There are curtains between the front and back of the factory building that will roll down and make sure that we have 200C in the casting hall and the one behind it which is the curing hall,” Krag says.

“Concrete is very sensitive to temperature. Most of the time we will not actually be heating but ventilating in the curing hall, because curing concrete, especially in these amounts, creates an enormous amount of heat that will need to be taken away.”

As the team will be producing so much concrete over such a long period, Krag notes that even small variations in the properties of the concrete matter. This is why FLC has carried out extensive materials testing at Rødbyhavn where it has produced a model of part of a tunnel segment that is the equivalent length, width and height to one and a half of the full scale railway tunnel’s tubes. Core samples were drilled into the segment to test the concrete, and the team finished testing last spring.

A key innovation in the tunnel design – and one that FLC has borrowed from the Øresund tunnel – is the 10 special elements. These 39m long, 47m wide, 13m high and 21,000t elements will be placed roughly every 2km along the tunnel route and will incorporate a basement to store electro-mechanical equipment needed for the operation of the tunnel.

This includes communication systems, ventilation equipment and a sub-basement pump sump that collects rainwater.

“One of the operational benefits of the special elements is that we don’t have to shut down a lane to do basic maintenance,” says Villemoes. “But also, we can put a lot of the technical gear down there that would otherwise need to be fitted into the tunnel elements themselves. And that means you can actually slim down the tunnel.”

The special element production line is expected to become operational by the end of this year.

The three tunnel element factory buildings at Rodbyhavn

Outside the factory buildings, three 500m long, 200m wide dry docks have been constructed. These are made up of a shallow basin and a deep basin. The floor of the shallow basin is 1m above sea level but when flooded water in the basin will be 11m deep. The floor of the deep basin is 11m below sea level and when flooded it will hold water which is 22m deep.

These are surrounded by interior dyke walls that measure around 500m long and 11m high.When a tunnel element is ready to be immersed, it is pushed forward into the shallow basin, and a sliding gate is closed behind it to seal the dock on the factory side.

On the other side of the dock is a floating gate which is used to fill the two basins. The 50m wide, 20.8m high and 19.2m deep hollow structure is made from reinforced concrete, explains Krag. It has eight internal chambers that can be filled with water to make it sink. When water is pumped out of the structure, the gate floats upwards and water from the Fehmarnbelt can flow into the dock.

This makes the floating gate essentially like “a plug”, says Krag. “It will let water in and out of the dam. It serves the same purpose as the sliding gate.”

The floating gate pumps seawater into the deep and shallow basins until the water level is 11m above sea level and the element is afloat.The water level in the dock is then lowered to match the seawater level and the floating gate will open to enable tugboats to enter the dock and transport the element from the shallow basin to the deep basin and eventually out to sea.

Work to construct the 18km long trench on the seabed is also progressing, with between 12M.m3 and 13M.m3 of material dredged so far from the seabed.

Work on the cut and cover approach to the Danish end of the immersed tube tunnel began last May. The approach is being constructed in a shallow 550m long excavation on an area of reclaimed land that juts out into the sea west of the tunnel element factory complex. There will eventually be 200m of cut and cover tunnel on the Denmark land side.

Villemoes explains that when tunnel construction finishes, the reclaimed area housing the cut and cover section will be reprofiled to match the original coastline.

“It’s because we will build the first 100m of the tunnel on land,” says Villemoes. “Then erect an inner dyke and remove the last 50m of the portal area. This leaves one end of the tunnel exposed on the seabed, ready to connect to the tunnel elements.”

Construction of the tunnel is expected to be completed in 2029.

Want to read more? Subscribe to GE’s enewsletters and follow us on Twitter and LinkedIn