S. Lykke, Project Director, Tunnel, Femern Bælt A/S, Denmark
J. Gimsing, Design Manager, Bridge, FemernBælt A/S, Denmark
A bridge or an immersed tube tunnel is set to create the fixed link across the Femern Bælt to connect Denmark and Germany. The Femern Bælt Strait is about 19km wide and the water depth varies typically between 20-30m. The feasibility study of 1999 indicates that a bridge solution all the way with cable stayed main spans is the preferred solution with an immersed tunnel all the way the preferred alternative solution. A combination of bridges and tunnels is not an option and any solution must include four road traffic lanes and two railway traffic tracks; two lanes plus one track in each direction. Both solutions have advantages and drawbacks.
- The treaty regarding establishment of a fixed link across the Femern Bælt was signed on 3 September 2008. The conceptual designs will be developed from spring 2009 and plan approval design will follow. It is expected that the client, Femern Bælt A/S, can start the approval process in Denmark and Germany in spring 2011. The tender process is planned to start in spring 2012 and construction works may start in autumn or winter 2012/2013. The link is expected to be opened for commercial operation in 2018.
Fixed road and rail link between Denmark and Germany
- The schedule is condensed, and from now until the start of construction, quite detailed and comprehensive pre-investigations of physical conditions in the area as well as of technically needed cutting edge technologies. Whether the preferred solution is a bridge or a tunnel, the fixed link will be setting world records.
The navigational requirements for a bridge solution are estimated to be at least two 700m openings with 65m headroom clearance. An alternative solution with one 1700m opening is also under investigation. In the first case a cable-stayed bridge is the obvious choice, in the second case, the choice is a suspension bridge.
- In both cases the bridge will be a world record for the bridge type. The link will be carrying both railway and road traffic. All existing cable stayed and suspension bridges with longer spans are only carrying road traffic, so the Femern Bælt Bridge will be a world record in respect to live load (railway + road) capacity for the chosen bridge type.
- The preferred bridge solution is a cable stayed bridge, mainly due to a 20% cost saving compared to the suspension bridge. The Øresund Bridge connecting Denmark and Sweden has a free span of approximately 500m and could be an inspiration to the Femern Bridge. However the main spans are planned to be approximately 50% longer than the main span of the Øresund Bridge. Due to water depths and other physical conditions similar increase is estimated to be economic for the approach spans, which are 140m on the Øresund Bridge. The ground conditions in the southern part of the Femern Bælt are inferior to the conditions in Øresund, so piled foundations are expected to be needed, which again makes longer spans economic.
Perferred bridge option
- The cross section of the bridge is planned to be similar to the Øresund Bridge with an upper road deck in concrete and a lower railway deck presumably as a steel box with a concrete plate acting compositely.
- Ship simulation studies have found the optimum arrangement of the navigation spans and studies to determine the risk of ship collisions with bridge piers or the bridge superstructure have been completed. It is the intention to secure safety for shipping by, for example, introducing a Vessel Traffic Surveillance (VTS) System in order to maintain, or if possible even improve, the safety of ships after construction of the bridge.
- On the Øresund Bridge the tight construction schedule was achieved by extensive use of the floating crane Svanen. For the Femern Bælt Bridge two such floating cranes with increased heights will be needed or one with double the lifting capacity of Svanen. Preliminary studies of the lifting equipment have been carried out. If it is decided to build a bridge, the client may even choose to get the lifting equipment constructed, so it is ready when the contractors need it.
- The approach bridge from the Danish side is approximately 9,500m long and from the German side it is approximately 6,000m long. Nevertheless it is expected that the time critical construction activity will be the navigation bridge with a length of approximately 3,000m, as this bridge has to be erected in shorter segments requiring more welding and concreting activities in the sea with increased risk of delays due to inclement weather conditions. Optimization of the layout of the navigation span bridge in order to shorten the construction time will be a key issues in the design process.
- If the bridge becomes the preferred solution it is estimated that more than 1 million m3 of concrete will be required.
