Immersed tunnel remains favourite

A detailed examination of the bored tunnel variant was already concluded last year and Femern A/S published a consolidated technical report with descriptions of the four structural variants examined. Based on these studies, the immersed tunnel and the bored tunnel have now been weighed up against each other in a technical comparison. The aspects compared include the construction method and construction risk, duration of construction period, consumption of resources, usability of the excavated material and costs.

Two different construction methods

In contrast to an immersed tunnel where prefabricated tunnel elements are lowered into a tunnel trench that has already been dredged and which is then refilled again, a bored tunnel is made by tunnel boring machines that drill through the subsoil. For the bored tunnel, three tunnel tubes about 20 kilometres long and about 15 metres in diameter would have to be bored through difficult subsoil.  Unlike the immersed tunnel, the construction of a bored tunnel of this size and under the given geological conditions cannot be seen as a tried and tested construction technology. The construction risk is therefore greater with a bored tunnel. Also, the construction period of eight years instead of six and a half is considerably longer for a bored tunnel.

Consumption of resources

The size of the area utilised on Fehmarn and Lolland differs depending on whether an immersed tunnel or a bored tunnel is constructed. The construction site set up on both sides would be considerably larger for a bored tunnel than for an immersed tunnel. However, with an immersed tunnel the area required for the production facility planned on Lolland to build the tunnel elements must also be taken into consideration. The area required on Lolland for construction site and production facility for an immersed tunnel comes to around 205 hectares, the construction site on Fehmarn to around 30 hectares. A bored tunnel requires a construction site of about 76 hectares on Lolland and 46 on Fehmarn. The total land requirement is therefore less, but the impact on Fehmarn is greater.

The energy consumption of the two construction variants also differs substantially. The total energy requirement for the construction of a bored tunnel is 1,230 gigawatt hours, compared with 166 gigawatt hours for the construction of an immersed tunnel.

What will happen with the excavated material?

For an immersed tunnel around 18 million cubic metres of earth will be excavated (14.8 million cubic metres for the tunnel trench and about three to four million for the access roads and work harbours). For a bored tunnel less material will be excavated – 12 million cubic metres for the tunnel excavation and about two to three million for the access roads and work harbours. While the material from the immersed tunnel will be pure seabed that can be used directly for land fill, the material excavated from the bored tunnel has to be extensively treated. This is because the material from a bored tunnel is very fluid and mixed with bentonite. Even after treatment, some of the material may have to be classified as contaminated.

Construction, operating and maintenance costs

The construction costs for a bored tunnel are EUR 6.8 billion, which is about 25 percent higher than the costs of an immersed tunnel at EUR 5.5 billion (both based on pricing as of 2008). Moreover, a bored tunnel incurs higher operating and maintenance costs. As the Fehmarnbelt Fixed Link is a user-financed construction project, in the event of increased costs the resultant rise in interest must also be taken into account. The longer construction period for a bored tunnel has an additional negative impact on this.

For the complete technical comparison of the two tunnel variants, click here (available in English only). The consolidated technical report, which contains a description of all four of the construction variants examined, is available here.