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Tubular Trials

Norway is building its first immersed tube tunnel in Oslo, as part of a plan to regenerate the city’s docks.

Oslo’s most striking building is without doubt the new National Opera House. Designed to resemble an iceberg, it is clad in white marble and acts as a beacon to the rest of the city. Unfortunately, most of the city is separated from the opera house, and neighbouring Bjørvika Fjord by the E18 motorway that carries 120,000 vehicles per day.

The motorway follows the shoreline and cuts between a railway station and the opera house, which is built out over the sea. As a result, engineers are working on a E575M (£516M) project to reconnect the city to these important amenities. "The E18 road and service roads make a 12-lane barrier between the city and the fjord. The goal of the project is to remove this barrier," explains Svein Roed, Bjørvika tunnel project director for Statens Vegvesen, the Norwegian roads authority.

"There are two plans – the first is the road plan and the second the city development plan. Once the road is out of the way, we can extend the city south and redevelop this land, which was a port area. A similar development in the next bay along in the 1980s and 1990s was a huge success," says Roed.

It was decided to divert the motorway in an immersed tube tunnel on the adjacent seabed to free up space in front of the opera house. "We could have built a bridge, but then really we have the same problem. A second solution was to dig through the underlying bedrock, but this would have been extremely expensive. The bay is mostly clay, so an immersed tunnel was the most practical solution."

Each end of the 675.5m long immersed tube tunnel comprising six segments will connect to existing tunnels bored through rock. An immersed tube had never been built in Norway so expertise had to be acquired from overseas. The roads authority chose Capita Symonds primarily due to experience the firm gained on the immersed tube section of the spectacular Øresund crossing between Denmark and Sweden.

Capita Symonds associate director and immersed tube expert Jon Baber says the working relationship between consultant and client has been very close: "They wanted the expert knowledge. We have worked with them since 1999, to develop the concept and preliminary designs, then to do detailed designs and to the point of tendering." Norway has little history of design and build and clients typically produce detailed designs and put projects out to tender with very specific criteria for the contractor to meet. "We are considering design and build, but for a project of this size we thought we would stick to what we know," says Roed.

Each tunnel segment is 112.5m long, between 28m and 43m wide and 9.3m to 10.6m tall. Heights and widths vary slightly from segment to segment to accommodate fans and emergency stopping spaces. "It made the design that bit more interesting and also made the project a little unusual. A typical immersed tube will have unique elements at either end but the remainder of the segments will usually be identical. Here, each element is a unique shape," says Statens Vegvesen project manager Ian Markey.

Before the segments can be positioned, the clay seabed had to be dredged along the tunnel alignment. This created problems because the route runs across an area of seabed thought to contain archeologically significant Viking remains. So, instead of using a vacuum dredger, a very large grab dredger was used to scoop up material before it was passed it through a metal screen which acted like a crude sieve.

It was not essential for all objects to be recovered, but a team of archaeologists were on site to salvage material from the grid. Some 8,000 objects were discovered over the past year, including 11 ancient boats. Stranger perhaps was the large pile of contaminated sawdust, carried into Bjørvika by Oslo’s Akerselva river. The sawdust had accumulated through centuries of carpentry in the city. "For the trench there were two elements – most is clays, which was very straightforward to dredge, but there was also contaminated material," explains Baber. "A huge 50,000m3 pile of wood sawings, which were quite alkaline, needed to be removed. But this was out of 600,000m3 to 700,000m3 of the total," he said.

Once the trench had been dug, a final dredge was necessary days before the segments were positioned to ensure it was free from debris. Divers then entered the trench and guided a roll of geotextile onto it. Diving was more time consuming than expected. "We thought they [the divers] would have good visibility, but as soon as they put their foot to the sea floor, it created a cloud of debris." The divers had to find their way through the murky water using a global positioning system.

Immediately before the segments were sunk into place, the contractor laid a layer of gravel onto the geotextile. The elements were constructed in a massive dry dock 800km away in Bergen. The dock was originally built to house offshore oil and gas installations when under construction. Once complete, the vast hollow tubes were sealed at each end then floated to Oslo over five days.

Some of the elements had been constructed up to two years earlier than they were needed, and were moored in BjØrvika, waiting to be sunk. Each element took two days to sink. First, ballast was introduced to make the elements sit lower in the water. A series of cables tethered to the shore were used to pull the elements into position above the trench.

Once an element was in place, it was sunk the following day by filling massive water ballast tanks inside the tube, a slow process that took several hours. The tanks were filled until the element began to exert a load of between 150t and 160t across four winches which were used to control the vertical positioning of each element. The elements lock into each other on the seabed via V-shaped grooves cast into the ends.

Once in place the elements are sealed and the bulkheads removed. All the elements had been sunk before Christmas and it was possible to walk almost all the way through apart from a small gap at the eastern end, which is cast separately to complete the tunnel. Once in place, the delicate business of replacing water ballast with concrete ballast then begins. This will ensure the elements stay in place on the seabed.

With the tunnel in place, work on the completion of the scheme can begin. "When the roads are submerged, [there will be a need to] rebuild the city streets and carry out town planning," says Roed. "Traffic will be removed in 2010, and phase two – the redevelopment of the city and extension of roads to Bjørvika – can begin in 2013."

  • Watch a video of NCE’s Ed Owen immerse himself in Oslo’s stunning new tunnel at www.nce.co.uk/video

WHO’S WHO

  • Client: Statens Vegvesen

  • Immersed tube consultant: Capita Symonds

  • Contractor: Skanska Norway, Bam Civiel, Volker Stevin

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