Blackfriars was always going to be one of the biggest headaches in the £6bn Thameslink upgrade programme.
The project, first proposed in 1991 during the final days of British Rail, envisaged services through the station rising to 24 trains an hour, with platforms lengthening to accommodate 12 carriages instead of eight.
But, given the existing station's cramped location on the north bank of the Thames, and the limitations of the Victorian-era railway bridge immediately to the south, creating the additional capacity has meant a major overhaul.
Work finally got underway on the scheme in 2009, and bar a few minor repairs which will be completed later in the year, the revamped Blackfriars will be ready for the Olympics this summer.
The station has been completely rebuilt, and now extends across the bridge, which has been widened and strengthened structurally. The track layout has also been substantially redesigned.
All this has taken place in the heart of London, surrounded by some of the capital's biggest tourist attractions, and working on and above a busy river.
No wonder Chris Evans, project director at main contractor Balfour Beatty, and in charge of 2,000 site workers at the project's peak, describes it as “one of the most logistically challenging jobs imaginable”.
The first part of the project involved the demolition of five platforms and roof of the existing station, which began in April 2009.
Elements of the original station are listed, and these are being retained. They include stones engraved with the names of one-time overseas destinations from Blackfriars, plus cast-iron heraldic 'cartouches'.
Balfour Beatty had to ensure trains could continue to run through Blackfriars Underground station during the construction work. “This meant creating a 350-tonne steel cover for the tube lines, known as a track protection structure (TPS) – essentially a set of steel arches connected to a steel base which was secured to the platform,” explains Evans.
The demolition works also included a link bridge which connects the station itself on Queen Victoria Street with the river crossing. This was being replaced as part of the new track alignment.
Under the old track layout, trains running from City Thameslink station to London Bridge had to cross the lines serving the terminal platforms at Blackfriars.
The realignment involved the bay platforms moving from the east to the west side of the station. “However, this meant a new link bridge would have to be created in a more westerly position,” says Evans. Balfour Beatty constructed the bridge entirely offsite and slid it into position over three days during the 2009 Christmas period.
Widening the bridge
With the new track alignment and platforms which now extend the full width of the bridge – 50% longer than in the old station – Blackfriars Bridge had to be extended by around three metres to the east and six metres to the west. When finished, with the roof complete, the refurbished bridge will be 20% heavier than before.
This has required extensive work to the bridge structure. Fortunately, the project team have been helped by an accident of history. To the west of the existing railway bridge is a line of disused piers which once supported the former St Paul's Railway Bridge (see Blackfriars history box). The easternmost line of disused piers has been strengthened, and tied into the revamped Blackfriars Railway Bridge and clad in stone.
Additionally, some 17 new rib arches have been installed to support the extra load. Each spandrel of Blackfriars Bridge is unique so every rib had to be engineered individually.
Extra concrete 'shoes' were added at the foot of the bridge to support the new ribs.
Balfour Beatty has tried wherever possible to make use of the river because of the project's access issues, and the arches were delivered to the site by barge in February and March 2010. In all, 14,000 tonnes of material has been transported to site by barge, and 9,000 removed the same way, Balfour Beatty estimates.
Each new rib arch weighs 45 tonnes and took a day to install. Heavy lifting could not be performed from the bridge deck so barges were used as lifting platforms. Constructing these was complicated by the tidal nature of the Thames, so an adjustable platform was created which could be raised or lowered according to the level of the river.
“We had to liase closely with the Port of London Authority, and take into account the tidal flow of the Thames, river traffic, and less controllable variables such as wind,” explains Evans.
Besides the new structural works to support the extra load, 200 repairs have been made to the existing bridge arches, some of the damage caused by shrapnel from World War 2 bombs.
The widening of the bridge has progressed on an east-west basis. The eastern half of the bridge was redeveloped first, with rail traffic running in the west.
Over Christmas 2010, during a six-day blockade, the track realignment took place, with the lines shifted from the western to the eastern side of Blackfriars Bridge. Since then, work has been concentrated on the western half of the bridge. This phase is now almost complete, with test trains running on the lines into the new platforms at the time of TCI's visit last month.
Raising the roof
In summer 2010, work started on the installation of the new station roof.
The roof, which of course spans the whole length of the bridge, is supported by a steel 'spine beam', which effectively acts as the backbone of the structure. It is held up by supporting columns 5.8 metres above the deck of the bridge.
The 26-metre spine beam sections, each weighing 18-tonnes, were also delivered by barge and lifted into position from tower cranes.
The Vierendeel roof trusses – each 38m long – were installed in April 2011, and transported onto the bridge via a works train. The trusses were offloaded and lowered into position via tower crane, and attached to the already installed spine beam.
The roof will include 4,400 solar panels, covering 6,000 sq m, creating the largest solar array in London. The roof will provide 50% of the station's energy needs, according to Network Rail.
Inside the new Blackfriars Station, it is noticeable how low the ceiling is one the platforms – a result of the protected sight lines to St Pauls.
“However, we have been able to make them just high enough to allow for overhead line electrification in the future,” says Evans.
2012 has seen the fruits of Balfour Beatty's labours over the past three year slowly come to life.
At the turn of the year, the new entrance on the South Bank of the river was opened – made possible by the longer platforms – the first station on that side of the Thames in 125 years. It meant demolishing the original arches on the south side of the bridge and constructing a new concrete box in their place.
In February, the revamped London Underground station was unveiled. This summer will see the switch-on of the 4,400 solar panels in the roof, and by July the new station will be fully operational, bar a few minor repairs that will continue through the autumn, largely invisible to passengers.