By 2028 crossing between Denmark and Germany will take only 10 minutes by car and seven minutes by train. That will be possible thanks to the Fehmarnbelt link, which is expected to be the world’s longest road and rail tunnel, connecting Rødbyhavn on Lolland and the German island of Fehmarn.
The Fehmarnbelt is an immersed tunnel, made up of hollow concrete elements, cast on land and assembled section by section to form the tunnel. The 89 elements that comprise the tunnel will be built on land and towed out to sea, where they will be lowered down into an excavated trench in the seabed and assembled. Each standard element is 217m long, 42.2m wide and 8.9m high.
An 18km trench for the tunnel needs to be dug in the seabed in order to build the Fehmarnbelt link. The alignment of the tunnel trench runs from east of Rødbyhavn to east of Puttgarden on the German side.
The tunnel elements will be lowered into the trench and assembled, hence the designation as an immersed tunnel. The trench is then refilled.
The construction budget is EUR 7.05bn (USD 7.97bn).
The consortium Femern Link Contractors (FLC) includes VINCI Construction Grands Projets S.A.S. (France), Per Aarsleff Holding A/S (Denmark), Wayss & Freytag Ingenieurbau AG (Germany), Max Bögl Stiftung & Co. KG (Germany), CFE SA (Belgium), Solétanche-Bachy International S.A.S. (France), BAM Infra BV (Netherlands), BAM International B.V. (Netherlands). Subcontractors are Dredging International N.V. (Belgium) and the consultant is COWI A/S (Denmark).
A second consortia is FBC, responsible for dredging and reclamation including Boskaslis (Netherlands), Van Oord (Netherlands) and the consultant is Sweco.
Upon request of the Danish Parliament, from 2013-15, and from 2018-19, Femern A/S has been preparing the primary construction site in Rødbyhavn. These are called ‘preparatory works,’ which include building a new pump station and drainage canals, demolishing existing structures, building access roads, utility lines and diversionary road and bike paths around the site.
The aim is to ensure that the construction site is ready for the contractors, as soon as the green light has been given from the Danish Parliament.
In 2020 the main tunnel works are expected to start and to last 8 years and half. Tunnel works include the work harbour, the tunnel element factory, portal and ramps, and the tunnel itself.
However, Femern A/S Technical Director Henrik Christensen of Femern says they expect the preparatory works at the construction site for the production facilities on the Danish side to be completed by the end of 2019. “We are preparing start up scenarios which we will present to our owner in the coming months,” he adds.
Extensive archaeological investigations on the main construction site are also being carried out. “It is a big task as they are the largest connected archaeological investigations in Danish history and there’s been a number of interesting findings from the Stone Age,” says Christensen.
“We need to finish these investigations, before we can start construction works, and so far we don’t expect any delays.”
In terms of geology there is clay on the German side and it has a good bearing capacity. On the Danish side, there is moraine and that is more harder material than clay. “There is a construction challenge on the Danish side because of the harsher material but we are going to use heavy equipment,” Christensen says.
“We will also use barges to transport the material to shore.”
Christensen explains that they did detailed geotechnical investigations with thousands of samples, giving a very detailed description of the ground conditions. The geological studies were done in four stages over several decades. Starting in the 1990 with the collection of known information and preliminary field studies. Detailed studies were carried out from 2008 to 2013, and verification studies between 2015 and 2016. “We collected enough material for a Geotechnical Library, and it is actually been the study of PHD-students,” says Christensen. “However, the conclusion of these investigations was clear. There is no geological or geotechnical obstacles to build the fixed link. These results more or less confirm the preliminary studies taken in the 1990s.
All this data was made publically available to bidders, so contractors could incorporate the geological landscape in their bids.”
Dredging operations
Christensen explains that before the tunnel elements can be immersed, they need to dredge a 46-100m wide, approximately 12m and 18km long trench in the seabed. “In total, around 15mn cbm of clay and sand will be excavated from the trench,” says Christensen. “The material will be used primarily for landreclamation on Lolland and, to a lesser extent, on Fehmarn.”
The trench in the seabed will contain elements of the Fehmarnbelt tunnel across the Fehmarnbelt from Rødby harbour on the Danish island of Lolland to the German island of Fehmarn.
Christensen explains that they will use very large dredgers to deal with every type of material. “At a depth deeper than around 25m, we will use grab dredgers, and if we encounter harder material, we will use trailer dredgers first for loosening the soil,” says Christensen. “Barges will carry the excavated material to shore, to be used primarily for land reclamation on the Danish side.”
As owner of very large floating excavators, Dutch contractors Boskalis and Van Oord have won the contract for the excavation work. Five of these huge machines will be used for the Fehmarn project.
The largest of these is a backhoe, which goes under the name of Magnor. The shovel of this machine can house an entire shipping container and simultaneously lift up to 67t of stone and sand, as well as being able to dig down to a depth of 40m.
Even at this rate, it will take 1.5 years of digging before the trench in the seabed is ready. A total of approximately 19mn cbm of sand, clay and stone will be excavated from the seabed, resulting in more than 3sqkm of new land appearing on the south coast of Lolland.
