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Inside the St Pancras labyrinth

PILING - The last of more than 2,000 piles is now being installed for the expansion of London's St Pancras train station.David Hayward reports on the three-year battle to avoid a maze of underground obstacles

EUROPE'S largest site hut, one of the smarter looking temporary buildings among the urban clutter behind London's St Pancras station, is an appropriate home for the geotechnical team overseeing the capital's most complex and challenging piling contract.

Later this month the last of some 2,300 concrete piles will be unceremoniously bored into the London Clay that underlies the site around the elegant Victorian train shed.Their role is to help more than double the size of St Pancras and create the London terminus for the Channel Tunnel Rail Link.

For almost the past three years, a fleet of up to seven rigs of varying capacity has been forming an equally varied array of piling.Temporary minipiles and 2 m-diameter giants now compete for space alongside a massive contiguous bored-pile cofferdam and vast under-reams sporting 4.5 m-wide bases.

It is not so much the pile inventory itself, but what lies above, below and around them that has made the works so challenging. A diverse inventory of obstacles includes above-ground mainline trains and road diversions, below-ground Tube and Thameslink tunnels and an intricate maze of sewers, culverted rivers, major gas and water mains, all interwoven with numerous heavy brick foundations that once supported railway arches, gasholders and workshops.

'We have been sinking piles within 1 m of rail, gas main and sewer tunnels; all of which had to remain live throughout the job, ' says Chris Merridew, civils director for joint-venture contractor Corber (comprising Costain, O'Rourke, Bachy Soletanche and Emcor Rail).

Banks of monitoring instruments in the tunnels and sewers, plus trial pits and sensitive probing, ensured that tunnels and their contents suffered no distress.

Above-ground congestion was exacerbated by the need to maintain a permanent major road route across the site.

'We have often had to totally dismantle a major piling rig and move it by lorry around the site perimeter, just to reach a new piling area a few metres away, ' recalls Mr Merridew.'It could take us two days to carry out a task which would be achievable in just half an hour on a conventional site.'

The departure of the last of those piling rigs over the next few weeks will allow most of the 600 staff in their three-storey site Portakabin (complete with lifts and coffee bar) to concentrate fully on the structures that have demanded such a complex piling blanket.

Designed and project-managed by Rail Link Engineering for client Union Railways, the £400 million CTRL contract 105 centres on increasing the size and capacity of St Pancras station by adding a 300 m-long, opensided elevated extension on to its northern end.

Supported on 6 m-high concrete columns, the platform extension will allow 400 m-long Channel Tunnel trains direct from France and mainland Europe to terminate in the centre of London, by 2007.

A linked network of passenger concourses, ticket halls and new underground subways will enable these European travellers immediate access to virtually all the UK's inter-city routes from St Pancras and King's Cross, or from other London stations via Underground and Thameslink services.

Creating such a major interchange on what has been a multiple-use, city-centre brownfield site for near 150 years involves far more than just an extension outside the 1886 Grade I-listed Sir William Barlow train shed.

A raised concrete deck, slightly wider than the original shed, is being built in two equal, full-length sections, 60 m wide, to carry 13 new platforms.Various-shaped columns supporting the eastern side are carried on individual pile caps, each founded on several concrete piles, measuring up to 1,800 mm in diameter and bored to a maximum of 34 m deep.

But snaking beneath the footprint of the platform's western side lies the project's major challenge: the curving brick-lined underground tunnel of the twin-track Thameslink route.Thameslink 2000, a planned major upgrade of this cross-London railway, remains on hold pending Government funding.

But construction of a new underground Thameslink station, linked with the facilities of both King's Cross and St Pancras, is only practical right now during the current above-ground extension work. Its construction was therefore included in the present contract.

The CTRL team has just finished enclosing a 365 m-long curved length of the live Thameslink tunnel within a large 12 m-deep contiguous bored pile cofferdam. Excavation of this cofferdam, to expose the 8 m-wide brick arched tunnel, is just beginning, so it can be broken out and converted into the new underground station.

It was the need to form a 980-pile cofferdam that dictated the project's two-stage construction programme. September 11 2004 became a crucial date to the joint venture's Bachy Soletanche piling crews.

'This was the start date for our 35-week rail blockade of the Thameslink line, during which time we need to complete the new underground station box, ' explains Mr Merridew, a Bachy Soletanche geotechnical engineer and the firm's senior manager on the Corber team.

In his role he has as clear an understanding as any on site of the urgency of this critical-path piling programme.He claims that it demonstrates one of the clear advantages of having a geotechnical contractor like Bachy Soletanche as an equal team player in the main joint venture contracting group.

'Having a management team with the back-up and resource of an independent geotechnical specialist is invaluable on a job like this, where a high percentage of the risk is in the ground, ' he says.

Six months before site work began in January 2002, Bachy engineers were key players in the Corber team as the joint venture and RLE valueengineered ways to ensure that the very finite blockade period proved as low-risk as practically possible.The major change was to swap an originally planned secant piled cofferdam for a quicker, lower-risk contiguous piled alternative.

With the main water table well below the piling, and an upper perched aquifer exhibiting lower water levels than expected, the need for a totally watertight cofferdam was reduced.As a result, some 420 fewer piles could be used, as the original hard-firm secant design was replaced by shorter contiguous piles.

The 24 m-wide cofferdam is propped at ground level by concrete Mbeams prior to top-down excavation beneath to reach the Thameslink tunnel.The revised design also allows these props to be placed at 15 m centres - double the spacing that would have been needed for a secant cofferdam.

The result has been a piling programme that is at least twice as quick.

Another value-engineered 'de-risking' decision included starting the whole piling programme months earlier than planned.A 40-arch Victorian brick rail viaduct carrying operational Midland Main Line tracks right across the site and into St Pancras created another major piling obstacle.

Originally it was to be left untouched until the tracks were diverted on to the first eastern half of the platform extension, which opened on schedule at Easter. Instead, the site team chose to nibble away at viaduct sides until only the minimum 3 m clearance was left either side of the tracks, thereby allowing early access for piling.

All piling for the Thameslink box, bar a few end piles needed to seal it after the demolition of the tunnel, was completed in time for last month's start of the rail blockade.As box excavation begins, so too can construction of the second half of the St Pancras extension directly above.

Some 40 per cent of the Thameslink box lies directly beneath this western side extension, and here the 6 m-high platform columns are founded directly over the box's ground-level slab on 100-tonne steel girders.That no piles are needed is little comfort for Bachy's senior man on site.

'As the man in charge of all civils work, I now have much more involvement with general construction rather than just the geotechnical challenge, ' explains Mr Merridew.'For me, the next date I may lose sleep over is May 15 next year, when the rail blockade ends and the new Thameslink station must be ready.'