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Fine tuning the Nordic Triangle

TUNNELLING & UNDERGROUND CONSTRUCTION

Design is being finalised for a major tunnel in Oslo which forms part of an important link between three Scandinavian capitals. Max Soudain reports.

The Baltic Sea is the only interruption in the E18 highway linking the Scandinavian capitals of Oslo, Stockholm and Helsinki. It stretches from the southern tip of Norway, through Sweden and, after it resumes in Finland, becomes the M10 to St Petersberg in Russia.

This important route forms part of the Nordic Triangle, one of 14 priority projects in the EU's transport integration programme, TransEuropean Transport Networks.

For years, traffic on the E18 met severe congestion as it passed through Oslo. Various schemes were proposed before work finally began on the Oslo Tunnel project in 1987. The aim was to allow a projected 100,000 vehicles a day to flow freely through the city by placing the E18 underground.

The 3. 3km scheme consists of two twin bored rock tunnels, linked by an immersed tube tunnel under Bjorvika Bay, the city's main harbour area.

Some 2. 6km of the route is underground.

While the two rock tunnels were completed in the early 1990s and partly solved the congestion, work on the immersed tube stalled. Eastbound traffic on the E18 must rise to the surface on the western side of Bjorvika Bay and skirt the northern edge of the harbour before returning underground to leave the city.

But now, with the alignment of the immersed tube tunnel chosen, construction of the final section of the project is moving closer. It has just entered the 'pre-engineering' phase and construction is set to start in 2005. Work is being carried out on the back of redevelopment of the harbour including a new National Opera House and public open spaces.

The present road configuration not only impedes traffic flow but also cuts the harbour area off from the city centre with a 'mess of concrete viaducts and bridges' according to Jonathan Baber, principal engineer with UK consultant Symonds.

The firm is acting as specialist tunnel consultant to Norwegian engineer Aas Jakobsen, providing technical advice and preliminary design for the tunnel structure and foundations.

Baber says the pre-engineering phase is expected to run until June 2002 and involves final design development, with eventual approval by the client, the Norwegian Public Roads Administration.

It will involve design alterations, including adjustment of junctions caused by changes to the length of the tunnel and the alignment, which has been lowered at the connections to the rock tunnels.

But the major design change has been the removal of the planned piled foundations for the tunnel. The geology beneath Oslo comprises thick deposits of soft clay, up to 55m under Bjorvika Bay, underlain by a complex sequence of metamorphic rock, penetrated by igneous dykes.

Baber explains that the original design assumed the tunnel would be supported on piles down to bedrock. 'Pretty much everything in Oslo is built on piles, so it was the natural assumption to start with. '

Loading on the tunnel was also expected to vary. Oslo is an international ferry port with large cargo facilities. The large variation in the depth of cover over the tunnel, which occurs because of the cargo handling areas in the centre of the harbour, would have caused significant differential settlement if piles had not been used, Baber says.

'But changes to harbour redevelopment plans, with more public open spaces, has meant that anticipated settlements have evened out and we believe we can do without piles - a major change which will have to be addressed over the next few months, ' he says.

A deeper tunnel also means the planned ship barrier, designed to protect the structure from collisions, will be much smaller and will require less earthworks, imposing smaller loads.

All this, says Baber, has led to a much more conventional immersed tube design of a concrete tunnel in a dredged trench.

The 600m to 700m long tunnel - 'depending on the final design, ' says Baber - will consist of six 100m to 120m long elements which in turn will have six segments, each 20m to 25m long. The segments will be cast in two stages - the base and then the walls and roof. These will be connected by groutable expansion joints cast into each one.

Each element will be prestressed and fitted with steel bulkheads, floated out to the alignment and sunk into the trench. Elements will be joined using rubber gaskets to form a watertight seal. Water between them will be pumped out and the bulkheads removed before final sealing of the joint with another rubber seal.

Construction will be very similar to that on the immersed tube section of the 0resund link between Denmark and Sweden (which Symonds acted as consultant on), albeit on a much smaller scale.

Unlike 0resund though, there is little or no repeatability in the design of the elements, explains Baber. In Oslo almost all of the elements will be different, as they incorporate slip roads and lay-bys. Only the central element will be the 'standard' width of 28m to carry the dual carriageway road.

Elements will either be built at the western end of the route, in a casting basin that will eventually form part of the cut and cover link with the bored rock tunnels, or will be built off site and floated to the alignment.

'There is an issue of whether or not to build the elements on site but this has programme effects, ' Baber says. 'If the tunnel becomes longer then the elements would have to be floated in - this would also require some local dredging to bring the elements on to position. '

The trench is likely to be dredged in two halves, allowing placing of the tunnel elements while the rest of the trench is formed. This will cut through the top 2m of clay and silt that has virtually no strength, Baber says, although the clay becomes 'fairly uniform' and its strength does increase with depth.

The tunnel is likely to sit on a gravel layer in the base of the trench to even out settlements and ensure that the elements are placed as accurately as possible.

'While dredging accuracy is plus or minus 200mm to 300mm in soft clay, the gravel will give a tolerance of 20mm to 25mm, which is preferable, ' says Baber.

'While settlement is likely to be high, the tunnel will still cope, ' he adds.

The problem will be if there is high settlement of a section. 'If the entire tunnel settles then it is not really a problem. ' The joints between the segments and the elements both allow some degree of movement, he explains.

Another site investigation is about to be carried out to gain further information for settlement predictions along the route. The plan is to design without preloading of the material below the tunnel, but Baber says if there is a need for ground treatment, stone columns and soil mixing will be the possible options - 'There is quite an interactive process between design and construction. '

Once the pre-engineering phase is finished and the client has approved the final design, final planning approval will take place and financing will be secured. The plan is that parallel construction and detailed design contracts will then be tendered. Baber says detailed design will be developed with the contractor to bring the project up to the start of construction in 2005.

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