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Stockholm Monster

Stockholm's new cross city rail line has taken 20 years to get off the drawing board and on to site. John McKenna reports.

An underground rail line that has taken 20 years to gain funding, requires extensive tunnelling and will cost billions to complete… sound familiar? No, it is not London's Crossrail, but Stockholm's £1.3bn Citybanan, or City Line project.

Apart from the scale of the two cities, the main difference is that Citybanan is now being built.

Construction of the Swedish capital's 6km line has just begun with sheet piles being driven into the ground ahead of the excavation of three of the four access tunnels on the project.

Stockholm has four underground rail lines and only two overground lines. Commuter, long distance and freight services all use the two overground lines to enter the city, causing congestion during peak hours.

Citybanan will double rail capacity through the capital, operating services mainly for commuters and freeing existing lines for other rail traffic.

There will be two new stations: City Station, next to Stockholm Central, Sweden's largest mainline Station, and the Odenplan, in the north part of the city.

There will also be a new platform for Citybanan trains at the existing Stockholm South Station.

Funded mostly by money from the Swedish government, the project is overseen by a team from client Banverket, the Swedish rail authority.
"We can't widen [the existing railway] above ground for historical reasons. The City Line has to be built as a tunnel," says Banverket project manager Kjell-Åke Averstad.

Since Stockholm sits on hard crystalline granite with a strength of between 250MPa and 300MPa, tunnelling was always the favoured option for the route.

Such hard rock is extremely stable but takes its toll on tunnelling machinery, wearing cutters and racking up maintenance costs. As a result, drill and blast was preferred to using a tunnel boring machine.

Citybanan has yet to reach the drill and blast stage, but in total the project will require 1M.m3 of rock to be excavated in this way. A joint venture of German contractor Züblin and Swedish contractor Oden is building the access tunnels ahead of the main tunnel construction.

Drill and blast work will get under way at the end of this year. As the work progresses, 25mm diameter rock bolts between 2.4m and 7m long will be inserted in the walls to hold them stable. The bolts and other fixings will be protected from corrosion by an epoxy coating.
The only section of tunnel to be built differently will be the underwater section in the south of the city where the line passes under a lake.
It has still to be decided whether this will be floated into position as an immersed tube or built insitu within a cofferdam.

The main tunnelling and stations works are divided into seven packages and UK-based consultant WSP is designing the works package covering the city centre, arguably the most complicated section.

This includes the design of City Station, 40m underground. It is located under a metro station on Stockholm Blue Line and at some points is within 3m of its neighbour.

At these points contractors will use shotcrete to stabilise the rock. The station will be built during three summer possessions from 2009.
For most of the scheme, the rock is considered stable enough to support itself with a 50mm thick lining of steel fibre reinforced shotcrete, explains WSP project manager Roberth Colliander.

"But underneath the Blue Line and City Station's central mezzanine floors, we will have 300mm of shotcrete. The rock thickness between the tunnels and mezzanines is only 3m so it's not going to be stable on its own."

Water ingress through tiny fractures in the rock is another potential risk. Aside from the problem of water entering the tunnel, there is also an issue with water affecting the stability of the city's buildings.

Many of Stockholm's oldest structures stand on timber piles. If groundwater shifts, or its level rises and reaches these piles, there is a risk of widespread settlement across the city. As a result, there are strict environmental controls on any type of activity that may disrupt the groundwater levels and if water appears unexpectedly, work must stop.

At the time of the site visit for this feature at the access tunnel by Stockholm Central Station, sheet piling had been suspended after muddy pools of water had appeared.

To keep de-watering to a minimum, Colliander and his team have proposed extensive pre-grouting to plug as many underground voids as possible.

"We're talking about 500,000m3 of drilling in the whole project just for pregrouting," says Colliander.
Holes up to 20m long are drilled 200mm apart ahead of drill and blast excavation. A coarse grain cement grout is then injected at high pressure to make fractures watertight (see diagram).

Holes 2m to 3m long are bored into the face and loaded with explosives that are then detonated. The process is repeated over 15m before pre-grouting begins again.
Sheffield-born WSP engineering geologist Robert Swindle is leading the geotechnical model for the project. "Even fractures that are very small have to be tightened up using grout," says Swindle.
"We will be using very fine grain cement to tighten small aperture fractures. This is nothing to do with stability; it's just purely down to water ingress."
WSP is waiting for an environmental permit so that work can continue on the access tunnels. The project is due to be completed in 2014, followed by two years of commissioning.
Citybanan will open to traffic in 2017.

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