Article - Issue 6, November 2000
The Øresund Bridge – linking Scandinavia to the continent
The Øresund Bridge, one of the largest infrastructure projects in the history of Europe, is set to change southern Scandinavia forever. A combined tunnel and bridge link carrying the motorway and railway has replaced the existing ferry routes between the Danish capital of Copenhagen and the Swedish regional capital of Malmö.
The Øresund Bridge comprises three major civil works projects: a 4 km artificial island, the world’s largest immersed tunnel for both railway and motorway, as well as the 7.8 km bridge with a cable-stayed high bridge as the impressive centrepiece.
Almost five years after the civil works began the Øresund Bridge was opened to traffic on 1 July 2000. This 3 billion USD project was completed on schedule as well as on budget.
The new and greatly improved infrastructure will further the integration of the area now known as the Øresund Region, a cross-border region with roughly 3.5 million Danish and Swedish inhabitants. Combining the best of Copenhagen and Malmö, the Øresund Region will have a major impact on Northern Europe in terms of education, science, finance and culture. But before full integration can be achieved, a substantial amount of legislation has to be introduced, not least within the labour market and education.
The skill with which the contractors successfully planned and executed the construction of the tunnel, the bridge and the major dredging works can largely be ascribed to the Design-and- Construct concept which forms a part of all the major contracts. Optimum use of prefabrication also played a key role. As the Design-and-Construct concept lays down precise functional requirements for the whole project, contractors are responsible for the detailed design as well as for the construction work.
Rigorous environmental requirements were in force throughout the construction period, governing both the construction of the link and the impact of the completed link on the surrounding environment. As the Danish and Swedish governments had stipulated that the link must not block the water flow through Øresund, careful trimming of the link’s design reduced the calculated blocking effect from 2.3 per cent of the water flow to 0.5 per cent prior to final adjustment. Compensation dredging in and around the alignment further reduced the blocking effect.
Dredging and reclamation
The dredging and reclamation contract included the initial construction works for the fixed link which began in August 1995. Over four years a total of 7.5 million cubic metres of (mainly) seabed material was dredged, two thirds in Danish waters and one third in Swedish waters. The seabed consists of gravel and stone, clay till and limestone with flint layers.
Dredging was carried out by a number of dredgers, traditional bucket dredgers as well as a specialised cutter/suction dredger for the tunnel trench.
The objective was for the completed link to have no net effect on the water flow and on the movement of salt and dissolved oxygen through Øresund to the Baltic Sea. Consequently the size and shape of the peninsula and the island were optimised to reduce the blocking of the water flow through Øresund. Further reduction to zero blocking was achieved by compensation dredging. All dredged seabed material was reused in the reclaimed areas which cut sediment spill to a minimum and, in addition, reduced the need for dumping sites as well as for import of sand and gravel.
The dredging works included:
Work harbours and access channels at the artificial island and the artificial peninsula
A trench for the immersed tunnel
Relocation of the Drogden Channel Relocation and deepening of the Flinte Channel
Compensation dredging to achieve zero blocking of the water flow
The reclamation works included:
The artificial peninsula at Kastrup
The artificial island which connects the tunnel and the bridge
The 0.9 km2 peninsula at Kastrup serves to bring together the passenger track from the underground station at Kastrup Airport with the freight track running north of the airport. The motorway along the northern edge of the peninsula joins the railway at the entrance to the tunnel portal. The peninsula also has a track leading to a railway maintenance area.
The 1.3 km2 artificial island south of the natural island of Saltholm forms the transition between the tunnel and the bridge. From the tunnel portal on the western end of the island, the motorway runs parallel to the railway to the eastern end, from where the railway runs under the motorway on to the two level bridge.
The reclamation areas were constructed as basins surrounded by coarse pebble bunds lined with a geotextile membrane. The basins were then backfilled with clay to prevent the suspended sediment from escaping. Reclamation was carried out in stages as the bunds were completed, surrounding the individual sections. During filling at the artificial island, the traffic corridor for the high-speed railway was compacted to minimise any differential settlements which could interfere with the passage of highspeed trains. The reclaimed areas were completed with revetments, comprising layers of armour stones and filter stones. The revetments protect the tunnel against flooding during storms and high water.
The immersed tunnel
The tunnel extends approximately 4 km from the Danish coast at Copenhagen Airport to the artificial island in the middle of Øresund. The tunnel comprises five parallel tunnel tubes – two for the railway, two for the motorway plus a small tunnel as an escape gallery. The sheer size of the tunnel and the shallow waters in this part of Øresund made an immersed tunnel the natural choice.
The immersed tunnel was constructed from twenty prefabricated concrete tunnel elements. The elements were cast at a purpose-built factory 12 km north of the tunnel site. Each element was towed to the tunnel site by four tug boats and lowered into the tunnel trench with enormous precision, using GPS satellite navigation.
The fact that casting took place under cover allowed for efficient and smooth production, regardless of the weather. As a positive side effect the number of work-related accidents was subsequently reduced.
Each tunnel element is 176 metres long, 49 metres wide, approximately 9 metres high and weighs around 57,000 tonnes. Each element was cast in eight sections at the tunnel factory each of which was cast in one 30 hour cycle. Each section was cast against the previous section and the element was then gradually pushed out of the casting hall and on to a ramp, in what was, in effect, a giant lock system. When all eight sections were cast and one complete 176 metre element was ready, a sliding gate was closed behind the element, and the basin was flooded until the element floated.
The element could then be pulled into the deep end of the basin to await being towed to the tunnel site. After the water inside the lock had reached sea level allowing the element to be towed out, the next two elements would be cast. Two parallel production lines allowed for maximum output and efficiency.
