Immersed Tube Tunnel

Immersed tube tunnelling is an efficient engineering solution for the Fehmarnbelt Fixed Link. An immersed tunnel is an underwater tunnel composed of segments, constructed elsewhere and then floated to the tunnel alignment to be sunk into place and then linked together.


The proposed solution shares many design features in common with the Oeresund Tunnel, where such features have proven their value (see picture below of Oeresund tunnel).


The cross section of the immersed tunnel has been designed to accommodate the combination of twin dual carriageway motorways as well as two rail tracks. Each direction of each mode has its own tube, in accordance with international best practice (see pictures below of standard elements).

For the Fehmarnbelt Tunnel a solution comprising a combination of multiple standard elements, with intermediate special elements has been adopted. The special element technique is a unique idea and has never before been used for immersed tunnels (see pictures below for a full description).

The overall tunnel is comprised of ten special elements and seventy nine standard elements that measure 46m and 217 m respectively.

In designing a tunnel, space for technical equipment is a challenge and the longer the tunnel is the greater the challenge. The Fehmarn tunnel has to fit both lights, ventilation, sprinklers etc. All these specific installations need support in the form of transformers, pump sumps etc. The common denominator for all these items is (other than they are nice to have in a tunnel) that they take up space and need to be serviced ever so often.

In order not to over congest the technical section, a concept consisting of special elements has been developed. All technical equipment that otherwise could easily block the service section has been moved to the special element sections consisting of a basement specially designed to facilitate all the technical equipment. Having the technical equipment concentrated in special element sections also significantly eases the task of maintaining the tunnel.

Special elements of larger cross-sections are provided approximately every 1.8 kilometres along the tunnel with standard elements in between. In this way special functions can be provided for at regular intervals outside the profile of the road and rail tubes.

The next post will discuss how the tunnel is constructed underwater – but before posting it I just wanted to hear your comments on how you imagine it to be constructed?

5 Replies to “Immersed Tube Tunnel”

  1. Hi Susanne – I’m enjoying your blog!

    Safety first…
    From the cross section there appears to be no access to the escape tube from the rail tubes. In the event a passenger train has to be evacuated what is the plan?

    Best regards Texx

  2. Hi Kasper
    The elements are designed to float with a well defined freeboard that facilitates transportation by tugboats to the immersion area. The element will be lowered into the predug trench by pumping water in to ballast tanks placed inside the element. The temporary water ballast will later be exchanged by permanent ballast concrete. As you point out big quantities of ballast are required but we will tell you more about this in a future post!

    The final backfilling of the trench and the layer of stone protection on top of the element will also help to keep the element in place.

  3. This is fascinating. My not being an engineer probably even more so! 2 related questions: If the elements are sealed and airtight until after assembly, the “upfloat pressure” must be enormous. 1) How is this handled, e.g. how are the elements pushed down to the sea bed? 2) And once there, how are they kept in place and secured?

  4. Hi Nigel
    The tunnelelements are airtight during floating, transportation and assembly as they are provided with steel bulkheads. These will be dismantled later when the permanent joints are installed.

    The connection and sealing of the elements is a very interesting technical challenge and you will hear more about this in a future post on this blog!

  5. I am very interested to hear how you connect and seal the tunnel elements together. I should imagine that this process offers significant technical and safety challenges.

    Are the tunnel sections flooded or airtight during assembly on the seabed?

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