Complex structural and foundations work to build the first railway station to straddle the Thames in London is underway. Ed Owen reports from the £5.5bn Thameslink programme’s Blackfriars station.
The £5.5bn Thameslink programme will free masses of capacity on commuter networks north and south of London. The jewel in the project’s crown is the £350M Blackfriars scheme, which will re-open as the first station in London to straddle the River Thames.
Just five weeks into the job, Network Rail senior programme manager for Blackfriars Dan Athol is buzzing with excitement. But he knows the job will be challenging.
Increasing capacity while the railway operates
“Everyone has major challenges − keeping passengers going in and out, holding both a site and a working station, in addition to working trains, the weather and the river.
“There are trains above, and boats below, and we are working in the middle of London − with some of the work done in the middle of the night.”
Dan Athol, Network Rail
“There are trains above, and boats below, and we are working in the middle of London − with some of the work done in the middle of the night, but hours are normally restricted to between 8am and 5pm Monday to Friday,” he says.
Blackfriars Thameslink station is currently located at the northern end of a viaduct which takes the line across the Thames. The northern end of the station runs above London Underground’s District and Circle Lines, which run along the Thames from east to west, and the Blackfriars underpass.
Blackfriars railway bridge: who’s who
- Contractors: Balfour Beatty, Cubic
- Consultants: Jacobs (north bank), Tony Gee (south bank)
- Stakeholders include Port of London Authority, Southwark, train operating companies and Network Rail
The Thameslink upgrade will result in the extension of existing platforms across the river to accommodate an increase in maximum train length from eight cars to 12.
The job is in some ways typical of the wider Thameslink project, which involves work to increase capacity while the railway is in operation (see box). Experience says this is not ideal, but there is really no alternative.
The existing Blackfriars station was completed in 1977. Trains terminate at platforms on the eastern side of the station or run through the station on track located on its western side. When work is complete in September 2011, this arrangement will be reversed so that trains terminate at new platforms on the west side and go through the station on the east side. All track will serve platforms under the new station configuration, and these will straddle the river.
With platforms over the river, a new entrance will also be built on the south bank. “The first new station on the south bank for more than 120 years,” Athol says.
Work has already begun, with track taken up on the eastern side of Blackfriars Bridge, protection for the District and Circle Line Tube station put in place and a significant temporary ticket office and access staircase − known as Key output zero − opened.
Protecting the line
Before work could begin, Network Rail had to satisfy the requirements of Transport for London, the Port of London Authority and others.
A portion of District and Circle Line track is exposed between buildings on the eastern side of the station and this must be protected.
“We will be demolishing over the top of the existing District and Circle Line − 8t steel shield protectors have been lifted in to protect the line.”
Dan Athol, Network Rail
“We will be demolishing over the top of the existing District and Circle Line − 8t steel shield protectors have been lifted in 39 sections to protect the line against accidental falls,” he says.
The line protectors are needed for when 167-171 Queen Victoria Street on the east side of the station is demolished to make way for a new glass ticket hall.
The protectors are also fitted with monitoring devices in case the demolition creates excessive ground movement.
“There is some concern about heave when the building is demolished,” he says.
A prelude to the real job
But all of this work − significant as it is − is a prelude to the real job of rebuilding Blackfriars railway bridge to widen it and add platforms.
Blackfriars rail bridge comprises a series of rib arches running across the river.
Reconstruction involves first dismantling the eastern half and rebuilding it with an additional rib arch. This will give the bridge more width so it can accommodate the new platform 1.
Work then shifts to the west side where use is made of piers left standing from the former rail bridge demolished in 1985. “We then widen the whole deck − mostly on the western side, where it will snatch piers from the adjacent bridge,” says Athol. This provides room for the new platform 4.
A crucial stage at Christmas
Once the eastern side is complete, the first half of the station can be built. Designs are not yet complete, but it will be a modular glazed structure.
To allow trains to run from their new terminus on the western side of the station, part of the station structure must be replaced so that through trains can pass behind the terminus platforms.
To do this a crucial possession takes place this Christmas. Two of the bridge spans at the northern end of the Thameslink bridge − above the underpass and Tube lines − will be removed and a new structure will be constructed and slid into place to give the extra width needed for the new alignment.
