Piling at a huge new retail and leisure development in London is always going to throw up a few complications. But work at the London Underground (LU) and Chelsfield site in White City in west London has called for particularly high levels of meticulous planning.
The presence of two separate tube lines and a siding for 16 trains have made the development an exercise in very close liaison with LU.
Because LU's tracks, power and signalling cables criss-cross the site above and below ground, 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.
Main contractor Multiplex senior construction manager Tony Hodder says: 'We have to ensure at all times that we're not interfering with the operation of the underground system, LU's assets or the public. It's been a long drawn out process.' This initial raft of authorisations included allowances to pile extremely close to LU resources such as rail lines - just a few metres in places - when the train operator normally insists on 115% of rigcollapse radius.
Cementation-Stent project manager Scott Hunter says: 'To conform to LU's engineering standards we can bring a 33m rig to within 38m of any LU property. But in places we are piling within 2m of the Central Line's open cut.
'So a major part of the approvals process was obtaining concessions against the LU's engineering standards for the use of tall plant and it took about 12 months to do.We got our first concessions in about four and a half months.' However, Cementation-Stent has still had to provide additional assurances around key property such as building reinforced concrete piling mats.
But 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 the piling 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 in the east, phase two in the west and phase three in the centre, all are divided by LU assets.
Each phase poses the sorts of complications liable to keep designers and contractors awake at night.
Key to the scheme is moving the railway siding from the centre of the site and burying it in a box in the western section, presenting some of the job's toughest challenges.
Hodder says: 'The new siding will be 6m below ground. The construction of this and the associated tracks leading into it are the critical part of the project.' Multiplex project manager Rod Eagle adds:'We don't get to build on the centre of the site until the trains can be decanted into [the western section].' Multiplex will excavate the siding box in 12 sections due to three main concerns. The biggest problems are three major bands of underground services running across the excavation area. These include traction power for the Central Line, high and low voltage power, communications and signal cables as well as other third party cabling.
A second problem is the overground eastbound Central Line following the site's western edge (which will be built over). Finally, Bulwer Street is just a few metres from the southern end of the box.
The cabling is the fulcrum around which work in this area moves. Hodder says: 'There's a hell of a lot [of cabling] and if you interfere with any of it you could close this whole system down. To the Central Line that would mean delay and costs. It's been isolated and it is of course dictating how we construct this siding and how we excavate.' Hunter adds: 'The original plan was to put the secant wall in all the way around to seal the box, excavate the inside down to 6m and then put the piles for the shopping complex down in the excavation.' However because there are areas that are effectively frozen until the service diversions take place, while the secant wall can be installed in one go, the box has to be excavated in sections as areas become free.
The wall is being installed as part of Cementation-Stent's initial £20M contract (which is subject to additional bolt-ons). The JV will eventually install 2,300 piles in this area, founded in a succession of made ground, sands and gravels and finally London Clay that extends beyond 40m.
There are three main types of piles being used. The first are 1,200, 750mm hard and firm CFA piles forming the secant perimeter wall at depths of up to 28m. Second are 950 rotary bored bearing piles from 450mm to 900mm diameter installed at 20m to 38m depths to support the above ground structures.
The last are 33 plunge columns, 1,050mm and 1,500mm diameter and between 20m and 40m deep. They will be installed at 8-10m centres in two groups to support two top-down slabs. The slab at the southern end, supported by 16 piles, will provide stability for Bulwer Street.
Hunter says: 'The slab will allow access when the rest of it has been excavated and will also act as a permanent prop to limit movement on Bulwer Street.' At the northern end, the other 17 plunge columns will be supporting a slab over the 'throat' of the siding where the 16 lines converge to form the single entry track. The slab will also enclose the diverted cables from the three bands of services crossing the western area.
Once the siding has moved, work can start in the middle of the site. Here, CementationStent will eventually install 2,000, 450mm to 750mm rotary bored piles to depths of between 20m and 38m.
The eastern section is less problematic, although hardly straightforward. Network Rail's West London line runs along its eastern edge and the westbound Central Line runs in a tunnel between the central and eastern sections of the site, emerging from the tunnel as it crosses the development.
A load transfer structure will protect the Underground line.
'We're effectively bridging the tunnel and shedding the load on the strata beneath, ' says Eagle.
The western edge of the reinforced concrete transfer slab will be supported by a contiguous pile wall comprising 600mm diameter, up to 25m deep CFA piles. The eastern side will first be supported by a temporary berm during excavation of an underground car park.This will be replaced with a permanent secant pile wall also of 600mm diameter, up to 25m deep CFA piles, tied back by the transfer slab above.
Construction in the eastern part is well under way. Cementation-Stent has already placed 2,600 rotary bored piles ranging from 450mm to 750mm, to depths from 20m to 38m. Using up to six rigs. work began in November 2003 and finished in May last year.
Hunter adds: 'We couldn't pile close to the [Central] line because of the berm, but once it's removed, another 900 piles will go in.' Work will begin in March this year.
To the north of the site where the Central Line goes into open cut, the line is being covered with a concrete slab to support the development above. In December, rig crews began installing the foundations for this using minipiling techniques for the 113, 550mm bearing piles which are up to 20m long.
The new viaduct span for the Hammersmith and City line will be slid in over a long weekend in July 2005 to accommodate the feeder track for the new sidings. In advance of this, crews using 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 minipiles of 245mm up to 28m down.
Piling will finish in autumn 2006, with project completion scheduled for October 2007.