An immersed tube tunnel design has made a comeback against competition from a bridge design for the new Fehmarnbelt crossing, NCE can reveal.
Project promoter Femern confirmed that while previously it had favoured a cable stayed bridge design for the proposed 19km long road and rail crossing between Denmark and Germany, changes to the tunnel design had put it on an equal footing.
The £4bn crossing will link Rødbyhavn on the Danish coast and Fehmarn on the German side and will comprise a four-lane motorway and a double-track electrified rail line.
Under earlier plans the tunnel option would have required a ventilation island in the middle of the crossing, which meant very high construction costs because of its location in 30m deep water and a busy shipping lane.
“Ventilation was the biggest challenge [to fire safety] and we considered one or more ventilation islands in the channel but we [now] believe we can build it without the island,” said Arup associate director Kim West. It is part of a joint venture (JV) with consultants Ramboll and TEC designing the tunnel while consultants Cowi and Obermeyer are working on the bridge.
This immersed tunnel will consist of 90, 200m long elements. The tunnel will house two tubes each for road and rail– with a service tunnel between the two road tubes. In addition to reducing costs via the ventilation redesign the team also took out one of the two original service tunnels planned.
Instead of relying on the island option, the JV has designed a ventilation system with 61 jet fans in each road tube placed at 400m intervals to help remove the pollutants. It is not decided whether jets will be used in the rail tunnels as well, or whether the trains can push the air through, in a similar manner to how ventilation in the Channel Tunnel works
In addition, the tunnel includes a deluge fire safety system. One or two large pipes will run along the length of each tunnel tube and discharge water in the event of a fire.
The tunnel has a larger environmental impact than the bridge because it requires a much larger amount of excavation – up to 20M.m³ will be dredged from the seabed versus a maximum of 5M.m³ for the bridge.
However, a major advantage in using the tunnel option is that it will have no impact on shipping, which currently has 40,000 crossings per year and is predicted to rise to 100,000 in 2030.
“Technically both options are realistic and quite close [in the competition],” said West.
Femern confirmed the new status of the tunnel. “So far, both are neck and neck in terms of technology, safety and finances” said a spokesman, adding that construction for either option was likely to take around five years.
Femern is expected to make its selection by the end of the year and determine the final route corridor by January next year.