“The slide is critical, as it opens up a new range of possibilities,” he says.
“The replacement structure will be built parallel to the existing structures and supported off temporary trestles. During a possession the existing structures will be removed and the new bridge slid into position.
Blackfriars bridge: Changing platforms
Building the new station involves a substantial realignment of track on the viaduct which crosses the Thames and the Blackfriars underpass. Through trains will switch from the west side of the station to the east.
To take longer trains, up from eight to 12 cars running 24 times per hour, the existing platforms are not long enough.
The only way to extend is along the bridge. Network Rail decided to use the full length of the bridge as the platforms, and open a station at the southern end.
To take platforms, the bridge must be reconditioned and widened. First, the eastern side will be closed, refurbished and rebuilt, with trains running along the western side.
When work moves to the western side, two bridge spans will be removed and replaced with a single bridge deck.
This will give the northern end extra width to accommodate through tracks.
The western side will then be refurbished and widened, taking nearside piers from the adjacent, disused bridge.
The bridge re-opens in September 2011.
The new structure will initially have a temporary deck section, which will be removed once the tracks have been slewed into their new alignment. The new structure will be a 25m single span steel/concrete composite bridge to replace the two existing supported structures.
The central pier beneath will need to be demolished to allow space for the new Network Rail concourse beneath.
The new structure will weigh 800t, constructed to the east of its final position, and slid 18m into position during a 147-hour possession.
“We have to remove the slab, and remove part of the station roof. The job is complex enough, without working under a live railway. The bridge’s south abutment is above the underpass, and we have to do a lot of strengthening work − again done with trains running,” Athol says.
Refurbishing the western portion
Once the new structure is in place, work can begin on refurbishing the western portion of the bridge.
This job is different to the eastern half as three additional ribs are added to the bridge using spare nearside piers which remain in place following the demolition of the original Blackfriars Bridge in 1985, leaving just its piers (NCE 1 May 2008).
These disused piers will be strengthened by infilling them with reinforced concrete and their foundations will be connected to the existing bridge piers by jet grouting beneath the river bed.
But before any of this work can begin, some repairs have to be made to the western side. “The rib ends, where pins join the abutment and piers need to be repaired, will be done with a walkway erected under the bridge deck,” says Athol.
Constructing the new station box
While the structural work to complete the bridge is underway, the work to construct the new station box will also be moving along. In addition to demolishing 167-171 Queen Victoria Street, piling will be underway to prepare the ground for the new station structure, which will include a new Tube station.
“On the north bank we are installing 484 contiguous and secant piles and 112 bearing piles. On the south bank we have to complete detailed design but half of the abutment will be removed to install the station box,” says Athol.
“The job is complex enough, without working under a live railway.”
Dan Athol, Network Rail
On the north bank the contiguous and secant piled walls are used to facilitate the basement excavation for the new London Underground station, the bearing piles form the foundations for the common entrance, escalator, Network Rail north station buildings and temporary bridge abutments. On the south bank traditional vertical load bearing piles support the Network Rail south station.
Pile diameters will vary between 600mm and 900mm diameter. “Due to the number of piles being installed we can deliver efficiency savings by reducing the pile diameter where it is not structurally required over and above that of using a common diameter across the project,” says Athol.
For the Thameslink upgrade to work smoothly, continuous running of trains is essential. Network Rail’s juggling act at Blackfriars will test the track operator to the limit but the results promise to be stunning and give London a new landmark.
The Thameslink programme is a £5.5bn investment in improving rail infrastructure and new rolling stock. It will deliver more capacity, and more journey options for passengers travelling through or to London from the north and south.
Work will be required at 50 stations and 4km of additional platform length will be constructed - the equivalent of eight times the length of Brighton Pier and around double the length of the runway at Luton airport.
Major works will be required in central London with new stations being built across the Thames at Blackfriars and London Bridge.
Farringdon station, to the north of Blackfriars, will have longer platforms, a new footbridge, new station roofing and new concourse facilities for London Underground and National Rail services.
Major works will also be required at Borough and east of London Bridge station where Thameslink services have to cross track used by busy lines feeding into London Bridge station.
In all the Thameslink programme will provide 50% longer trains across the current Thameslink route by 2012.