Tunnel to Germany: The world’s biggest assembly line spanning half a kilometre of concrete beams

To verify the production process, a trial casting of half a segment was carried out this summer. The quality is now being reviewed. The concrete that will be placed at the bottom of the Fehmarn Belt must have a durability of 120 years. Illustration: Femern A/S

During the construction of the connection between Copenhagen and Sweden - the Øresund Tunnel- it took approximately 10,000 man-hours to produce the first tunnel segment.

When the twentieth and last concrete element left the production line, the required man-hours had been cut by three quarters. Tomas Skjold is betting on the same improvements at the element factories in Rødbyhavn, from where the finished building blocks for the Fehmarn Belt tunnel are shipped out.

He was responsible for reinforcement on the Øresund Tunnel approximately 30 years ago and is now a construction works manager on a much larger project for Femern A/S, the developer of the upcoming 18-kilometre-long submerged tunnel between Denmark and Germany.

“When we built Øresund, we were constantly optimizing. It’s often very small details, such as a new place to put a tool, so it’s a few seconds faster to get hold of it the next time you need it,” Tomas Skjold says from an overview platform at the construction site.

“Added together, the many small ongoing optimizations give big time gains at this scale.”

From V pylons to tunnel tubes

Tomas Skjold has agreed to show us around what turns out to be a highly industrialized construction site.

Before we get to the setup, however, we must briefly present the final product. As you know, it is an 18-kilometre-long submersed tunnel with four motorway tracks and two railway tracks. It has to be ready for the first drivers and train passengers in 2029.

European consortium FLC is responsible for the production, while Tomas Skjold and Femern A/S must ensure that it meets the requirements through continuous inspections. Illustration: Femern A/S

Originally, the plan was to build the link as a cable-stayed bridge, similar to the Øresund Bridge, but with V-shaped pylons. Another feat of engineering prominently displayed in the landscape. But in the end, an efficient but less eye-catching solution was chosen: a submerged tunnel.

The final version consists of 79 so-called standard concrete elements, which measure 217x42x9 meters (l x w x h), and 10 special elements measuring 39x50x13.5 meters, which are wider and deeper in order to make room for maintenance facilities in the tunnel.

Broadly speaking, the process goes on as follows: The elements are produced in a factory with over 1,000 tunnel workers at a newly constructed working harbour in Rødbyhavn, after which they are equipped with waterproof bulkheads so that they can float. One by one, they are then transported to the Belt with tugboats and submerged in an excavated tunnel trench. When a certain number of elements are placed, tunnel asphalting and railway and facility construction begins.

Elements are produced in three halls with two production lines each. Five lines are for standard elements, and one is for special elements. For practical and technical reasons, the production of a 217-meter-long standard element weighing around 73,000 tonnes is divided into nine sub-segments that are approximately 24 meters long and weigh 8,000 tonnes, which are assembled before being shipped to the Belt.

This is practical because it is easier to manoeuvre them. When it comes to the technical side, it is because subsidence may occur when the element is placed on the bottom of the Fehmarn Belt, which is easier to handle if it is divided into sections, Tomas Skjold explains.

From the factories, the finished elements are taken to a dry dock that can be filled with water. From here the elements, which can float when sealed at both ends, can be towed to the Belt. Illustration: Femern A/S

Half a kilometre from start to finish

And then we come to the assembly line itself. Because it is truly an assembly line, where the first teams assemble the pre-produced reinforcement at one end of the hall, after which the finished reinforcement skeleton (shaped as a segment-sized tunnel element) is placed into a mould.

Here, concrete from one of the factory’s two agitators is pumped in, the concrete is vibrated, and when it becomes strong enough, the segment is pushed on and ends up in a waiting area at the other end of the hall on the same assembly line. The concrete then needs to harden further until it has achieved the required strength.

Work on segment 2 begins on the same conveyor belt shortly after segment 1, so that the nine assembly blocks are produced successively. Between two finished segments, approximately 50 cm of rubber is placed to act as a joint, and when all nine segments are lined up against each other, prestressing elements are passed through holes all the way from segment 1 to segment 9, and then the segments are pressed together into one 217-meter-long element with hydraulics.

It all takes place on a conveyor belt—in this case concrete beams that, at a length of 470 metres, stretch far out through the 240-meter-long hall and onto the dry dock.

Above all, it requires skills and experience in logistics to make sure it runs like clockwork. A major challenge will be to ensure that problems in a single section are not allowed to spread to other workstations.

“That’s why there are queuing zones in the halls, so that there is, for example, room to put a finished reinforcement skeleton aside if there are problems with the mould, so that work on the next segment can continue,” Tomas Skjold says.

It is clearly visible that he is looking forward to getting started. The moulds are referred to as the “beating hearts of factories.”

“It’s crucial for the pace that casting works,” he says.

The casting of a segment, which requires around 3,000 cubic meters of concrete, must be continuous and without breaks. With breaks, cracks occur in the concrete, which weakens its quality. For that reason, the three factory halls can also be connected to both concrete agitators if one breaks down.

Mould for standard tunnel element with two tubes for railway, two tubes for highway and one tube for service. The process from the start of reinforcement to the point where the concrete has hardened is expected to take around ten weeks with a seven-day working week and multiple shifts. Illustration: Femern A/S

Five years from start to finish

The production of a single standard element should last 10-11 weeks on average.

This means that the 79 standard elements can be produced in just a few years if the five production lines are utilized to the maximum. However, a buffer has been added to the schedule due to, among other things, the weather and unforeseen issues.

The first element is expected to go into production around the turn of 2022/2023, but according to the plan, it will not be shipped out until well over a year later, allowing time to build up a stock of finished elements.

The last element is expected to be shipped out and connected with the other 88 in 2027.

Another factor is that there is limited storage space for finished elements, and that only one element at a time can be shipped out and submerged. There is room for one element in the extension of each production line in a dry dock area, i.e. five standard elements and one special element in the waiting position.

In addition, a similar number can be stored in the production halls, but it will take up space that would otherwise be used for producing new elements. The aforementioned process must therefore not just run like clockwork on each production line, but also overall, from the first reinforcement being cut and tied to the element’s voyage out into the Belt. There needs to be a flow between the production speed, storage, and shipping.

An advantage of the mass production method is that there are no fixed places for each element, apart from a special element having to be placed at a certain interval.

Not high tech, but it works

Apart from cranes and hydraulic jacks, which are responsible for lifting and pushing reinforcement, segments, and elements, element production is carried out with good old-fashioned manual work.

“The system is based on having the same people do the same job over and over again, so that they are constantly getting better,” Tomas Skjold says.

At present, he denies that it is possible to completely replace human hands with robots.

“It’s almost impossible in a project as large and complex as this one.”

Professor Søren Wandahl, who researches construction processes at the Department of Civil and Architectural Engineering at Aarhus University, agrees with that sentiment.

“The technology to automate the entire production process exists, but the investment will not match the yield, because these are very special dimensions that are only to be used for a limited period of time.”

He also mentions that the production facilities at the tunnel factory at Fehmarn will be able to be used for the production of offshore wind turbines and other large construction projects that require element production in the future.

“It’s not super high-tech, but it’s what makes the most sense in terms of time and price,” Søren Wandahl says.