A clever mix of extensions, innovation, structural improvement and new infrastructure is needed to draw out the potential of the tightly constrained Farringdon station.
More from: Delivering Thameslink: Major Project Report
Farringdon station, which sits on the edge of the City of London close to Fleet Street and Smithfield meat market, has become increasingly important in recent years with a big increase in offices, banks, restaurants and shops.
Now, with its London Underground (LU) and much enhanced Thameslink services set to be supplemented by a new Crossrail connection, Farringdon will also become a major central London interchange.
The existing four-platform station cannot handle much greater passenger flows than those already jamming the stairs to its single street exit in rush hour peaks. But adding capacity has meant some complex design and construction.
“Like most of the work, piling is done at night and weekend possessions and is out of sight and notice of passengers. It is incremental improvement by stealth.”
Richard Walker, Network Rail
This is because the station is tightly constrained in a long cutting next to the Farringdon Road, with Thameslink lines disappearing underneath buildings into tunnels as they head south and the Tube lines curving off to one side to Moorgate. In between the national rail network and Tube lines are three more short tracks, used for LU train stabling.
All of these rail lines are at natural ground level in the old Fleet River valley, explains Network Rail senior programme manager Richard Walker.
In 1855 the river was diverted into a large brick sewer tunnel and the valley was built up with a mass of complicated arches, brick retaining walls and iron and steel bridge structures carrying the station entrance building and the road outside. As a result, many buildings running either side of the tracks have double layer basements.
To extend the station in this space has meant constantly juggling design and construction sequences, not least because both Tube and railway must remain in use throughout.
Within this context, the “easiest” part of the job has been constructing a new ticket hall on a site opposite the existing entrance where a short branch line previously ran out to Moorgate.
Meanwhile, the original Victorian ticket hall and entrance and its 1920s façade − both Grade II listed − are being renovated and reshaped inside.
Further capacity is being added by creating a second concourse using a long narrow space formerly occupied by a small terrace of shops on Turnmill Street, which bounds one side of the station cutting. Passengers from this side concourse will arrive at an entrance halfway along the cutting, connected to platforms by a new steel footbridge.
The footbridge was one of the first new components of the station upgrade to be delivered, and was installed in March 2009 using an 800t crane. The bridge is already in use with a temporary entrance to relieve the main stairway and entrance during the construction.
Still to come is a new steel and glass roof, on steel columns, to cover the currently open air sections of the platforms where they extend northwards beyond an existing Victorian train shed roof.
All of these activities have their own complications.
The new ticket hall, for example, requires some complex phased demolition of old brick walls and basement structures, including structural walls that held up the now demolished Cardinal Tower office block.
The job is made more difficult by the presence of a power substation within the site boundary that must go too, but cannot be taken out until a replacement is connected nearby.
In addition, support walls alongside the branch line tracks − which have now been demolished − were the fixing point for rail overhead power lines, so these have had to be reconfigured and supported elsewhere.
Interactions with the operating railway are constant, says Walker: “Altogether, we have to move overhead lines and reposition them in nine phases.” The Thameslink tracks running out on one side of the site also had to be supported with minipiling along each side before the wall was demolished.
Originally the new ticket hall was to be a curving, convex building over the Thameslink tracks and alongside Cardinal Tower. But with the go-ahead for Crossrail, plans have evolved − meaning a neater and better value engineering solution.
Now the new ticket hall will house both Thameslink and Crossrail services, with construction able to take place on a larger site extending all the way to Farringdon Road.
Once the ticket hall is open for Thameslink services, work on the Crossrail platforms will continue unobtrusively behind a temporary wall, ready for 2017.
Network Rail will be delivering much of the initial work for Crossrail at Farringdon, including the challenge of installing piling among the tight constraints set by the new tunnels.
There is other work going on at platform level insde the station, including a new cabling route beneath the Tube platforms that is being constructed “Great Escape-style” by hand excavating the invert downwards
None of the 2.1m diameter piles for the new buildings − and the eventual redevelopment of high rise above − can interfere at all with the new running tunnels below, nor the historic Fleet sewer, which means the piles have to be placed in a rather unconventional configuration. “The pilecaps have some very high bending forces to deal with as a result,” says Walker.
Piles also have to be double sleeved and painted with bitumen to prevent any movement from the settlement induced by the Crossrail tunnelling activities. Therefore, most of their work is done by skin friction below the Crossrail tunnels and end bearing in chalk 40m below ground level. Above it, they pass through mixed ground of alluvial silts and clays.
Site access has required significant work, with a butcher’s shop demolished on Charterhouse Street, above the site, to make an entrance for a steel framed concrete ramp. Even this 90m long access is cramped, and a special steel turntable is being made at the bottom so concrete and delivery trucks can be turned round to leave again.
Farringdon’s platforms are being extended southwards to take 12-car trains. The extension work has to be done at this location because northwards the track declines steeply for the St Pancras tunnel on a 1:29 gradient − the steepest on the network.
But to the south is a clutter of arches and supports for five different bridge structures that form the base of the station and hold up the road outside. The new lines must pass through these supports.
Four of the five bridges have to be removed and rebuilt to make space, with the extended platforms taking a split path around widened brick arch abutments. The work is being done in two phases, reducing the road access above firstly by 60% and then 40%. The platform extensions will cut across the points for the old Moorgate branch line, which is why the line has had to be closed.
Basement station accommodation, including the station manager’s office, also has to be relocated to make way for the widened and lengthened platforms. Initially a high grade temporary facility will be built one level up on the station concourse, followed eventually by a permanent new office. Ticketing has also been moved into a temporary building outside.
The new side concourse is being built over arched brick buttresses that make up a big Victorian retaining wall along the old valley side, supporting the road at the top.
“The arch spaces will make very useful places for plant and storage,” says Walker, although the structures themselves cannot be altered for heritage reasons.
To extend the station in this space has meant constantly juggling design and construction sequences, as both Tube and railway must remain in use throughout the project
There is other work going on at platform level inside the station, including a new cabling route beneath the Tube platforms that is being constructed “Great Escape-style” − hand excavating the invert downwards, and building side concrete walls and a steel top.
New piles are being installed for the columns that will eventually support the new roof. “Like most of the work, this is done at night and weekend possessions and is out of sight and notice of passengers,” says Walker. “It is incremental improvement by stealth.”
Minipile rigs are used for the pile installation, tracking in across a temporary deck formed from polystyrene blocks with a rolled steel decking mat on top. “The blocks can be cut with notches to fit over the track and power rails and are light enough not to need any plant to put them in during a possession,” explains Walker.
The new steel frame and glass roof has also been value engineered “to reduce the size of its bays”, says Walker, which means its prefabricated sections will require smaller crane lifts when it is assembled.
It was far from easy to locate the big crane that lifted the footbridge into position in the narrow Turnmilll Street, particularly when it came to finding firm points to position the outriggers.
So many bridge and basement structures exist in the area that every heavy plant move has to be checked carefully for ground bearing.
For that crane some permanent foundations, to be used later, were built up further with temporary piers to take the outriggers.
The project designer is Atkins and the contractor Costain. Monthly spending has risen to £10M as the peak of the work was reached this summer. Substantial completion will be in December 2011 at a total cost of about £250M. Crossrail, and the eventual redevelopment of the office space above, means activity alongside will go on for some time yet.
Farringdon in figures
cost of new station
area of new ticket hall
maximum pile load
maximum pile depth