Blackfriars station is being rebuilt on a bridge right over the River Thames. With a four-lane road to the north, live railway lines on all sides and the river below, it’s not the easiest site on which to deliver a complex project. So how was it done? Thomas Lane buys a platform ticket
Last week, passengers alighting at London’s Blackfriars got a sense of what the new station will feel like when it’s finished in 18 months’ time. They now step off trains onto brand new platforms sheltered by a shiny white canopy, although they still exit the station via temporary walkways. Regulars will notice that they now get off on the east side of the station rather than the west but probably don’t realise that workers gave up their Christmas holidays to make the switch from old to new.
Over a six-day period, while the line was closed, main contractor Balfour Beatty worked frantically to connect the new lines between the new platforms to the existing lines running north out of the station. They also had to connect up tannoys, train time indicators and fire alarms. Another big job was installing new power and signalling cables for the trains as well as connecting up the data and power cables for buildings both north and south of the Thames - this is a station built on a bridge right over the river.
Why rebuild this station so it straddles the Thames? The upgrade is all part of the Thameslink project to increase capacity on the line between Bedford and Brighton, which includes longer, 12-car trains. The only way to fit these in was to extend the station from the north end all the way over the bridge. A replacement station also serving the underground is being built on the north side and a new station is being built on the south, meaning this scheme is really three projects in one. Shifting the lines from west to east will allow the station to be kept open, as Balfour Beatty can work on one half of the station while the other remains in use.
Sites don’t get much more constrained than this. you are working over a live road and the river, next to a live railway line
Chris Evans, project director, Balfour Beatty Civil Engineering
But access is difficult and dangerous, with trains going in and out and a four-lane road running under the bridge’s north end. “Sites don’t get much more constrained than this,” says Chris Evans, the project director for Balfour Beatty Civil Engineering. “It’s very congested and you are working over a live road and the river, next to a live railway line. There is also a very complex series of works to maintain passenger flows and not inconvenience the travelling public.”
Dividing the bridge in two is the key to the success of this job. There are four lines on the bridge, two of which originally terminated on the east side of the bridge, while the west lines are a through route. The first half of the job has been tackled by keeping the through lines to the west running, with temporary access for passengers at the north end. This has freed up the east side for the work including repairing the 125-year-old bridge.
Longer trains mean more passengers, necessitating wider platforms, so the bridge is being widened on both sides by adding additional structural steel ribs to match the existing structure, one on the east and two on the west. The bridge deck has been replaced and new track laid, plus new platforms, canopies and all the paraphernalia needed for a modern railway.
Repairing and upgrading the bridge
The first job was to strip off the old deck to expose the riveted plate wrought iron ribs. But the team immediately hit an unforeseen snag. The structure had corroded at the edges of the bridge where it is exposed to the weather and was worse than initial surveys indicated. Fixing this meant removing the rivets and the top flange of the rib so corrosion on the ribs could be repaired. “We didn’t anticipate removing the flange,” says Lindsay Vamplew, project director for Network Rail. “We could only take a limited number of rivets out at a time because of the risk of de-laminating the whole structure.” He adds that the job was made more challenging because taking weight off the structure meant it moved and changed shape, and the nuts and bolts used to replace the rivets cracked the fragile wrought iron when tightened. The team refined a repair technique after completing a couple of sections but the programme was knocked off track.
Widening the bridge meant extending the concrete support piers in the fast-flowing river. To prepare the piers for new, cantilevered extensions, the faces of the existing piers were demolished, working from scaffolding cantilevered off the bridge structure. The extensions were precast on the deck above ready for installing below.
The piers were now ready for the new ribs. These were installed in three sections from the river using a flat-topped barge. Two 70-tonne crawler cranes worked in tandem from the bridge with two mobile elevating work platforms on the barge to lift and position the rib sections. “It wasn’t static, it was a dynamic lift as the barge is moving up and down on the river,” explains Evans. “We did trials to make sure this was going to be 110% safe.” With the ribs in place the team could crack on finishing the new deck. Struts spanning between the deck and the ribs were installed and topped by steel plates. Time lost on repairing the corrosion was made up by optimising the work sequence and by parallel working where possible.
The new bridge deck
Moving the through lines from west to east wasn’t done just to make Balfour Beatty’s job easier. It is a key part of the Thameslink upgrade as it means the trains going into London Bridge station merge more efficiently with trains coming from Charing Cross, speeding up journey times. But moving the lines means the trains need to cross diagonally from one side of the bridge to the other to connect up with the existing lines going north. The bridge section carrying the lines where these cross over is just north of the old wrought iron structure and wasn’t up to the job. A new deck was needed.
