Highway crossing for Oslo harbour

Construction work for the E18 crossing of the Bjørvika in Oslo harbour started in 2005 and is planned for completion in 2010. NGI has worked on this project for the Norwegian Public Roads Administration since 1995.

The main highway crossing Oslo from east to west, E18, will finally come up to standard when the crossing of the Bjørvika in Oslo harbour is completed. This will also provide space for a new development of the area around the new Opera-house in Oslo.

The total length of the Bjørvika tunnel crossing between the existing Festnings tunnel and Ekeberg tunnel will be 1100 metres, with three lanes in each direction. The middle part is a 675 metres submerged tunnel, the first of its kind.

The dotted line line (above) shows the route for E18 crossing Bjørvika, from the Festningstunnel in the west to the Ekeberg tunnel on the east side (just below the picture frame. The picture below shows a cross-section along the tunnel route.

NGI's work
The initial work for NGI, which started already in 1995, included geotechnical evaluations for alternative solutions for a submerged tunnel crossing under Bjørvika. The main geotechnical challenges were associated with the soft sediments to great depth.

Detailed engineering started in 2001, followed by constructions plans in 2003. NGI had the responsibility for geotechnical design for the sink tunnel and for the cut and cover excavation across Sørenga on the eastern end to connect the tunnel to the existing Ekeberg tunnel. Since the construction work started in 2005, NGI has been in charge of geotechnical control and follow-up.

Challenging ground conditions
The ground conditions in the Bjørvika and on both sides in connection with the two existing tunnels, are difficult, and gave great geotechnical challenges to the project. In the transition zone between the sink tunnel and the Ekeberg tunnel on the east side, a solution with open excavation with slurry walls of 230 meter length was chosen.

The excavation for the slurry walls was made by using a specially developed grab, dredging in full width of 2,2 meters down to required depth. Subsequently, the reinforcement was placed and the slurry walls were cast in place. The last stretch to the Ekeberg tunnel was constructed in an open excavation with heavy sheet pile wall anchored to rock.

  
Illustrations showing (left) dredging of the tunnel route on the sea bed of Bjørvika, (centre) placement of a gravel mattress and (right) principle for placement of the six precast concrete tunnel elements which, after hook-up, will form the sink tunnel.

Floating sink tunnel
Crossing of the Bjørvika was made by the design of a sink tunnel which will be placed below existing sea bottom. The tunnel roof will be at elevation -10 metres to avoid potential collision from ships in the harbour. The soil conditions consist of soft clay down to bedrock and the tunnel will float in the clay without piles down to rock. The tunnel route is dredged by a grab from a barge. The slopes are made 1:2,5 to obtain stability.

After excavation, a textile fibre lining is placed on the bottom of the tunnel route. On top of this a gravel layer is placed as a foundation for the tunnel elements. The gravel pad is placed with very high accuracy for the thickness and level, in order to avoid tension in the concrete tunnel elements. The sink tunnel is made up of six elements of 112 meter lengths. They are precast in a dry-dock iutside Bergen, on the west coast of Norway, and towed along the coast to Oslo. During 10 weeks in October and November 2008, the six tunnel elements will be placed and connected.

The dredging and excavation of the tunnel route will imply a removal of load on the clay along the tunnel route. Upon placement of gravel and tunnel elements, the area will experience a reload. This process will cause a change in material behaviour, which is vital for the design of the interaction between the tunnel elements and enclosed soil. After the tunnel elements have been placed and connected, gravel is placed along the sides of the tunnel in order to secure stability.

  
(left) An example of FEM analysis of sink tunnel and sea  bed, (centre) principle for construction of slurry wall, and (right) the open excavation at Sørenga on the east side, with entrance to the cast tunnel with slurry wall.