Oresund Bridge
A marvel of modern engineering and a combined railway and motorway bridge spanning the Oresund or ‘The Sound’ between Denmark and Sweden, the Oresund Bridge is the longest bridge in Europe. The bridge runs nearly 8 km (5 miles) from the Swedish coast to the artificial island Peberholm in the middle of the strait. The crossing is completed by the 4 km (2.5 miles) Drogden Tunnel from Peberholm to the Danish island of Amager.
First opened in July of 2000 as a primary means of transit for the entire region, the structure carries a data cable through its infrastructure that serves as a connection hub for much of the internet access across Europe. It is a symbol of many and sometimes opposing things: Nordic cooperation in the economy and engineering, as well as the solving of grisly real or imagined crimes; freedom of movement, with thousands of commuters travelling both ways on a daily basis.
At its completion in 2000, the bridge was advertised by a massive half-marathon and on 1st July the King of Sweden Carl XVI Gustav and the Queen of Denmark Margrethe inaugurated it in a widely-broadcast event that was central to showcasing the tangible success of the transnational cooperation.
Design And Concept
The engineers behind the bridge had to design a transit structure that was tall and wide enough to let shipping traffic transit the channel – thus the bridge portion. However, the bridge also couldn’t be so tall that it obstructed the nearby Copenhagen Airport.
Engineers were concerned that the high shipping bridge would be too much of an obstacle to landing and taking off planes that they decided to transition the relevant portion of the bridge into a tunnel. The biggest fear of engineers was that after construction, a plane might crash into a support tower of the bridge, creating large interruptions in international travel.
In the initial scoping phase of the project, a design competition was held to determine how the bridge and tunnel structures should be built. The Oresund Bridge project involved building an underwater tunnel, an artificial island, an artificial peninsula and a bridge.
Design - Cable-stayed Bridge | Length - 7,845 m
Width - 23.5 m | Height - 204 m | Clearance Below - 57 m
Designer - Jorgen Nissen, Klaus Falbe Hansen, Niels Gimsing and Georg Rotne
Engineering Designer - Ove Arup & Partners
Construction Span - 1995-1999
Construction Cost - 19.6 billion DKK 25.8 billion SEK
2.6 billion EUR
The tunnel connects to the Denmark side and construction began there in late 1993. Workers created an artificial peninsula named Kastrup to house the entrance into the tunnel and over the next five years a tunnel trench was dredged in the seafloor. Further, twenty prefabricated concrete elements were lowered underwater and connected which resulted in a type of tunnel known as a tube tunnel, as it’s essentially a long concrete tube.
The finished tunnel is 2.5 miles long. It comes back to the surface and transfers traffic onto the bridge thanks to an artificial island named Peberholm, created out of dirt and materials dredged for other parts of the project.
Construction
In November 1995, the design-build contract for the Oresund Bridge, was awarded to Sundlink. The 7485 m long bridge consists of two approach bridges and one high bridge. It is a two-level structure with an upper concrete road deck on steel trusses and a lower concrete deck for the railway. In 1996, it became evident that the heavy lift vessel Svanen would be ready for use in Summer 1997, which would fit very well into the overall time schedule for the Oresund project.
As a result, it was possible to increase the sizes of the concrete elements forming the superstructure and to handle fully equipped superstructure elements. Concrete caissons, pier shafts and troughs for the railway deck of the approach bridges were prefabricated at Malmo North Harbour. The two pylon caissons of the high bridge were much too heavy to be handled by Svanen, so that they were produced in a dry dock near the bridge site. All the approach caissons were founded directly on limestone below. Both approach and pylon caissons were placed on concrete support pads grouted on the limestone. The eight girders for the high bridge were made in Sweden.
Another notable constraint to the bridge was that it needed to have a rail line under the bridge. The Oresund Bridge needed to transport everything, cars, railcars and international internet cables. The rail constraint kept the engineers from going with a traditional cable suspension bridge, which would’ve been too shaky for trains. Rather, they settled on a cable-stayed design, which transfers weight through multiple cables back to main towers.
This island was the main transition point and focal point of the entire transit system, but it was made entirely of dredged seafloor material. This meant that the tunnel couldn’t be drilled, rather the tunnel had to be prebuilt into the island.
Engineers created 55 million total kilograms of tunnel segments that were sealed shut using specially designed ships underwater, where they were then dropped into place and backfilled. In total, the Oresund Bridge cost 4.5 Billion USD at the time, which was paid for through government funding and loans initially, but is now being recollected using tolls from drivers that cross the bridge. The toll system is expected to completely pay for the bridge within 30 years of its completion date, or 10 years from now, in 2030.
As for the construction of the bridge supports, there are 4 main towers. These massive towers were poured in sections that were placed using specially built rigs. Another aspect of the structure that is quite unique and interesting is that a maintenance system and vehicle were built into the structure. Since daily inspections were needed on the structure to ensure safety, there’s a motorized gantry under the railway portion of the bridge. This vehicle is equipped with a variety of tools, even a large hydraulic arm that can reach the road’s surface.
While many major construction projects can be quite detrimental to the environment, the major care was taken during construction to not harm the surrounding underwater ecosystem. The artificial island is now home to over 500 species of plants and has become a haven for biologists.
Tunnel
With a portal to portal length of 4.05km, the tunnel is the longest of its type in the world, for road or rail. It runs from the Kastrup headland to Peberholm beneath the Drogden navigation channel, and consists of a 3.15 km immersed tube with 270 m entry tunnels at each end. Building it involved pouring almost 500,000 cu m of high-strength reinforced concrete in just 24 months. At peak, some 8,000 cu m of concrete per week was being placed.
The immersed tube section comprises 20 precast concrete tunnel units, manufactured in a purpose-built casting yard at Copenhagen’s north harbour. Each unit is 175.5m long, weighs up to 55,000 tonnes, and consists of eight segments 38.8 m wide and 8.6 m high. Parallel production lines enabled two units to be constructed at a time.
The casting was done on dry land, with new segments cast against the previous ones as the units were slid gradually into a split level dry dock. Steel bulkheads were installed at the ends of the units to keep them watertight. As each unit was finished, a sliding gate closed the dock, water was let in and the unit was floated out to sea. On 8th August 1997, the first unit was towed to the pre-dredged tunnel trench and sunk into position using GPS navigation.
On 4th August 1998, one of the bulkheads failed while a unit was being placed. The flooded unit was lowered into the trench, several metres out of position. Repair and relocation took eight weeks. Once all were in place, the units were connected into a single tube and the trench backfilled at the sides. A layer of stone ballast was placed over the top for protection, and lies some 10 m below the water surface.
The south side of the tube carries two train tunnels, each with a single high-speed track. The north side has two road tunnels, each with two lanes and ceiling mounted ventilation fans. Within the wall between road tunnels are three voids, one above the other, for services, an escape route and cabling.
On 16th March 1999, the first official vehicle — a bus carrying the Danish transport minister and the Swedish minister for industry, employment and communications — was driven through the finished tunnel, from Copenhagen to Peberholm.
Reference
https://interestingengineering.com/how-and-why-oresund-bridge-was-built
https://www.oresundsbron.com/en/node/6738 » https://nordics.info/show/artikel/a-short-history-of-the-oresund-bridge
https://study.com/academy/lesson/oresund-bridge-underwatertunnel-construction-facts.html
http://www.engineering-timelines.com/scripts/engineeringItem. asp?id=1448
By -
Tuhina Chatterjee, Associate Editor Civil Engineering and Construction Review
