In shorter tunnels machinery and installations – like power supply and pump sumps for discharge – are put directly under or beside the road and rail track. However, these areas may be hard to reach when maintenance is required, thus causing traffic delays because all or part of the tunnel has to be closed.
The single-storey solution would have been too cumbersome for the Fehmarn tunnel, which runs 18 km below sea level with no external access points but the portals at each end, one on the Danish coast and one on the German. The Ramboll-Arup-TeC Joint Venture thus recognised the need for special features.
After a lot of thinking and discussions with the Oresund and Great Belt tunnel maintenance teams, the project team came up with an innovative and unprecedented solution: A tunnel with intermittent two-storey elements, the lower of which would be dedicated to installations.
“Traditionally, all tunnel elements are a single storey. With two storeys, there is enough room for the power supply, the pump sumps and machinery on the lower level – so you don’t have to stop traffic on the upper level when you do maintenance,” explains Susanne Kalmar Pedersen, Project Director at Ramboll Transport.
The tunnel will be constructed from 79 standard elements and 10 special elements to be located at 1.8-km intervals. All mechanical and electrical equipment requiring space and frequent maintenance will be gathered in these special elements, thus simplifying the technical production of the standard elements, which will be more uniform and, therefore, better suited for
The tunnel elements will be manufactured on land, towed to the tunnel alignment and immersed one by one in a pre-dug trench.
Once in place, the elements will be covered with sand and a protective stone layer, and in a few years’ time, the natural seabed will have been completely re-instated.