Piling on a huge new retail, leisure and transport interchange project in London is always going to throw up a few complications. But work at the £1.36bn London Underground (LUL) and Chelsfield site in White City has called for particularly meticulous planning.
The presence of two separate Tube lines and a siding for 16 trains has made the development an exercise in very close liaison with LUL.
Because LUL's tracks, power and signalling cables crisscross the site above and below ground the piling subcontractor, a joint venture of Cementation Foundations Skanska and Stent Foundations, needed 57 separate approvals for the western side of the site alone, and there are more to follow.
This initial raft of authorisations included allowances to pile extremely close to LUL resources such as rail lines when the train operator normally insists on a distance of 115% of rig-collapse radius.
Cementation-Stent project manager Scott Hunter says: 'To conform to LUL's engineering standards we can bring a 33m rig to within 38m of any LUL property. But in places we are piling within 2m of the Central Line's open cut. So a major part of the approvals was obtaining concessions for the use of tall plant.'
A giant development does not necessarily mean acres of construction space. 'People think White City is a big site but on the western side there isn't enough room to swing a cat, so reinforcement cages were prefabricated off site by Romtech, ' adds Hunter.
Multiplex won the £600M design and build contract for the development and has split it into three main areas. Phase one is in the east, phase two in the west and phase three in the centre, at present largely occupied by an existing LUL siding.
Multiplex project manager Rod Eagle adds: 'We don't get to build on the central part until the trains can be diverted.'
Each section poses the sort of complications liable to keep designers and contractors awake at night.
However it is in relocating the siding to a new underground depot in the site's western section that the contractors face their toughest task.
Multiplex will excavate the new siding box in 12 sections because of three main concerns.
Top of the list are the three major bands of underground services that run across the area. These include traction power for the Central Line, high and low voltage power, communications and signal cables.
A second problem is the overground eastbound Central Line along the site's western edge, which will be built over during development. The third is the location of a road and houses just metres from the southern end of the box area.
The cabling is dictating 'how we construct the siding and how we excavate', Hodder says. 'If you interfere with any of it you could close the whole system down.'
The original plan, he says, was to put in a secant wall all the way around to seal the box, excavate the inside down to 6m and then install the piles for the shopping complex.
However because some areas are effectively frozen until the service diversions take place, the box has to be excavated in sections as areas become free.
The area will eventually include 2,300 piles, founded in strata consisting of made ground overlying first sands and gravels and then London Clay that extends beyond 40m.
Three types will be used.
Some 1,200, 750mm hard-firm CFA piles will form the secant wall for the box at depths of up to 28m. Second in are 950 rotary bored bearing piles from 450mm to 900mm diameter installed at 20m to 38m depths. Finally, Cementation-Stent is installing 33 plunge columns at either end of the site, 1,050mm and 1,500mm diameter going to depths from 20m to 40m, and at 8m to 10m centres, that will support two top-down slabs.
At the southern end, this provides stability for houses in Bulwer Street. Hunter says:
'The top down slab will allow access across the end of the box when the rest of it has been excavated and also to act as a permanent prop to limit movement on Bulwer Street.'
At the northern end, the plunge columns will be supporting the 'throat' of the siding where the 16 lines converge to form the single entry track. The reinforced concrete slab will also enclose the diverted cables from the three bands of services that cross the western area.
The central section of the excavation area will be the last to start, as the existing siding is still in use to house trains.
The piling JV will eventually install 2,000, 450mm to 750mm rotary bored piles to depths of between 20m and 38m.
The eastern section of the site is less problematic, although hardly straightforward. Network Rail's West London Line runs along its edge, and the westbound Central Line runs on its border with the central section, emerging from a tunnel to run overground as it crosses.
A load transfer structure will protect the line, consisting of 600mm diameter piles up to 25m deep, supporting an overlying reinforced concrete slab. On the western edge these will be contiguous CFA piles, but along the eastern side, the ground will be cut away to make room for an underground car park.
A berm supporting the eastern side of the tunnel will be removed and replaced by a secant CFA pile wall, tied back by the concrete slab above.
Cementation-Stent has already placed 2,600 rotary bored piles from 450mm to 750mm, down to 38m depth, starting work in November 2003 and finishing in May this year.
Hunter adds: 'Once the berm is removed, another 900 piles will go in.' Work will begin in March next year.
To the north of the site where the Central Line goes into open cut, the line will be covered with a concrete slab to support the development above. This month rig crews start installing the foundations using mini-piling techniques for the 113, 550mm bearing piles which will be up to 20m long.
The new viaduct span for the Hammersmith & City Line will be slid in over a long weekend at the end of July next year to accommodate the feeder track for the new sidings.
In advance of this, low overhead rigs will install 100, 450mm or 600mm diameter piles to depths of 28m using both tripod piling and case and auger methods. They will also construct 181 mini-piles of 245mm, up to 28m down.
Piling at the site will finish in autumn 2006, with project completion due in October 2007.