How is the Fehmarn tunnel constructed underwater?

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.

  1. Having positioned the element on the tunnel alignment, the ballast tanks inside the element are filled with water making the element sink (immersion process).
  2. 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.
  3. 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.
  4. 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.
  5. 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.

7 Replies to “How is the Fehmarn tunnel constructed underwater?”

  1. Are the tunnel elements “dry” all the way from the construction bay and down to the instalation/connection?
    Will all installations – electricity etc. – be installed before sinking and connecting to the tunnel?

  2. Hi Rienk
    I believe that the sandflow method in general will be preferred for shorter IMT’s as the Limerick tunnel. For long immersed tunnels as Oresund and Femern it may be beneficial to avoid the temporary foundation and place the tunnel directly on a prepared foundation bed. The equipment used for this method is highy specialized and expensive and therefore more suitable for long tunnels.

  3. Hi Pedro
    The excavated trench will follow the seabottom and the vertical curvatures can be accomodated in the joints between the straight elements.

    The element sinking method is more than 100 years old but the first rectangular concrete immersed tunnel was the Maas tunnel in Rotterdam completed in 1942.
    The Oresund tunnel is the longest combined rail and road tunnel ever built (involving a width of the tunnel elements of more than 40 m). Oresund tunnel was furthermore the first IMT where a highly industrialised element production was used.

  4. Hej,can you explain why a gravel bed has been chosen rather than sandflowing (like in Limerick)? thanks, rienk

  5. Hey, thanks for the reply Susanne!

    In my comment i wrote “horizontal” but i meant “without curvature in the vertical plan”

    But i’m guessing even if it goes “straight” (no turning left or right too much) it will still have to go “up and down” enough to accomodate for the sea bed altimetry (which i have to idea how sinuous it is because google earth only displays altimetry on land last time i checked

    using landfills and digging to make it as straight as possible as it is done when building a regular train line on land?

    also, was the Orseund tunnel the first of its kind? i tried googling for submerged tunnel but i didnt find any other already built

  6. Hi, cool blog, i liked the youtube vids explainning why stuff floats on the other one.

    Now some questions 🙂

    -Is it the same technology used in the Oresund “bridge” tunnel? Overall it seems like it, but i guess some new ideas might have come up.

    -Will the tunnel be in a straight line? And does it have to be horizontal? I’m thinking if it is it will be easier because all the segments can be equal, but depending on the sea surface it might involve more digging or in general more “land” being moved.

    1. Hi Pedro Happy that you like our blog and thank you for the relevant questions.
      We have used the Oresund Tunnel technology for our time and cost assessments but the future contractors may find even better methods.
      The tunnel will be a straight line all the way if possible as this is beneficial for a factory production method. Anyhow we also have to stay within certain corridors on land due to environmental reasons which may induce a slightly curved alignment nearshore. The tunnel as such is not horisontal but each element is straight and changes are made in the element joints.

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