The towing of the elements demanded great skill and precision. The 57,000 tonne element was almost completely immersed during towing and the changing currents in Øresund had to be calculated in great detail before the towing-out. This operation took place in the Drogden Channel in Western Øresund, which is an international shipping route used by some 40,000 ships per year. Early in the construction phase a radar station was established to monitor shipping and guide vessels through the work areas thus avoiding any major accidents. During operations only a handful of collisions and minor incidents took place involving work vessels and shipping, causing material damage only.
Not once was a tunnel element damaged due to a collision. However, on 4 August 1998, the accident that everybody was dreading took place: a tunnel element was flooded while being positioned in the tunnel trench. Before tow-out the tunnel tubes in the element were sealed off with steel bulkheads. Due to a number of mistakes one bulkhead was not properly secured and succumbed to the water pressure. In a matter of seconds all five tunnel tubes in the large concrete element filled with water, and in accordance with procedures the element was immediately lowered into the trench, but several metres out of position. It took Øresund Tunnel Contractors eight weeks to inspect, repair and eventually move the tunnel element into its correct position.
During this period the lowering gear and platforms were attached to the flooded element so the contractor was unable to save time simply by building the tunnel from both sides at the same time. However the tunnel factory continued to produce the tunnel elements and once the flooded tunnel element had been repaired and placed in the correct position, the tow-out and construction of the rest of the tunnel elements was carried out at high speed. As a result, despite this major setback, the tunnel was constructed ahead of schedule.
For the bridge section, which connects the artificial island with the Swedish coast at Lernacken, a cable-stay design was chosen for the high bridge due to the weight of the combined railway and motorway link. The entire bridge is 7.8 km long of which the high bridge accounts for 1,092 metres. The 490 metre long main span of the high bridge crosses the Flinte navigational channel, which is used by approximately 10 per cent of the vessels passing through Øresund. The majority of shipping passes through the Drogden navigational channel along the Danish coast.
As was the case with the tunnel, a high level of prefabrication was used in the construction of the bridge. Apart from the pylons which were cast in situ, all other components – caissons, pillar shafts and bridge girders – were produced on land and transported to the bridge by the large floating crane, the ‘Svanen’. This self-propelled crane, a unique and highly versatile tool, positioned the heaviest concrete caissons and pillar shafts with great precision. For two years ‘Svanen’ made regular trips between Sundlink Contractors’ bridge factory in Malmö North Harbour and the bridge line, constructing the two approach bridges and the high bridge like a giant Lego kit.
The superstructure of the bridge is a composite steel-concrete structure with truss girders. The upper deck carries the motorway and the lower deck contains the railway. The bridge is the largest of its kind in the world carrying both passenger trains, freight trains and motorway traffic.
The cables of the high bridge are arranged in a classic harp pattern and anchored to the superstructure truss at 20 metre intervals. Supported primarily on two pairs of pillar shafts the bridge is symmetrical from the centre of the navigation span. It has intermediate side span piers to limit the deflections of the 490 metre main span. Each pylon consists of twin, cast-in-situ concrete towers extending to a height of 204 metres above sea level. Cables are anchored in the pylons at 12 metre intervals. The foundations for both the pylons and the side span piers are prefabricated concrete caissons, located in the hard limestone, 14 to 17 metres below sea level.
With a height of 204 metres above sea level the pylons of the Øresund Bridge are the tallest concrete structures in Sweden. The construction of the pylons began with the placing of the pylon caissons which had been cast in a dry dock in Malmö Harbour and towed to the bridge line. On top of the caissons the casting of the pylons was carried out by means of selfclimbing formwork. The entire casting process took seven to ten days for each 4 metre lift.
About 4,335 m3 of concrete went into each pylon leg – 220 m3 per lift in the first stage and about 34 m3 per lift near the top. The concrete was provided from a batching plant on a barge moored next to the pylon cofferdam. Each pylon leg required 800 tonnes of reinforcement which was prepared in prefabricated cages, and then lifted into position. The pentagon shaped pylon legs are solid for the first 17 metres and hollow above that level.
The reinforcement cages for the cross beams, which connect the pylon legs approximately 45 metres above sea level, were built ashore, then lifted into position for casting. Steel anchor boxes were also cast into the pylon legs, to which the cables are attached.
The eight bridge girders for the high bridge were produced at Karlskrona shipyard on the East coast of Sweden and then towed to Malmö, where the concrete motorway deck was added. The forty-nine girders for the two approach bridges were produced by Dragados in Cadiz in Southern Spain and subsequently towed to Sweden in pairs, complete with concrete motorway deck.
The high bridge is carried by a total of 80 pairs of cables, ten in each direction from each pylon leg. Each cable consists of between 68 and 73 strands. A strand consists of seven 5 mm wires. The wires are galvanised, waxed and encased in plastic casing. Each cable has a tensile strength of 2,000 tonnes and the combined length of all cables is about 25 km. The total weight is about 2,300 tonnes.
Following the inauguration of the Øresund Fixed Link on 1 July 2000 approximately 500,000 vehicles crossed the Øresund Bridge during the first month. Information about traffic and toll prices etc. can be seen at www.oeresundsbron.com. The photo files of the Øresund Bridge are available at www.bridgephoto.dk.
Main contractors on the Øresund Bridge
Dredging and Reclamation
Öresund Marine Joint Venture:
Per Aarselff (DK)
Ballast Nedam Dredging (NL)
Great Lakes Dredge and Dock Co. (USA)
Øresund Tunnel Contractors:
NCC International (SE)
Dumez-GTM International (F)
John Laing (UK)
E. Pihl & Søn (DK)
Boskalis Westminster Dredging (NL)
Højgaard & Schultz (DK)
Monberg & Thorsen (DK)
Banverket Industridivisionen (SE)