TBM tunnels considered for Fehmarn sea link Dec 2011
Shani Wallis, TunnelTalk
Despite the environmental advantages and Denmark's past experience of building the TBM bored tunnel under the Storebælt, technical investigation of a TBM bored tunnel option for the 18km long undersea crossing of the Fehmarnbelt between Denmark and Germany has not shaken preference for the immersed tube solution.

Could TBM bored tunnels create the Fehmarnbelt undersea link?

With an estimated cost of €6.8 billion, a TBM bored tunnel concept, based on two highway tubes and a twin-track railway tube, is more than 25% higher than the 2008 estimated €5.5 billion immersed tube option and is considered to carry more construction risk.
According to a news release from Fehmarn A/S, the Danish client authority, the bored tunnel is a higher risk due to the combination of a large tunnel diameter, long boring drives and difficult ground conditions. The release goes on to state: A bored tunnel solution is believed to be pushing the boundaries of what is considered proven construction technology.
"The results of our TBM tunnel investigations do not change our preference for an immersed tube from the technical point of view," said Steen Lykke, Technical Director of Fehmarn A/S. "On the contrary, they have confirmed that an immersed tunnel has a number of advantages within a range of parameters. This applies to technical risks, construction time, construction costs as well as operation and maintenance costs."

An immersed tube is preferred over the bridge options

Public hearings on the scoping report in 2010 returned recommendations that the bored tunnel should be considered and the technical investigation of the alternative is now included in the Consolidated Technical Report. The report also includes evaluation of the originally favoured cable-stayed or suspension bridge alternative.
For reference, Denmark completed the twin bored tunnel of 8km long through similar geology for the 18km long fixed link across the Storebælt in the 1990s. Comprising two bridges for the road link and the bored tunnels for the railway under the east channel, the tunnels opened first in 1997, and the roadway in 1998. Tunnel construction, at between 12-40m below the sea bed and a maximum 75m below sea level and using four Howden/Wirth TBMs of EPB style, overcame various geological challenges as well as two serious construction accidents, a flooding of both tubes at the start of two drives and a fire that burned out one of the TBMs out under the sea. The DKK 21.4 billion cost (US$3.75 billion) (in 1988 prices), were split almost equally between the road and the rail links.
The release stated that the Consolidated Technical Report is solely a technical examination of the different solutions in that all solutions are currently subject to an environmental assessment.
The final approval of the project in Denmark will be in the form of a construction act passed by the Danish Parliament. In Germany the final approval will be made by the Landesbetrieb für Straßenbau und Verkehr Schleswig-Holstein authority in Kiel.
The approvals are expected in 2013/2014. Construction works are then expected to start in the Summer of 2014 towards an opening of the Fehmarnbelt fixed link in 2020.
References
Political backing to Denmark-Germany link - TunnelTalk, Feb 2011
Cost comparison for Femarnbelt link options - TunnelTalk, Nov 2010
Deep geological data on Fehmarn Link - TunnelTalk, May 2011

           

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