Promoter Femern has played down the possibility of a bored tunnel for its Fehmarnbelt road and rail crossing between Denmark and Germany.
In December Femern resurrected the possibility of using a bored tunnel following a public consultation. It had previously announced that its preferred option was an immersed tunnel for the four lane motorway and dual track rail link. The bored option had been previously rejected due to difficult geological conditions, which in combination with the need for a large diameter and long boring drives would “be pushing the boundaries of what is considered proven construction technology”, Femern had said.
Femern has now produced a technical comparison of the two tunnel variants that it says confirms the immersed tunnel once again emerges as the preferred solution.
A detailed examination of the bored tunnel variant was already concluded last year. 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.
The immersed tube option comes in £1bn cheaper at £4.6bn, and would take 18 months less to build than the bored option. The construction time for the bored tunnel includes the time taken to manufacture the six large tunnel boring machines (TBMs) that would be required. Femern expressed concern that fabricating these machines will demand a large production capacity and to keep to the overall programme the machines have to be fabricated more or less simultaneously. It fears that this will limit the competition as only a few manufactures are able to produce this size of TBMs and fewer still, six TBMs simultaneously.
Immersed tube vs bored tunnel
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, for the bored tunnel, three tunnel tubes about 20km long and about 15m 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 used 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 205ha, the construction site on Fehmarn to around 30ha. A bored tunnel requires a construction site of about 76ha on Lolland and 46ha 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 1,23GWh, compared with 166GWh for the construction of an immersed tunnel.
What will happen with the excavated material?
For an immersed tunnel around 18M.m3 of earth will be excavated (14.8M.m3 for the tunnel trench and about 3M.m3 to 4M.m3 for the access roads and work harbours). For a bored tunnel less material will be excavated – 12M.m3 for the tunnel excavation and about 2M.m3 to 3M.m3 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 cost for a bored tunnel is €6.8bn (£5.6bn), which is about 25% higher than the costs of an immersed tunnel at £4.6bn (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.