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The tunnel will rest on a solid foundation

The tunnel elements will be placed firmly on the seabed and kept in place by rocks, gravel and sand.

The foundation for an immersed tunnel has an essential role. It has to be stable and secure, so the covered tunnel is solid and secure in its trench.

When the trench is dredged a foundation is therefore laid on the bottom, on to which the elements are lowered. The foundation is precisely laid using sophisticated equipment. After this, the tunnel elements are lowered down on to the bottom, assembled, locked into place with filler material and covered.

  • An up to one metre thick foundation of fist-sized stones, crushed stone, on which the elements will rest, will be laid on the bottom of the tunnel trench as a base for the elements when they are lowered into place.
  • A layer of coarse gravel will help to keep the tunnel elements in position once they are in place.
  • There is a sand filling on top of the gravel layer; this is more granular than gravel.
  • An approximately one metre thick layer of small and large stones will be laid on top. The rocks are gradually covered in sand, and the rock layer will be flush with the seabed along most of the tunnel section.

THERE IS CAREFUL THOUGHT BEHIND THE CHOICE OF FOUNDATION FOR THE FEHMARNBELT TUNNEL.

Did you know that...

... the water that gathers between the tunnel elements is pumped away while they are underwater

... a hydraulic arm pulls the elements together underwater

... at its deepest point, the motorway tunnel will be nearly 40 metres below sea level

... the special elements are an innovative approach in the Fehmarnbelt project

Thorough preparatory work

There is careful thought behind the choice of foundation for the Fehmarnbelt tunnel. Before the Øresund tunnel was built, it was commonplace to pump sand under the element to support it when it was lowered into place. For the Øresund construction itself, the engineers developed a more accurate and effective solution. Instead of pumping in sand after the element had been lowered into place, selected contractors laid a foundation of crushed stone prior to immersion. The method is now a proven and often preferred alternative.

Guided by laser technology and echo sounding

A layer of crushed stone is deposited as the tunnel elements are immersed so that crushed stone is always laid out ready for the next element.
 
The crushed stone is laid from a barge designed for the purpose. It is poured from the barge into the dredged trench through a special downpipe. The pipe and the supply of crushed stone can be controlled so that the applied amount is very accurate and the layer on the bottom is even and perfectly flat.

Laser signalling ensures that the pipe is held in the correct position all the time during the work. After this, advanced echo sounding is used to measure whether the layer of crushed stone is completely flat and level. The accuracy of the finished layers is to within about 2-3 centimetres in water depths of up to 40 metres. This really is precision work.