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Tunnelling: Immersive opportunities


Contract award on the “progressive and innovative” Fehmarnbelt Tunnel presents opportunities for other immersed tube tunnel developments

Design of the tunnel to link Germany and Denmark has pushed the boundaries of immersed tube tunnel design and the level of innovation this has called for will benefit future projects. Not only will it be the deepest of its kind ever built at 40m below sea level, it will also be three times the length of any existing immersed tube tunnel.

When complete the 18km combined road and rail Fehmarnbelt Tunnel will connect continental Europe with Scandinavia and replace the existing ferry links with a four lane motorway and a double track rail line. It will reduce travel time by sea from 45 minutes to approximately 10 minutes by car and seven minutes by train, or save more than 150km on the journey via the Storebælt connection.

When the concept was first proposed along with a bridge alternative few could have predicted that the tunnel option would have reached this stage and driven the level of innovation that it has. It is a project that will shape immersed tunnelling techniques as it sets new records in terms of scale and dimensions that will establish the introduction of pioneering techniques and solutions.

The 42m width, depth of construction and length of the link has called for an innovative approach to design to overcome the challenges of constructing an immersed tube tunnel on this scale.

The costs of this innovation will be financed by future users of the tunnel rather than Danish or German taxpayer. The Fehmarnbelt link will be financed according to the Danish state guarantee model, which also financed the fixed links across Storebælt and Øresund. This model is based on state-guaranteed loans from the Danish state, which will be repaid over time using revenue from users. In addition to user payment, the Fehmarnbelt project receives substantial co-financing from the EU – approximately €800M has been awarded so far and this does not need be repaid.

Ground risk

A comprehensive geotechnical investigation was conducted with the initial scope designed to inform the design of both the bridge and tunnel options. The scope of the geotechnical investigation developed in parallel to the development of the project design and included phases of geophysical surveys, off-shore boreholes, advanced laboratory testing, large-scale testing and a high resolution seabed elevation model.

Fehmarnbelt geological section

Fehmarnbelt geological section

The scope of this investigation was recognised when it won the Award for Technical Excellence at GE’s sister publication’s NCE International Tunnelling Awards. These investigations informed the development of a complex a three dimensional ground model of the mixed Quaternary deposits overlying Tertiary clays and Cretaceous Chalk.

The portion of the tunnel near the German coast is founded in the Palaeogene Clay, a high plasticity clay with a high swelling potential during the unloading through excavation of the trench for the tunnel. This was a key project risk and to investigate this more thoroughly it was decided to conduct full scale tests.

Fehmarn 1

A 1:1 scale instrumented trial excavation was undertaken offshore within the clay

A 1:1 scale instrumented trial excavation was undertaken offshore within the clay. Five driven steel tube piles and five bored cast-in-place concrete piles were constructed adjacent to the excavation. The results from the large scale tests were interpreted and proved that the Palaeogene Clay had a lower swelling potential than anticipated. This was significant for the control of differential settlements along the tunnel and at the portals - providing significant cost savings. Few projects take the time and spend the money for large scale tests and so this is a good example of the value that such tests offer.

Design considerations

All tunnels require a variety of mechanical, electrical and control systems to function and these usually have large space requirements, with associated structures such as transformers and sumps, often grouped together periodically along the tunnel in cross passages and niches. The key to an economic design is creating the required space for this plant while minimising the tunnel cross section. The solution on the Fehmarnbelt Tunnel was to create “special elements”.

In addition to the standard immersed tube tunnel elements, the tunnel will have a total of 10 special elements that are installed at regular intervals – approximately every 1.8km – between the standard elements. Each of these special elements is 39m long, 45m wide and 13m high.

The special elements contain two levels: an upper road/rail deck level with an installation level beneath. The installation level provides the space required for all the mechanical, electrical and control systems required to operate a road and rail tunnel. The special elements also provide a number of other key benefits including parking access in a layby for maintenance vehicles from Denmark; transformers which can be replaced from road level; access to the mechanical and electrical equipment without interfering with traffic; and a transverse underpass with access to the longitudinal gallery in each road tube and rail tube

Ventilation design has a huge influence on the arrangement of a tunnel and many road and rail tunnels require intermediate ventilation shafts. For a subsea tunnel, the implications of such intermediate shafts are costly, introduce navigational risk and can be controversial from an environmental point of view.

On the Fehmarnbelt tunnel the ventilation is designed as a longitudinal system reducing the required tunnel cross section and also eliminating the need for intermediate ventilation shafts. Sophisticated ventilation modelling was undertaken taking account of the growth of traffic usage in relation to a reduction in car emissions as vehicle technology improves. This demonstrated that the system is capable of keeping conditions below internationally recognised threshold values throughout the whole lifetime of the tunnel.

Fehmarn internal tunnel image

Driver perception and comfort have been analysed in great detail to improve tunnel safety

In the event of a fire in the tunnel, the traffic behind the fire will stop but the vehicles in front of the fire will continue to drive out of the tunnel, considerably faster than the flowing smoke layer. Upstream of the fire the tunnel will be kept smoke free by maintaining a minimum critical air velocity with fans, thus preventing back-layering of smoke. Passengers in the traffic behind the fire may evacuate into the adjacent central gallery and the other road tube, via escape doors spaced at intervals of about 100m. The short distance between the escape doors improves the capability for self-rescue.

Driver perception and comfort have been analysed in great detail to improve tunnel safety. In the conceptual design moving light images and coloured light portals were introduced. The images to be projected on the tunnel walls and the light portals will help maintain driver awareness and develop a sense of progression through the tunnel during the 10 minute journey.

Future inspiration

When the first feasibility study was done in 1996, few people would have given an immersed tube tunnel option much chance in comparison with a bridge solution. The client, Femern was key in encouraging this through use of a design competition between the bridge and tunnel options to find the best solution.

A series of major design innovations, by the client’s design team led by Ramboll with Arup and TEC, tipped the balance in favour of the tunnel, notably the shift to longitudinal ventilation and the use of special elements to house the mechanical and electrical plant rooms. Many of the innovations from the design and construction of this project will inspire future long sea crossings using immersed tube technology.

Richard Miller is technical director at Ramboll

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