“It is undoubtedly the largest marine excavation work ever undertaken in Danish history, which is why there are major advantages in using the largest machines in the world,” says Jørgen Andersen, project manager at Femern.
“They are more cost effective and at the same time limit the environmental impact, as they deposit less sediment into the water.”
Logistical challenge
The Fehmarnbelt production facility will be for the next few years the heart of a great logistics challenge.
The production facility will churn out the building blocks of the immersed tunnel, requiring a constant flow of material.
Thus, the construction of a dedicated work harbour is one of the key phase of the project to ensure an uninterrupted flow of raw materials. It is estimated the factory will require 25,000t of cement and 160,000t of gravel for watertight concrete production every month.
The whole project will require 3.2M cbm of concrete and 350,000t of steel. The tunnel-factory and construction site will have limited storage capacity, and it would be impractical to transport the necessary materials by road, as it would require thousands of truckloads every month.
Thus, the vast majority of the raw materials for the project will be transported via a work harbour.
A distinct method will be used to unload and transport each material including a large amount of sand and aggregates. This method consists of unloading via a conveyor belt system while the cement will be rapidly blown out through a pipe to nearby silos. Reinforcing bars will be delivered by vessels; the production can start once materials have arrived on site.
As the plan is to ensure a continuous production process of segments in the casting hall, engineers decided that this method would drive the overall production cycle. The rebar assembly hall is optimised to this workflow with three rebar stations, two adjacent pre assembly areas and a buffer station in front of the casting bed. This outfitting work can start before reaching the upper basins.
Production of rebar cages
The production of the standard elements will start in the two-rebar assembly halls ensuring factory light conditions. The halls are designed to ensure a maximum efficiency and safety. The reinforcement bars will enter the hall already cut and bent aside. Fixers and welders operating two shifts a day will start to assemble the reinforcement for the bottom slab following a robust sequence.
All reinforcement work will undergo extensive quality controls before moving on to the next station. The rebar cage will be skidded to the top slab assembly area supported by both the lifting table and the launch scaffolding systems. Once the top slab is assembled, the completed rebar cage can be skidded to the buffer area where additional works can be completed when necessary.
This approach allows the assembly of the next cage to start without any delay and thereby removes this process from the critical path as soon as the bottom and external formwork is ready. The rebar cage will be lifted and skidded into the casting bed using specially designed inflatable rails.
Casting and curing of concrete
The formwork process is designed to offer the advantage that the entire segment can be cast monolithically and without anchors in the external walls.
This continuous casting operation requires the production and distribution of a hundred cubic meter of concrete per hour on average. Stripping the bottom formwork will start and the segment will then be pushed into the curing area by a robust skidding process.
At this point of the process, outfitting works can already start in the curing hall and continue in this extended work area thus boosting efficiency and robustness. The remaining outfitting will take place once the element is completely pushed in the upper basin.
Float out of elements
The first marine operations are the floating up and tugging out the elements and this process will be different for standard and special elements. After floating in a special thruster pontoon and closing all gates the basins will be filled up. First the standard elements will be winched to the lower base according to a precise switching sequence
Followed by the special element, which will be firstly lifted by the thruster pontoon to give it extra lift to float. After aligning the water level the floating gate will be removed. The standard elements are tugged out underneath two immersion pontoons.
The two pontoons will swiftly hook up to the four hoisting tows so the elements are ready to be towed out of the harbour following a strict procedure.
The last phase of the marine works starts with the immersion of the element into the final position. As each new element arrives on location above the tunnel trench, the immersion pontoons will be anchored.
Six anchor wires will connect anchor buoys to an unembedded bollard on the roof of the preceding tunnel element. The immersion starts as soon as the new elements approach the preceding elements transponders will guide it into position until the capture of the bulkhead of the new elements connect with pin on the preceding element.
Finally the Gina seal will be aligned and the water pumped out. The hydrostatic pressure will exert over 10,000t of force compressing the seal. Last marine operations placing the locking fill and protection layers will be executed by a spread pontoon and directionally controlled by immersed fusers first placing the backfill followed by two protection layers, one with gravel to keep it in position, then a layer of sand and finally a protective layer of stones is laid on top.
In proximity of some shore areas, the elements will be also protected by bigger boulders, which will function as an artificial reef for marine life. Over the years the natural sedimentation will completely cover the Fehmarnbelt tunnel elements and make it invisible to people and animals.
Christensen claims the importance of foundation for all immersed tunnels like Fehmarnbelt, mentioning the Øresund project. It is a combined railway and motorway bridge across the Øresund strait between Sweden and Denmark. The bridge runs nearly 8km from the Swedish coast to the artificial island Peberholm in the middle of the strait. The crossing is completed by the 4km Drogden Tunnel from Peberholm to the Danish island of Amager
“Before the Øresund tunnel was built, it was commonplace to pump sand under the element to support it when it was lowered into place,” he says. “For the Øresund construction itself, the engineers developed a very effective solution. Instead of pumping in sand after the element had been lowered into place, the contractor laid a foundation of crushed stone prior to immersion.”
The method, he says, is now a proven solution and an often preferred alternative.”
