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Welcome diversion

A new underground rail route through the centre of Stockholm reflects the ideas and challenges of Crossrail. Jo Stimpson reports.

Congestion on Stockholm’s overcrowded twin track commuter railway is soon to be eased with the construction of a massive new underground line. Built up city centre areas make expanding the line above ground impossible, so the new line will run underground.

“Here you cannot build any more tracks,” says Swedish transport authority Trafikverket project manager Kjell-Åke Averstad.

The chosen solution is City Line (named Citybanan in Swedish), a new £1.5bn underground commuter railway that will offer commuters more trains and connections to the metro network and bus stations.

Analogous to Crossrail in London, the City Line project features two tracks running through a 6km drill and blast tunnel between 10m and 40m below the surface, from Tomteboda on the north side of the city centre to the existing Stockholm South station on the opposite side.

Drill and blast

The drill and blast section moves into an immersed tube south of the city centre, taking the line under the sea to Stockholm’s Sodermalm district.

“Sixty per cent of City Line commuters are expected to change to the subway rather than get out on the surface”

Kjell-Åke Averstad, Trafikverket

 

The project also includes work to replace tracks and build a new 1.4km long flyover bridge between Årsta and Älvsjö station, further south.

Along the route of the drill and blast section, two new stations will be built - Stockholm Odenplan and Stockholm City, which will be located adjacent to the existing Karlberg and Stockholm Central stations respectively, relieving them of commuter traffic.

The number of City Line tracks will widen from two to four at Stockholm City and there is an option to expand the other two City Line stations to four tracks as well in the future.

The new Stockholm City station will be located directly beneath the T-Centralen metro station, creating speedier connections for commuters. Similarly, the new Stockholm Odenplan station to the north will offer a connection to the existing Odenplan metro station. “Sixty per cent of City Line commuters are expected to change to the subway rather than get out to the surface,” says Averstad.

Station design

The City Line stations are designed to feel light, safe and easy to navigate. The platforms will feature platform screen doors separating waiting commuters from the tracks - “for safety,” says Averstad, “but mostly for the quality of the air and to keep the station clean at all times.”

The stations will also feature lengthy escalators taking passengers from 40m below ground up to street level.
The nine- year construction programme started slowly in 2008 and began in earnest
in 2009.

The work on the drill and blast section is split into eight different contracts, with consultant WSP taking on the design of the most complex part - the central part of the route and Stockholm City Station - to be built by Swedish contractor NCC.

The first jobs have been access tunnelling for the central tunnelled section, and sinking an immersed tube tunnel into the water at SÖderstrÖm near Gamla Stan, the old town.

The access tunnels are now complete and drill and blast work is now underway on the main railway tunnels.

These comprise one tunnel housing the tracks, and a narrower service tunnel at the western side. Where the number of tracks increases from two to four at Stockholm City station, there will be an extra tunnel housing the additional tracks.

The tunnel’s cross sectional area varies between 25m2 for single track, 110m2 for double-track, 220m2 for stations and 40m2 for the service tunnel.

Tunnelling started from the surface at the sites of the two new stations. One of the access tunnels passes underneath a railway line and a highway - “we are blasting just beside [them],” says Averstad. The main tunnelling work is set to start gearing up this summer and come autumn contractors will begin blasting in both directions from the Stockholm City station site.

In fact, when blasting is fully underway there will be a point at which it is happening simultaneously on 25 fronts - “building from every direction”, says WSP deputy managing director Eskil Sellgren.

Drill and blast is the chosen method for central tunnelled section, because although the rock is stable it takes its toll on tunnel boring machines, increasing maintenance costs.

As the work progresses, care must be taken to ensure the rock’s integrity. Rock bolts of 25mm diameter and between 2.4m and 7m long will be inserted in the walls to stabilise them, and they will be protected from corrosion by an epoxy coating. There are also some concerns about the proximity of the Stockholm City Station to the metro system’s Blue Line, built in the 1970s.

Crossing the Blue Line

“Blasting for the Blue Line was a bit careless, so in the bottom there could be cracks,” says Sellgren.
Meanwhile, Odenplan sits above the metro’s Green Line, which was constructed as a cut and cover tunnel through city streets in the 1950s.

The City Line running tunnels will come underneath the metro ones in the rock. “We will construct the station from the surface downwards but the tunnel will be blasted,” says Averstad.

Excavation of Odenplan station was a point of great contention with Stockholm’s citizens, who feared that subsidence could damage the historic Gustav Vasa Church. “We have had some problems with protesters ” says Averstad. “But today we have all the necessary permits. I think most of them are happy we are doing it.”

“And they will be much happier when it is finished,” adds Sellgren.

“The plan was to construct the access tunnels early and have them all finished in good time”

Kjell-Åke Averstad,Trafikverket


Further challenges have come from poor ground conditions on part of the route, as the geology above the rock surface in this part of Sweden is very varied. There are weak parts, predominately in the east-west direction, with extensive cracks. ButWSP deputy project manager Jan Waage is optimistic about the main tunnelling work.

“We have predicted the rock to be good in most parts of these constructions,” he says.

Water ingress in access tunnels has caused further delays. “The plan was to construct the access tunnels early and have them all finished in good time,” says Averstad. “We had to stop the work to one of the access tunnels.” Groundwater was also problematic during preparatory works last summer for the new Stockholm City station ticket hall.

However, Averstad says the water problems have been overcome.

The tunnel sits on 60m deep piles and is constructed in three 100m long sections.

The steelwork of the sections is built in Estonia, then floated in. Two thirds of the concrete walls are cast outside of the city, with the remaining third completed in place in the harbour.

“We can’t finish it outside [the city], we must take it in,” says Averstad.

The immersed tube tunnel is being built by the Danish/ German consortium of Pihl and Züblin. “It connects to
the rock tunnel, which goes under some historic buildings,” says Averstad.

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