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Fall and rise of London Bridge

Radical plans to redevelop one of the capital's oldest and busiest stations are under way. Diarmaid Fleming finds out what could be in store for London Bridge.

It probably has the dubious distinction of London's most unpleasant major rail terminal; its sad plight brought sharply into focus by a new elegant Jubilee Line Station built underneath it. But ambitious plans by consultant Alan Baxter & Associates and architect TP Bennett for Railtrack's £400M development of the station mean that London Bridge as we know it will fall down, to be replaced gradually with a station appropriate to the 21st century rather than the 19th.

The station's ramshackle appearance belies its importance. Handling around 40 million passengers a year, it is the fifth busiest station in the country, with around 60,000 people weaving through its maze of platforms and warren-like tunnels during the weekday rush-hours.

The station serves local trains from south-east and south London, Kent and the south coast.

But the patchwork of different platforms and services is not part of any well-worked integrated transport scheme.

Instead, it is a relic of ad hoc development since the station was built. Starting life as a terminus in 1836, the station then spread with platforms built off its north side.

Developments were carried out with little reference to their effect on the surrounding urban areas, leaving today a sprawling combination of terminus and through platforms, an ugly block which inhibits urban development and the much-needed regeneration in one of central London's more seedier parts.

'There were seven or eight distinct phases of development after the original station was built. Now there are terrible capacity problems and these act as a barrier to development, ' says Alan Baxter and Associates senior associate Alan Fleet.

'Different train companies have conflicting requirements which the station can't meet.

Through lines have been squeezed in over the years and these platforms have insufficient capacity to deal with passenger volumes, while the terminal station has complete overcapacity for its numbers.'

The construction of the JLE station, opened in 1999, has made the capacity problems even worse, with a much higher flow of people.

'We found that most people who use London Bridge station want to go somewhere else.

We've aimed to shift the balance to create a larger through station which will cater for this, with a minor suburban terminus to one side, ' says Fleet.

The through station will be on the route of Thameslink 2000, running trains from Kent and the south coast to Bedford, Peterborough and further north. It will need nine through platforms to cope with predicted passenger numbers. Stations which combine through and terminus platforms will always mean complex passenger movements, with separate flows of people from each, and passengers needing to be accommodated while they wait for trains.

Baxter's solution has been to separate people and train movements on different levels by designing a concourse below or above the tracks, with easy high volume access to the platform areas.

With London Bridge station already 8-10m above street level, an over-track solution would mean a long vertical distance for passengers to travel. So an under-track concourse serving both terminus and through platforms will connect streets running north and south that are, at present, cut-off by the station.

This would help to regenerate the deprived areas south with the increasingly prosperous areas to the north.

Trying to construct what is in effect a new station within the current one with its haphazard sum of parts and huge volumes of passenger traffic represents a massive engineering challenge.

'We can't close the station down because the disruption would not allow it, ' says Fleet.

'We have developed the structure to enable it to be built in a planned and careful manner, which has been a major constraint on the design.' Construction is to be phased, to enable new platforms to replace the old and minimise disruption to services.

As well having to run trains and people through the station as the construction evolves, constraints above and below ground add to the complexity of the work.

Below ground are tunnels for the Northern and Jubilee Lines.

And there's plenty planned for above - with space in central London like goldust, the project includes a new 10-12 storey office development over the station. Baxter's design proposes cutting a huge slice out of the existing station viaduct structure and then spanning the gap with a series of platform-track 'bridges'. The concourse will occupy the space underneath, while a mezzanine level will enable easy changing between trains just below platform level.

While the present station provides for six through and nine terminus platforms, the new design will reverse this to six terminating and nine through platforms. These will be much wider to accommodate the anticipated 25% increase in passenger numbers when Thameslink 2000 comes on stream, with wide escalators to move people from the different levels.

An elegant structural form was devised for the main supports to fulfil constraints posed by the Tube tunnels, movement of people on wide escalators and supporting the office block above.

Running along the length of the construction are a grid of branching supports, which Baxters describes as 'like a row of hands with fingers outstretched to carry the deck of the office development'. The geometry allows them to splay out, straddling the escalators, while acting like trestles they carry the platform-track 'bridges' spanning the concourse.

'Speed of construction is absolutely vital. We are opting for concrete rather than steel, and have gone for pre-cast shells with an in-situ core which controls the quality of the finish and deals with fire risks, ' says Fleet.

Complex reinforcement in connections may give way to welded plates with reinforcement connected to these for more ease and speed of construction.

'We can't get many support points because the geometry is fixed by the office block above, the mezzanine and the escalators, ' says Fleet. Piling was dictated by the need to bridge over the JLE tunnels below. The result is 2.4m diameter 50m long piles down to the Thanet Sands.

'We looked at 900mm diameter continuous flight auger piles but the number of piles which would be needed meant the pilecap would have been like a continuous raft foundation.' There was also concern that this would have overstressed the JLE tunnel running underneath.

While the new station will represent the latest in structural design, part of the fabric of the old will remain in the shape of some of the existing train shed wall near the terminus platforms. The listed train shed itself - built between 1864 and 1867 under London Brighton and South Coast Railway resident engineer Frederick Dale Banister - will, however, have to come down.

Full planning consent for the scheme has been granted. How soon work starts depends on funding and the outcome of Railtrack's discussions with the Strategic Rail Authority and the Government on investment.

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