Experience of delivering the Øresund Crossing’s immersed tube tunnels is being applied to the Fehmarnbelt Fixed Link between Denmark and Germany.

W ork under water and on land is well underway on the 18km Fehmarnbelt Fixed Link, which will connect Denmark and Germany by road and rail when it opens in 2029.

Once complete, it will be the longest immersed tube tunnel in the world, more than 10km longer than the tunnelled section of the Hong Kong-Zhuhai-Macau Bridge which currently holds the title.

The tunnel is currently the largest construction project in Northern Europe. It will cross the Fehmarnbelt, a strait between the Danish island of Lolland and the German island of Fehmarn, housing a four lane motorway with northbound and southbound traffic in separate compartments as well as a high speed electrified railway.

The € 7.1bn (£6bn) project is being overseen by Femern, a specially-created subsidiary of the Danish state-owned transport management firm Sund & Bælt. It is seen as the strategic continuation of the Øresund Link, the 8km road bridge and tunnel between Sweden and Denmark, which opened in 2000. The combination will create a faster route from Scandinavia to Central Europe.

At present a car journey between Denmark and Germany involves an hour long ferry crossing of the Fehmarnbelt or a 160km detour via the 7km Storebælt bridge which links the Danish islands of Zealand and Funen before heading down into Germany via Jutland. When the Fehmarnbelt Fixed Link is open, journey times will be slashed by at least two hours, with drivers able to cross the strait in 10 minutes. Trains travelling at 200km/h will be able to make the journey in seven minutes.

The form that the Fehmarnbelt Fixed Link would take was debated, as many in the Danish Government preferred a bridge. But it was determined that the risk of shipping collisions was too great and that the north-south alignment of the route would have made it vulnerable to prevailing east-west winds.

A bored tunnel was also considered, but ultimately deemed unfeasible as well. Femern chief executive Henrik Vincentsen explains: “The length and geological conditions are difficult for a bored tunnel and since we have to carry freight trains as well you have to look at the gradient. It can’t get too steep for the trains to get uphill, so that would mean you need more land to construct the tunnel [approaches].”

Instead the immersed tube tunnel solution was chosen. Having built the Øresund Link, the Danes had developed some strong expertise in immersed tube tunnel construction. In addition, ground conditions in the Fehmarnbelt were better suited to it, and there was enough space at the site of the tunnel entrance on Lolland to build the precast factory where the immersed tube elements could be made. Ultimately, the immersed tube tunnel solution was agreed when Denmark and Germany signed the treaty to enable to project in 2010.

Femern awarded the contract to dredge the trench into which the precast tunnel segments would be sunk to Dutch contractors Van Oord and Boskalis in 2016. They are working together under the name Fehmarn Belt Contractors (FBC). The Fehmarnbelt is 35m deep at its lowest point and dredging for the 12m deep, 100m wide trench got underway in July last year. Approximately 15M.m3 of soil, sand and stone removed from the dredging will be repurposed to create a new wildlife refuge on Lolland.

Meanwhile, Femern has awarded contracts worth Ä 3.4bn (£2.9bn) for the immersed tube, the precasting factory and tunnel approaches to Femern Link Contractors (FLC). Its members are Vinci, Aarsleff, Wayss & Freytag, Max Bögl, CFE, Solétanche Bachy, Bam Infra, Bam International and Dredging International.

FLC board chairman Sébastien Bliaut says: “FLC gathered a core of contractors previously involved in the Øresund or Storebælt, supplemented by an extra German partner and two partners specialised in marine works. In total, the consortium consists of nine highly experienced international contractors.

“Because we have gathered so much expertise under one roof, we hope to be able to solve problems very quickly and competently.”

While Femern gave FLC a series of specifications and quality requirements, the contractor will decide on some design details as it undertakes the project. These include details such as the layout and construction of the precasting factory, the dimensions of the elements, the reinforcement design, the formwork and the work methods.

Bliaut explains: “On the basis of the specifications [provided by Femern] and permit requirements, FLC is in charge of the design. During the course of the design development there has been a constant exchange with the owner and consultants.”

What is known is that the tunnel will consist of 89 enormous concrete elements, with 79 of them being 217m long, 42m wide and weighing over 73,000t.