Building a new deck while keeping the trains running was an impossible task. The lines would have to be closed but Network Rail would only allow a six-day closure window over the 2009 Christmas break. “We planned this two years ago as we knew we had this six-day window and missing it was not an option,” says Evans. The solution involved building the new bridge deck to the east of the existing lines. The 22m-wide deck is built from steel with a steel and concrete base and took three months to construct. During the six-day closure the existing bridge deck would be demolished, including part of a supporting pier. The whole, 350-tonne deck would then be slid into position along a special track, ready for connecting up to the lines. The careful planning paid off. “In six days we lifted the railway line, removed the ballast, lifted out the supporting bridge deck, demolished the top of the central pier, completed the trestle and the slide track, slid the deck into place, connected the ends and re-laid the track,” says Evans.
The north station
The bridge works have been done in tandem with the north and south stations. To the north an office building needed demolishing so the station could be built. This station also serves the tube, and the District and Circle lines ran beneath the office, so the lines and trains had to be protected before work could start. The solution was to enclosed the lines using n-shaped metal sections so the trains could continue running in a protected environment. Mini piles were installed along the tube platform edges at night and steel beams placed over these. The steel sections were dropped down a shaft at one end of the platform and slid along the beams into position, allowing demolition work to proceed - including removing a massive 2.5 m deep, 40 tonne steel beam originally used to support the building over the tube lines.
Secant piles have been put in around the perimeter of the station to allow the basement to be excavated. Even this simple job proved more challenging than expected as the team discovered foundations from an earlier railway. Short lengths of railway line had been used as reinforcement, which couldn’t be piled through. Shafts were sunk, and workers armed with cutting gear removed the old bits of line to allow piling works to continue. The base slab of the station has been cast, as well as the sides of the box that will house the tube platforms.
Precast concrete sections will top these, enabling the steel track protection sections to be slid out from one end. From here, building the station is a relatively conventional job.
The south station
The south station is being built in the space originally occupied by three brick viaducts supporting the lines going onto the bridge. These need to be demolished as they aren’t up to the job of resisting the additional forces generated by longer, heavier trains braking as they approach the station. Also, apertures are needed to take escalators from ground up to platform level. Reinforced concrete boxes have been constructed to support the railway and house the new station, a job that has been done partly from Building’s car park (which just happens to sit under the railway line). Two of the arches have been demolished and the boxes built. The final one is under the west lines and can be completed now the trains are running on the east side over the boxes done last year.
Now that the lines have been successfully switched over, work can start on the western side of the station. This isn’t quite a carbon copy of the works to the eastern side, as two rather than one additional ribs are needed on this side of the bridge to widen it. Originally, the railway ran over an older bridge just to the west of the existing structure and although the deck has gone, the piers and columns, which are arranged in groups of three, remain. The plan is to remove the column next to the existing bridge and use the old piers to support the new ribs.
The rest of the work involves replacing the deck as before, putting in the platforms, a prefabricated Aggregate Industries solution to save time and working on the bridge.
Work on the canopy has already started on the east side and this will be repeated on the west.
By the end of this year the station will be able to handle 12-car trains and the new station will be ready for passengers in spring 2012 with the project fully complete by July, making this the first station to be built over a river in 185 years of railway history.
Blackfriars Project Briefing Video
Logistics - getting materials to and from site
How does Balfour Beatty get materials in and out of the site given the only access is down live rail tracks at each end of the bridge? Initially the team thought about using trains to get goods in and out. There is also road access to the lines at the south end of the bridge which meant road/rail vehicles could be used to get goods off lorries and up the line. Then the team realised they were sitting on what was once the main traffic artery in and out of London. “The Thames was a big risk to us but it turned out to be an opportunity as we could use it to bring materials in and out,” says Lindsay Vamplew, project director for Network Rail.
The team negotiated with demolition specialist Keltbray to use their wharf downriver to bring goods in and out by road. From here barges transport goods up to the bridge, and are then unloaded by cranes. But the river offered more than simple transportation of goods. Flat-topped barges are moored next to the bridge and used to store materials. These can also be used as a platform to work on the bridge. This took careful negotiation with the Port of London Authority (PLA). “The PLA was very cautious about having all these barges in the narrowest point of the river,” says Evans.
The PLA imposed strict rules on the project team. For example, only one arch of the bridge can be blocked off at a time, to maintain traffic flows. The central arch can only be blocked off in the winter months when there are fewer tourist boats on the river.
Balfour Beatty leases the barges and has had up to eight working at a time. Three of these are flat-topped storage barges and there are hopper barges to take away demolition waste. There are also working boats equipped with cranes and safety boats to keep an eye on things. These have been used twice to rescue people making suicide bids by jumping off the adjacent Blackfriars road bridge.
client Network Rail
principal contractor Balfour Beatty Civil Engineering
architect/engineer for stations Jacobs Engineering
engineer for bridge Tony Gee and Partners
steelwork Watsons Steel Structures
piling Balfour Beatty Ground Engineering
structural steelwork Bourne Steel
concrete frame MPB
kilnbridge bridge painting Pyeroy