The Fehmarnbelt link between Denmark and Germany will be the world’s longest road and rail tunnel when completed, thanks to some remarkable design solutions.
The Fehmarnbelt crossing is set to reshape the European map, by providing a link under the Baltic Sea between Scandinavia and central Europe.
It will be more than five times the length of the tunnel under the Øresund strait linking Denmark and Sweden.
The planning and construction of the fixed link is being undertaken by the state-owned Danish company Femern A/S, with a Ramboll / Arup / TEC joint venture serving as the main consultant for Femern A/S since 2009.
The construction of the link was approved in Denmark in April 2015 and in May last year contracts for the four civil works contracts with a value close to €4bn were signed.
However, work is being held up due to the complicated approval process in Germany. The construction phase is estimated to be eight-and-a-half years, which would see it opening in 2028.
When complete, the combined road and rail tunnel across the Fehmarnbelt, comprising a four-lane motorway and a double-track train line, will cut journeys across the sea from 45 minutes to about 10 by car and seven by train.
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This project will set new records in terms of scale and dimensions. The tunnel is unique in scale, boasting a 42 m width. At 18 km, the immersed tube tunnel (IMT) will be by far the longest of its kind in the world. Its foundation depths reach more than 40 m below sea level – one of the deepest tunnels of this type ever built.
There have been a number of major design innovations that have made it possible to overcome various technical challenges.
Special elements in the IMT
To produce a cost-effective design, the tunnel’s cross-section had to be minimised while housing the necessary mechanical and electrical systems essential for the tunnel’s operations.
Ramboll’s solution was to install a total of 10 double-deck ‘special elements’ positioned about every 1.8 km. Each of these elements is 39 m long, 45 m wide, 13 m high and contains an upper road / rail deck level with an installations level beneath.
Fehmarn Cross section Perspective
These elements provide other benefits including parking access in a lay-by for maintenance vehicles from Denmark and transformers that can be replaced from road level.
They will also allow access to mechanical and electrical equipment without interfering with traffic and a transverse underpass with access to the longitudinal gallery as well as the road and rail tubes.
Ventilation Design and Safety
Ventilation design has a huge influence, with many road and rail tunnels needing 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.
“At 18 km, the immersed tube tunnel will be by far the longest IMT in the world”
On the Fehmarnbelt tunnel the ventilation is designed as a longitudinal system, reducing the required tunnel cross-section and eliminating the need for intermediate ventilation shafts.
Sophisticated ventilation modelling was undertaken, taking into account the growth of traffic in relation to a reduction in car emissions as vehicle technology improves. This demonstrated that the system would keep conditions at globally recognised levels during the whole lifetime of the tunnel.
The main challenge during construction will be the logistics.
Fehmarn crossing Portal Area
There will be six construction lines running in parallel producing 79 standard elements (each with a length of 217 m), as well as 10 special elements. The bridge will require the casting of more than 3m cu m of concrete and the placement of 360,000 tonnes of reinforcement.
This massive concrete production will be delivered by several batching plants, necessitating a large harbour to ensure that cement and aggregates will be available in sufficient quantities. Up to 3,000 workers are expected to be involved in the construction during the peak years.
The Fehmarnbelt tunnel will push the limits of submersed tunnel technology and show what is viable with clever engineering.
Richard Miller is a technical director at Ramboll