Engineers in Sweden are putting the finishing touches to the new £1.51bn (Kr16.8bn) underground City Line in Stockholm, one of the country’s most ambitious infrastructure project to date.
The new 6km long, tunnelled line between Tomteboda and Stockholm South, will open on July 10, doubling track capacity through the congested city centre.
Currently all rail traffic including commuter, long distance, regional and freight trains run on two, ageing overground tracks creating delays. When the line opens, all commuter trains will go through the tunnels relieving pressure on the existing line.
Construction started in late 2009, taking three years to drill and blast through the hard gneiss rock. Fit out took a further 18 months, and testing of the systems and tracks has been carried over the last 18 months.
As part of the new line, two new stations – Odenplan to the north and City Station – have been designed by consultant WSP. The larger of the two and Sweden’s largest underground station, City Station is built directly under Stockholm’s existing T-centralen metro station and in places, it is only 4m below its existing blue line.
At City Station the tracks separate out to create two pairs of two tracks with 12m wide, 240m long platforms running through the middle of each pair creating the two, 27.5m wide platform rooms.
New track screen doors have been designed to withstand loads from the more onerous long distance trains allowing them to be diverted through the tunnels if the overground tracks need to be taken out of action. However only commuter trains will be able to activate the doors allowing passengers to get on and off at the station.
The new station comprises a vast central concourse with escalator and lift access connecting to ground level and each of the existing metro lines. At each end of the concourse escalators and lifts connect to the two new platform voids. Both were created by blasting sections the full 27.5m wide and 5.8m long.
“Here [the platform rooms] we did the full 27.5m x 5.8m in one go,” said Rosengren Bergkonsult rock mechanical and rock engineering manager Lars Rosengren. “It was shaking!
“But to take out a round like this, it takes several days. You’ve got to scale it, reinforce it. You can probably do one explosion like this in a week.”
Due to the quality of the rock, instead of a secondary lining, much of the surface of the tunnels has been simply covered with a shotcreted over drainage layer. This is held in place with rock bolts, drilled between 2.4m and 6m into the rock.
The tunnels sit below the water table so to stop water coming through fissures in the rock, 4m long boreholes were drilled into the rock. Grout was then injected under high pressure to seal the cracks.
WSP rail director Eskil Sellgren said: “After more than 20 years of work spent on the preliminary studies, planning, design and project management, the City Line is now ready to open to the public on 10 July. We are all very happy and proud to have been part of this challenging project, which our fantastic staff have not only delivered on time but also below budget in a very fruitful partnership with the contractor and the client.”
WSP project director Roberth Colliander added: “We are extremely proud of this amazing project, which has come through many technical challenges in a fantastic way. We are very proud of what we achieved together.”