The City Line, Citybanan, is a 6 km-long commuter train tunnel through Stockholm, with two new stations. Most of the tunnel passes through hard rock, such as gneiss and granite. Beneath Riddarfjärden Bay, the bedrock surface is located at some 40 to 60 metres below sea level. Therefore, a submerged concrete tunnel is being constructed. The concrete tunnel will be connected to rock tunnels, with a span of 22 metres. On the South side, the concrete tunnel will be anchored in the rock mass with cable anchors, with lengths of 25 to 40 metres. The rock cover is locally as low as 3 metres, where protected buildings are founded on bedrock. Design included 3D FEM and block stability analysis. The rock tunnel is supported by rock bolts and fibre reinforced shotcrete. Bolt design was restraint by rock anchors, to be installed later.
The project aims at augmenting the railway system in Stockholm. To the South of Stockholm City, commuter trains, freight trains and inter-city trains have to share a double track since 1870. Furthermore, four tracks at Stockholm Central Station are presently used by commuter trains, but will be needed for inter-city trains.
Most of the 6 km long tunnel will be excavated in hard rock. But in Söderström, the waterway between the islands of Riddarholmen and Södermalm, the rock surface - due to a Precambrian thrust fault movement and subsequent erosion processes - is found at some 50 m below the rock tunnels. Therefore, a submerged precast concrete tunnel piled to the rock surface will connect the Norrmalm rock tunnel in the North with the Södermalm tunnel in the South, as shown in Figure 2.