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Engineering feat of the month: Øresund Bridge and Drogden Tunnel

22/03/2016
This month’s instalment of our ongoing ‘Engineering feat of the month’ features will focus on the famous Øresund Bridge and Drogden Tunnel.

Overview
The project was a 16 km long road and rail link between the Swedish town of Malmö and the Danish capital, Copenhagen. It consists of three sections: The 8km Øresund Bridge, a 4km artificial island and the 4km Drogden Tunnel, which transports both passengers and freight. The project was proposed with the intention of bringing Europe and Scandinavia closer together, also allowing the Swedes better job prospects in Denmark and the Danes the opportunity to buy more affordable housing in Sweden.

The project began in 1991 and was completed 3 months early and under budget in June 2000.

The Bridge & Tunnel
A competition was run to find the best design for the bridge, however the complexities of the project meant that many ideas weren’t feasible. ‘The Largest Arch Bridge in the World’ was proposed but then rejected on the basis that its sweep down into the water would reduce the space available for large ships to pass through, running the risk of these vessels hitting the bottom part of the arch. A suspension bridge idea was also vetoed due to the flexibility caused by the cables holding up the carriageway- trains can’t run along tracks that might bend slightly under their weight.

The most viable option was to build a cable-stayed bridge. The extra stiffness of such a bridge would be ideal for carrying rail traffic, and was a much cheaper option than a suspension bridge.

The cable-stayed bridge carries rail traffic beneath the road carriageways - This option was more than likely chosen to reduce the extent of any accidents. It would have been possible to have both road and rail run alongside each other, however its current form allows for extra road carriageways to be added in the event of increased traffic load in the future.

One of the most obvious problems with a bridge made up of 80million kg of steel is rust, and soon after the bridge was opened it was noticed that the bolts in the guard rails were corroding. To stop the 16,000 bolts rusting through they were individually removed, blasted clean, coated in gel to keep out any water, and reinstated using better fitting washers. Dehumidifiers were also installed to prevent rust within the structure of the bridge itself.

Most of Øresund Bridge’s elements were built on land and then hauled out to sea by floating crane. The immersed Drogden tunnel was also built by concrete elements cast on land and then towed out.
Each of the 20 sections of the Drogden Tunnel is home to 7.5 trillion litres of concrete, and stand at the length of almost 2 football pitches and the height of a three story house. No machine was strong enough to lift each segment so the pieces were plugged and turned into rafts. A loch system was then built so that water was brought to the segments (rather than vice versa). That way each piece could be guided through the man-made basin and out into the channel ready to be sunk into trenches dredged in the sea bed.

Fabrication of the link’s elements on shore meant that a number of contractors could bid for the projects, driving down the overall price of project. It also reduced the risk of high pollution of channel water.

The Island
The man-made island section of the 16km link between the two countries was necessary to enable the two tiered traffic from the bridge to transform into the one level of traffic required for the underwater tunnel. Peberholm Island was conveniently built from the materials dredged from the seabed to accommodate the bridge piers and the tunnel, however the process of dredging was far more complex than the initial plan.

The underwater Copenhagen Limestone which needed removing was incredibly difficult to cut through and the dredger used at the beginning of the process wasn’t powerful enough for the job. An investment in a more powerful, more expensive dredger was required. The process of dredging was also made more time consuming by the rule imposed by both the Danish and Swedish governments which stated that if any more than 5% of debris was spilt into the water then all dredging should stop.

To add to the complications, 16 unexploded WW2 bombs were found on the sea bed where the dredging was supposed to be taking place. Locating them all and then having them safely detonated added both safety and timescale problems.

International barriers
Connecting two countries via tunnel-bridge link comes with more challenges than if the same structure was within the same country. For example, Denmark and Sweden run their trains on different electrical voltages, and on different radio signals and frequencies. Train operators also speak different languages. To combat these issues a large investment was made in electrical equipment to switch trains from 25k volts to 15k volts whilst in transit. Engineers also had to design a computerised Rosetta Stone to create a standardised wording so that operators on both sides of the link could understand spoken instructions.

The 16km link between Denmark and Sweden was one of the most ambitious civil engineering projects in the world, and it currently stands as the longest road and railway bridge in Europe. The project is considered a huge success and the economic value added to the area by joining Northern Europe and Scandinavia makes the $3billion investment incredibly worthwhile.


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