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?