Why an immersed tunnel?
There are more than 30 subsea tunnels in Norway. All but 1 are bored through bedrock. In Bjørvika, however, the bedrock lies deep, some places more than 40 metres below sea level. A bored tunnel in Bjørvika would have to lie 40-50 metres below sea level, whereas an immersed tunnel, built in elements onshore, could be assembled on-site and positioned at just 20 meters below sea level. The immersed tunnel easily aligned with the existing tunnels.
The seabed of the inner Oslo fjord mainly consists of clay and in some places bedrock is as deep as 50 meters below the top of the clay layer. This means that if a sub-sea rock tunnel were the adopted solution it would, even with very steep gradients, be several kilometres long. NPRA has therefore chosen to build an immersed tunnel. This type of tunnel is constructed by building in elements on land that are subsequently floated into position and lowered onto the seabed. It is the first time that the technique has been used for a tunnel with passenger traffic in Norway .
What is an immersed tunnel?
An immersed tunnel is built on land and submerged under water to its final position. In Bjørvika, six tunnel elements were built in a dry dock in Hanøytangen in Western Norway. Each element was 112 metres long and weighed approximately 35 000 tonnes. Bulkheads at each end ensured that the elements were watertight for transport. The elements were floated to Oslo from the 800 kilometers from Hanøyangen by tugboat.
Tunnel construction and transport
The immersed tunnel is made up of six elements. Each element is 112.5 meters long, 28–43 meters wide and 10 meters high. The tunnel roof will lie eight to eleven meters below the average water level. The immersed tunnel consists of two tubes, with three lanes in each direction and sufficient height inside the tunnel for signs, fans, surveillance systems and lighting.
The tunnel elements were built at Hanøytangen dry dock in Askøy municipality outside Bergen . Construction started in 2005 and was completed in 2008. The two first elements were towed to Oslo in August 2006. The last two were delivered in the spring of 2008. The journey from Hanøytangen to inner Oslo fjord takes around five days.
The open sea sections expose the tunnel elements to the highest loads they will experience in their lifetimes. Prestressing steel tendons are stretched through ducts in the roof and bottom plate toreinforce the elements during the tow. Where wave heights of more than 5 meters are forecast, the convoy will find harbour shelter. The tunnel has sufficient strength and built-in flexibility which enables it to withstand earthquakes. All six elements were anchored alongside the Bjørvika quay-side by the spring of 2008. The submersion and installation work could then begin. This took around two weeks per element.
The element is equipped with an access tower and a land surveying tower. Marine based operations will be carried out using special equipment from a Dutch operator. A vessel will lay a gravel foundation on the excavated fjord bottom with an accuracy better than +/- 3 centimetres.
The immersed tunnel lies on the gravel foundation without any other form of foundation being required. The tunnel elements have an unladen weight of 1.1 which means the load exerted on the fjord bottom is marginal.