Page 9 - TunnelTalk Annual Review 2010 Preview

Contrary to expectation, an immersed tube across the 20km Fehmarnbelt between Denmark and Germany has come in a whisker less than a cable-stayed bridge. Technical risks, long term environmental impacts, navigational safety and developments
toward more carbon efficient transportation played a large part in reducing the estimated cost of the undersea alternative and elevated it to the preferred solution.
Tunnel beats bridge for Fehmarnbelt Fixed Link
The numbers are in and the tunnel has it by a nose! As large as the numbers are for the scope of the project, a cable-stayed bridge across the Fehmarnbelt for a fixed connection between Denmark and Germany comes in at DKK 38.5 billion (about US$7 billion or E5 billion) while the estimate for an immersed tube across the 20km strait is just slightly less at DKK 37.9 billion.
After agreeing the fixed link concept in September 2008, two teams in the Danish owner organisation, Femern A/S, developed conceptual designs and cost estimates for a four-lane highway and two- track railway connection across the sea on a cable-stayed bridge and in an immersed tube tunnel. Of these, the bridge had been considered the less expensive option through the process and therefore the favoured plan. Announcement of the cost estimates in November 2010 produced a surprise result with the immersed tube revealing a lower estimate than the bridge.
The result illustrates the tremendous work achieved by the tunnel team in exploring new concepts to reduce the
cost of building, operating and maintaining an undersea link. Changing attitudes and new techniques for reducing polluting substances into the atmosphere also played a role in the outcome.
One of the most significant cost savings was elimination of an intermediate man- made island designed to accommodate a ventilation shaft and equipment installation. Projections of low traffic volumes in the initial years, together with significant and rapid technical advances in reducing toxic emissions by road vehicles, have allowed the adoption of longitudinal ventilation in the long four-lane traffic tunnel. Instead of large ventilation buildings and the intermediate vent station island, fans will be installed in ceiling recesses at 400m intervals along the 20km link. This also optimizes the design of the tunnel’s cross section eliminating the need for separate transverse or semi-transverse ventilation ducts. “The change reduces the volume of concrete in the immersed tube elements by some 10%, which is a significant saving on a project of this scale,” said Steen Lykke, Project Director Tunnel for Femern A/S.
TunnelTalk reporting
Since release of the cost comparisons in November 2010, Danish politicians have adopted the immersed tunnel as the preferred option. “The decision means that Femern A/S has reached an important milestone,” said Leo Larsen, CEO, Femern A/S. “As our conceptual design projects are based on a thorough technical foundation, we can now focus on ensuring that the authorities approve the project, including from an environmental perspective.”
Making the case
Fewer risks, all told, in both the construction and operational phases than a cable-stayed bridge is how leaders of the project say they arrived at recommending the immersed tunnel.
A cable-stayed bridge across the Fehmarnbelt, with two free spans of 724m each, would be the largest spans ever constructed for either road or rail traffic. Compounded by the high shipping traffic in the area, this would pose significant risks in the construction phase in terms of cost overruns, delays and industrial accidents.
Immersed tube tunnel (left) comes in slightly less costly and considered less risky over all than the cable-stayed bridge alternative (right)
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