Immense precasting operations are to be performed in the giant factory-based offsite construction works, in Denmark, to construct what will be the world’s longest immersed tube tunnel. The almost 18km-long concrete tunnel on the Fehmarn Project will sit on the sea bed and within it will house large longitudinal cell voids, running the length of the tunnel and big enough to house separate road and rail infrastructure.
Future road and rail traffic will go back and forth between Denmark and Germany on the Fehmarn link, which will be a strategic connection in the expanding European transport network – especially for high-speed rail.
Fehmarn Project was in planning development for years. Early on, the concept of a tunnel solution to create the important transport had to first overcome a competitive challenge from a bridge alternative. Then, plans developed further, and costings, and then progressively the client Femern A/S moved the project into the procurement phase.
Always, the enormity of the concrete construction challenge for such a world-beating immersed tube tunnel filled the minds. The colossal factory became the solution – a facility that is to outlive the project and serve future, but far smaller, immersed tube projects in the region.
But, first, casting for the tunnel needs to take place.
Planning for doing so is married to the reality that the coast-to-coast, very long hollow concrete tube will be constructed in long sections – called ‘elements’. In total, the immersed tube will be formed from 79 elements that are ‘standard’ in cross section (more than 42m wide, and 8.9m high), plus some slightly larger ones to house the key M&E systems, placed at intervals along the tunnel.
Those hollow elements are the same in cross-section, all the way along – consisting of five cells: two are for two-lane roads; two are for single-lane rail lines; and, one is a service and escape tube.
An element will not be cast in one go; it is way too long. At 217m in length, that is far, far too long to do so. A whole standard element equates to approximately 73,500 tonnes of reinforced concrete.
They will, therefore, be built in parts to be joined together.
The element will be cast in shorter slices, or what are termed ‘segments’. While but pieces of the whole element, the segments are only small in the scale of the overall larger tunnel for, in themselves, each is significant. Each segment, or slice, is 24m long with the same cross-section throughout as the full element, of which it is a part. That calls for more than 8,000 tonnes of reinforced concrete to build such a slice, and there are nine such segments. They will then be bonded together to form a full element.
To that end, in the enormous casting factory, multiple production lines will be performing the same tasks, each contributing segments to form full elements to come out at the end of each of their chains of activities.
At the end of the casting lines, the open cells through each completed full element are then sealed off with specially designed bulkheads. The elements are then moved out of the factory toward the awaiting drydocks.
The drydocks are lined up to receive the factory output and the deep trenches were constructed as part of a specially built basin in a new harbour with breakwater. Within the drydocks the sealed-off elements are fitted out to work with flotation pontoons. When ready, and scheduled for the construction sequence, a drydock is flooded and the awaiting element is floated. Gates are at the ends of each drydock. They open, and then the main gate that holds back the sea is open to give the passage to the floating element.
Out at sea, the elements are tugged to holding zones and then, to schedule, submerged and placed in a specially prepared trench. One by one, the giant hollow concrete elements will be added to the extending line of the tunnel. Only when sealed and watertight will the bulkheads – now internal structures in the extending line of the tunnel – be removed.
That is the overall cycle.
Getting the concreting cycle working effectively on so many production lines is key to the success of the venture. Inside the factory, casting the concrete is an enormous physical and logistical task. To that end, formwork plays an important role.
Peri developed a formwork concept for the central production of the standard tunnel elements.
The main contractor building the tunnel, portals, ramps, and seabed trench to hold the long tube, is Fehmarn Link Contractors (FLC), a joint venture of Vinci, Per Aarsleff, Wayss & Freytag, Max Bogl, CFE, Soletanche Bachy, BAM. The JV is supported by Cowi as consultant and subcontractor Dredging International.
For the volume of work to be done, Peri has supplied more than 10,000 tonnes of special formwork. Each steel assembly has been designed to cast one of the concrete segments that make up a full element. The formwork enables a volume of 3,000m3 of reinforced concrete to be constructed for a segment.
Peri’s heavyweight formwork solution “forms the foundation for monolithic concreting”, it says, reducing “the risk of water penetration during tunnel operation.” It adds that the approach also means the segments are cast without ties in the outer walls.
“At the same time, the approach means considerable savings in material and labour costs, thereby reducing the use of raw materials,” it says.
The formwork specialist fabricated the steel components in facilities in Poland, Italy and at its plant in Weissenhorn, Germany. It took more than 250 truck loads to transport the equipment to the 12 ship transports. This involved massive supply chain planning and execution in its own right, and tight co-ordination with the client says Peri, so as to feed into the complex construction programme.
Getting this far has taken a long time in the overall project development and timeline. The many parts of the construction jigsaw are coming together in a highly visible way, and on a gargantuan scale. Once completed, though, such a long preparatory time will likely come to seem distant when travellers come to almost zip through the world’s longest immersed tube tunnel – taking seven minutes by train, and only slightly longer by car, at perhaps barely 10 minutes.
