In 2014 the construction of the Fehmarn tunnel will commence. This post will describe how a tunnel, consisting of 79 standard elements and 10 special elements, will be immersed and assembled under water.
The construction phase begins with the dredging (digging underwater) of the trench where after a gravel layer will be placed on the bottom of the excavation. This gravel bed will form the foundation for the elements.
The concrete elements are closed in both ends with steel bulkheads and are designed to float with a small freeboard. Once the trench is ready, the elements are transported by the aid of tug boats to the position where they shall be immersed.
The pictures above show preparations before immersion and as the immersion process unfolds.
- Having positioned the element on the tunnel alignment, the ballast tanks inside the element are filled with water making the element sink (immersion process).
- Following the elements immersion onto the gravel bed (close to the previous element), the element is dragged towards the previous element by help of jacks thereby closing the gap.
- The element to be immersed is provided with a Gina profile (a custom made very large rubber seal) on the connecting end of the element. This rubber seal creates a water filled space between the elements as shown.
- The water between the elements is pumped out and the water pressure acting on the opposite end of the newly joined element pushes the elements close together and makes the Gina joint fully watertight.
- The bulkheads are then removed and for added safety against ingress of water an Omega rubber profile is installed.
On paper the scale of the immersed tunnel elements can be difficult to grasp. A standard element weighs 73,500 tons corresponding to the weight of more than 14,000 elephants; or if you find it difficult to imagine then imagine Queen Mary 2 (QM2), the largest ocean liner ever, which is approximately the same weight. Above, Queen Mary is seen as she passes Kronborg Castle at Helsingør (Denmark).
I have now described how the element is joined to the previous element, however, the trench still needs to be filled and the temporary water ballast tanks need to be replaced with permanent ballast concrete. More about this in a future blog.