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Technical Excellence: Open heart surgery at London Bridge

Next year is a big year for the team working to improve transit through London Bridge station as part of the Thameslink project is now nearly a third of the way through.

Network Rail has a neat analogy to describe the £1bn project to upgrade London Bridge train station.

The rail infrastructure operator describes the process of excavating an area roughly the size of Wembley arena and building 15 new bridge decks on the existing footprint - while simultaneously maintaining a peak daily flow of roughly 122,000 passengers through the commuter hub - as being “like conducting heart surgery while the patient’s awake”.

Senior project design manager and project engineering consultant WSP director Adrian Tooth says the complexity of the project lies in synchronising each phase of construction with a major reconfiguration of the track and the signalling systems outside of the station. In essence, the project consists of a staged “slicing away” of a series of existing viaducts and arches that were built between 1830 and 1890, and which currently support large parts of the station, to make way for the stadium-sized new concourse and 7,500m2 of retail space. As the sections are removed, 15 new bridge decks will be systematically constructed above the concourse to carry more trains into and through the station. In addition, the project involves constructing a new 300m long viaduct through Borough Market to create space for two more tracks.

Overall, the objective is to change the station from one that has nine terminating tracks and six through tracks to one that has six terminating tracks and nine through tracks.

“The project is effectively split it into three parts,” says Tooth. “There is one construction stage to finish the terminating tracks and two stages to deliver the through tracks.

London Bridge

Pressure job: Work on platforms 10 and 11 has to be finished by 5 January

“We’ll have completed the six terminating tracks by January, hence we are a nearly a third of the way through the project.”

The Christmas window will allow the construction team to complete work on platforms 10 and 11, so that they can be operational for an immovable 5 January deadline. “They’ve got possessions that are planned years in advance, so if they missed this Christmas, the job would go back a year,” says WSP joint venture partner and Hyder project director James Dessain.

Both Dessain and Tooth agree that the Thameslink upgrade has, out of necessity, been the most collaborative project they have ever worked on. “We’ve got people from Network Rail all sitting with the team on the same floor, all working with each other; there are no boundaries at all really,” says Tooth.

The imitation game

WSP design package manage Claire Gott explains that a key objective in the project was to maintain the aesthetic of some of the original features of the station.

This included coming up with a solution to mimic a series of quadripartite arches that couldn’t be kept or maintained economically because they wouldn’t be able to carry the loads of the new track sections.

“The solution was to come up with a concrete shell that mimics the shape of the quadripartite arches with a fine board finish,” says Gott. “To fit with programme requirements we are forming a traditional flat slab on top of columns and then in-filling below with the quadripartite arch - but the issue with that is if you create a void, Network Rail needs to inspect it.”

The collaborative solution was to eliminate the need to inspect.

“The lining panel is designed to act compositely with the top slab as a single structural member in the end condition and because it’s like that, it’s accepted that they don’t need to get access because it isn’t a void - and so the problem goes away.”

 

He describes the integrated 3D model of the whole station as the most complex BIM models that he has seen. In addition, collaboration between the design team and main contractor, Costain, has ensured that there have been no snags in the construction phase.

“All of the canopies for the platform are made in cassette systems which are manufactured off site,” says Tooth.

Again, the consequences of a delay to the project were so unthinkable that a mock-up of the canopy was put up and taken down three times off-site to allow the construction team to perfect the process. All of the main steelwork from this mock-up was then re-used in the real project.

London bridge sequence

For a larger version of this image click here

Perhaps the most innovative features of the overall design are the shock transmission units for the new steel viaduct over London Bridge bus station, possibly a world first. When a train driver slams on his brakes significant longitudinal forces are induced in the tracks, which are normally transferred through the sleepers to the ground below. But the columns that support the viaduct are extended from those that were built as part of the Jubilee Line Extension. These original columns couldn’t support the braking forces from the new viaduct, so it was necessary to transfer these loads into the concrete structure to the east via the shock transmission units.

These are basically giant hydraulic dampers, which can accommodate thermal movements and the sudden shocks from the braking trains and transfer them to the concrete structure rather than the supporting columns.

In areas where is has not been necessary to remove the existing arch features, the design team has had to make provisions for carrying rail tracks above existing arches or listed parts of the station (see box). Again this has entailed minimising loads but also in-filling sections with lightweight concrete and polystyrene - thought to be a first for a rail project.

Bridge deck supporting column design was driven by the need to free up as much space as possible for commuters. “Originally the design was for paired columns,” says Tooth. “But that was honed down to a single major column with a cross head”

However, there were still concerns that the noise generated by the trains passing over these bridge decks would reverberate so loudly in the cavernous new concourse that announcements would be inaudible.

So the design team conducted sound recordings of trains going over different types of structures, applied damping factors to the different noises that were generated and then played them back to the station team to create a sense of the noise that they could expect according to different design solutions.

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