The other 10 “special” elements will be placed roughly every 2km. They will be 39m long and 47m wide, weigh 21,000t and contain a basement. The extra width allows for a layby in which maintenance workers can park before they descend into the special element’s basement. From here, they will gain access to the maintenance passage between the two motorway carriageways without crossing the road.

Including a way to access the maintenance passage is something learned from the immersed tube tunnel section of the Øresund Link, on which FLC members Vinci and Aarsleff worked. The maintenance passage in the Øresund tunnel also sits between the road tubes, but it was only after the tunnel opened that it was realised that accessing it requires shutting down one of the lanes.

Another area in which experience of working on the Øresund Link has influenced the Fehmarnbelt Link contractors is the construction of the factory that will manufacture the tunnel elements. For Øresund, the factory created was too small for the required output. This will not be an issue on Fehmarnbelt as the precast factory adjoined to the temporary harbour at Rødbyhavn, close to the tunnel site on Lolland will be a much larger 220ha. “It is of an unprecedented scale in our industry,” Bliaut says.

The Lolland factory is currently under construction and will be the world’s largest concrete production site, with a cost of almost Ä 1bn.

“At the moment we are erecting the first of the three halls,” Bliaut says. “Together with Femern, we have set ourselves the ambitious goal of starting production of the first segment in the first hall by as early as the end of this year.”

The elements will be cast into large casting moulds containing steel reinforcement. Bliaut adds: “Once the learning curve is behind us, it will take nine weeks to make one tunnel element per production line.”

Once an element is completed, it will be pushed out of the factory towards the dry dock. It is then sealed with bulkheads at each end and the water level in the dry dock is raised, leaving the element to float. Four tugboats then towed it to its position.

Once in place, the ballast tanks placed inside the element will be filled with water, causing the element to sink into the trench. Each element will be sunk into position next to the preceding one with 15mm precision. Remaining water is then pumped out, causing a partial vacuum to ensure a watertight connection.

The tunnel segments will be manufactured at a factory close to the tunnel site on Lolland island in Denmark

At this point, after a watertight connection and safe working conditions inside are secured, the bulkheads can be removed. The elements are then covered with ballast concrete to hold them down before water is then removed from the ballast tanks which can then be taken away. When all the elements are in place, a protective layer of stone will be laid on top and in time the tide will ensure it is covered by sand.

Vincentsen believes that all elements can be put in place over three and a half to four years.

The specifics of the concrete for the tunnel segments are currently being decided. The contractor and Femern have been working for several years to find the mix that is ideal for this project.

“On the scale that we are working, one of the things you need to be sure of is workability; it has to be concrete you can pump because we have to pump it over big distances,” Vincentsen says.

Bliaut adds: “The Fehmarnbelt Tunnel is designed to operate for at least 120 years, so the concrete must be durable and also meet the sustainability requirements.” These requirements include using environmentally sound technologies, recycling more waste materials and using cleaner production processes to reduce pollution. The final concrete will also be low carbon.

The lower carbon concrete will reduce the project’s overall carbon footprint. With the concrete and design elements still in the final stages of clarification, an accurate carbon cost cannot yet be calculated. However, an environmental impact assessment carried out in 2013 estimated that the equivalent of 2M.t of CO2 will be emitted during its construction.

According to Femern, the main carbon reduction benefit will come over the tunnel’s 120 year lifecycle. By offering this new route and a high speed electrified railway, the Fehmarnbelt Fixed Link will reduce the need for flights and shorten road journeys between Scandinavia and Europe, slashing emissions.

The project has already weathered the setback of the Covid-19 pandemic, it is susceptible to the fluctuations in material availability and over the course of its creation it will inevitably suffer delays due to stormy weather.

Nevertheless, dredging is now more than half complete, work to create the tunnel entrance at Rødbyhavn has begun and Femern has started looking for input from the rail industry ahead of the bidding process for delivery of the railway. Vincentsen is confident that the Fehmarnbelt will be finished and operational by the promised delivery date of 2029.

Vincentsen and Bliaut express great excitement about what the construction of the tunnel means.

“We’re building a world record tunnel,” Vincentsen says.

“And, with a lot of the things we’re working on, we’re going to create a lot of new best practice that everyone can benefit from.” 

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Tagged with: Fehmarnbelt tunnel Immersed Tube Tunnels Marine Precast Concrete Tunnelling

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