Immersed tunnel solution
A tunnel solution will in many ways have advantages over a bridge solution. The long-term influence by noise on the local environment compared to a bridge will be minimised. There will be only minor visual and marked disturbances of the landscape in the local area. The risk for ship collision with structures in the Femern Bælt will be considerably lower. The only physical obstruction in the Strait will be a protected ventilation island (which has since been eliminated).
Double deck road and rail bridge
- However, there will be other challenges to be solved before a tunnel solution will be superior to a bridge solution. The preliminary estimations of costs show that a tunnel solution may be in the order of the €600 million more expensive. The temporary adverse effects on the marine environment may be slightly higher. The safety for the passengers and staff during emergency situations will require higher alertness in the development of the design due to the mere fact that the tunnel is very long.
- The new EU directive for tunnels on the Trans European Network will have to be applied, and it has to be combined with the ever increasing demands in the Euro Codes Emission Standards. Car driver behaviour and psychology must be given special focus and special solutions due to the size of this tunnel will have to be found.
No obstructions with an immersed tube tunnel
- The feasibility study from 1999 concludes that an immersed tunnel solution will be superior to other tunnel solutions. The conceptual design will have to be developed over the period to come. However, some key-features of the solution are to become reality as follows.
- The closed portion of the IMT will be approximately 20km long. This will be a world record since no other IMT of this length has ever been built. The deepest point of the tunnel structure will be approximately 43m under the sea surface. This will also be setting a world record. No other IMT carrying both railway and road traffic will have been built to these water depths. The Bosphorus Tunnel in Turkey - the deepest IMT ever built - is approximately 58m of water depth, but it only carries rail traffic. The Busan-Goeje tunnel in Korea - the second deepest IMT ever built is in 50m of water depth, but it only carries road traffic.
Six cell road and rail immersed tube design
- Most likely, the cross section will be in the order of 40m wide by 10m high, and it will likely include 6 tubes; 2 tubes for road traffic, two trubes for rail traffic and two tubes for emergency and installation purposes.
- To maintain an acceptable air quality under all operating conditions and during emergencies will be a challenge. It is therefore likely that one ventilation island has to be constructed in the Femern Bælt. Due to local geotechnical conditions it is likely that the island will be best placed on a chalk/salt dome, which is present a bit north of the centre point of the tunnel. The water depth here is approximately 30m. With this layout the length of the southern portion of the IMT will be about 11km whereas the length of the northern part is about 9km.
Plan and section of the proposed immersed tube alignment
- Over the coming 10 years, it is expected that the emissions from road traffic vehicles will be reduced dramatically, but it is also expected that the requirements to clear air in tunnels will be increased over the same period. One of the major challenges therefore will be to make reliable forecasts of these developments and make fail-safe traffic and ventilation arrangements that will make optimum use of the full cross section of the IMT and at the same time ensure the required traffic capacity.
- The volumes of dredging and reclamation on the tunnel project will be in the order of 20 million m3, and most of these must be dredged at water depths of between 25m and 44m. It is expected that some 70% will be sand, till and 'normal' clay, 10% will be layered clay/silt/fine sand, 15% highly and very highly plastic clay, and 5% organic silt and peat.
- The likely equipment best fit for the dredging job is cutter-suction equipment, but other types may also be considered. However, today we are not aware of cutter-suction equipment with the required capacity to go as deep as 44m and at the same time maintain productivity. The pumping distances to sedimentation basins will be considerable and as far as 12km.
- The Femern Bælt area has a rich flora and fauna. Any spill of materials into the water column as a consequence of the dredging and reclamation activities has to be controlled and limited to a level which is acceptable. What that level is, is not known today, but a good decision basis will be created by performing a number of baseline studies. Such studies were initiated in winter 2008 and spring 2009.
- It is estimated that more than 4 million m3 of concrete will be needed for construction of the Femern Bælt immersed tunnel.
- This paper was prepared for, and presented at the Fifth Symposium on Strait Crossings in Trondheim, Norway, in June 2009
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