Work being carried out at London Bridge will see the station undergo a dramatic transformation, as Jon Masters reports.
Work now under way at London Bridge has been described by Network Rail as “the most ambitious station redevelopment in a generation”.
Between now and 2018, London Bridge will be rebuilt in nine phases at a cost of around £400M, while the station remains operational. Starting on the south west side, successive sections running the length of the station and about the width of three platforms, will be cordoned off, demolished and rebuilt.
So far, contractor Costain has set up temporary works and removed sections of the roof from over the St Thomas Street side. In May, platforms 14-16 are due to be taken out of commission to allow Costain to establish its first elongated work zone sectioned off from the rest of the station. Then, over the next five years, the contractor will work its way across with the rebuilt station gradually emerging behind.
The new London Bridge is billed as the next showpiece of the £6bn Thameslink project. For this, and its promise of 24 trains of 12 coaches every hour, the existing platform arrangement at London Bridge of nine terminating and six through platforms will become six terminals and nine through platforms.
They will occupy roughly the same positions they are in now, but other than that the new London Bridge is going to look completely different to its current shabby appearance.
Key to the design is the creation of what, at 165m by 77m, will be Europe’s largest station concourse at ground level beneath the platforms and running the entire width of the station between pedestrian entrances on St Thomas Street and Tooley Street.
“We will still want to complete subsequent phases on time, but the pressures will be off. Over the next six months there will be lots of pressure”
James Dessain, HyderWSP
Some of the station’s Victorian masonry arches will be demolished to open up the space while a number of quadripartite arch structures will be preserved housing retail space at the outer edges of the concourse.
The permanent way and precast concrete platforms will be carried on a series of massive bridge decks overhead.
There will be eight of these structures. Each will carry two tracks and two platforms and consist of three or four simply supported spans of 22m to 26m, typically made up of six plate girders 970mm deep by 600mm wide, forming a composite deck with a 250mm reinforced concrete slab.
The structural steelwork will be encased in concrete to deaden noise on the concourse and there will be new roofs supported from each bridge deck, of individual steel and glass canopies over the platforms.
Described by architect Grimshaw as an “eyebrow” design, each canopy rises as it passes over the concourse to help light cascade to the lower level.
Building three additional through platforms means more heavy civil engineering to the east and west of the station.
A new Western Approach Viaduct will be built, effectively extending London Bridge’s recently built Railway Approach Viaduct to the north west of the station. The new structure will consist of a threespan steel viaduct, a twin-span reinforced concrete structure and a railway deck supported on cellular concrete box and polystyrene construction over existing station arches.
For the project design team, a HyderWSP joint venture working for Costain and Network Rail, the key target at present is getting the detailed designs complete for the first three main phases of Costain’s programme.
“Things change, a lot changes, often at short notice, but that’s the nature of the work”
James Dessain, HyderWSP
“We are up against tight deadlines for producing the detail for phases one, 1a and 1b. Then, once we’ve got to that stage, we are ahead of the contractor,” says HyderWSP project director James Dessain. “We will still want to complete subsequent phases on time, but the pressures will be off. Over the next six months there will be lots of pressure.”
The strain is likely to be increased by the nature of the project, Dessain says in that the overall scheme consists of nine individual station rebuild projects, not one. “Each phase has to work as an operational station in its own right,” he says.
That means all pedestrian movement, fire and emergency evacuation modelling has to be done for every stage.
HyderWSP also faces the demands of a contractor needing designs altered to suit exactly how the structure will be built. “Things change, a lot changes, often at short notice, but that’s the nature of the work.
“As long as we work together, and we are working very closely together, then the project will go forward with no problems,” says Dessain.
Network Rail’s London Bridge team is keen to stress the collaborative nature of its approach.
Costain’s contract has been procured differently from most, with client and contractor working in partnership. The design team - in a temporary four-storey office building across the road in St Thomas Street - is a mix of Network Rail, Costain and HyderWSP staff.
“On our main floor here, in the middle we have the design assurance team of Network Rail and HyderWSP and some Costain people as well,” says HyderWSP senior project manager Simon Regan.
“Surrounding them the project splits out into groups working on the station concourse, platforms and canopies, and back of house, which is all the remaining work on the arches.”
“So when we come to do design review meetings, as assurance people are having an input into the design they shouldn’t see any surprises when it comes across their desk to be signed off, which should reduce approval time and make for a smooth design process,” he adds.
Design and construction processes are also being aided by the project’s 3D building information modelling (BIM) model, described by Regan as “at the forefront of what’s being done anywhere” with construction modelling. “The BIM model is the focal point of how we are working collaboratively in technical terms,” he says. It’s the model that is leading the design - using 3D BIM modelling how it’s supposed to be used rather than just as a 3D visualisation.”
Making the most out of an area
Network Rail’s plans to transform London Bridge into a state of the art interchange are ambitious and, before work could get underway, a major ground investigation was called for.
Soil Engineering spent two years carrying out a phased ground and structural investigation out of public view and during night time possessions so station operations were not disrupted.
The £4.7M price tag for this investigation reflects not just the complexity of the work, but also the scale of the task. Nonetheless, Soil Engineering’s involvement in the scheme started off as a much smaller role.
London Bridge station was built in 17 phases during the 19th and 20th centuries and is built around a central viaduct spine, which is the original station. This was gradually widened through the addition of other arches. Each phase of development resulted in modifications to the brick arch viaducts which carry the rail lines above street level, so an in depth ground investigation was essential in order to be sure the redevelopment would work.
“The work on the project started with a £500,000 contract to carry out structural and geotechnical investigations at London Bridge station in 2010,” says Soil Engineering site agent Andy Melling.
This first stage of the work primarily used trial pitting techniques to expose the footings and look at arch connections, as well as investigations at track level. This phase was completed in 2011 but Network Rail added to the scope of the project, which has resulted in a total of five phases of work being carried out.
The second phase expanded the work carried out in the initial phase. The third phase added investigation of five bridge structures. Phase four saw archaeological investigations added to the brief and the final, fifth, phase covered installation and testing of geothermal technology.
While the main challenge was to keep the station operational throughout the work, the site team faced a number of logistical challenges.
Locating utilities was a major undertaking and a number of unrecorded, disused sewers were found during the work. But also directly below and following the alignment of the station’s main viaduct is London Underground’s Jubilee Line. The tunnels are located 25m below ground and with some of the boreholes in phase one and two going to 45m depth, careful location was vital.