THE FLEET of seven piling rigs at St Pancras - ranging from a hightorque HT55 and BG30, down to low-head tripod units - has virtually formed Bachy Soletanche's entire arsenal of bored piling.The total 2,300 piles were delivered in 10 different diameters, from 270 mm to 2.1 m, and in lengths up to 40 m.
Around 350 mini-piles, averaging 300 mm in diameter, helped support temporary retaining walls, while over a dozen under-reamed piles, with 4.5 m-wide bells, allowed crucial end-bearing piles to stop well short of a major water ring main 30 m deep.
The critical path for the piling crews lay in the completion last month of the Thameslink cofferdam box surrounding a 365 m length of rail tunnel and requiring some 950 contiguous bored piles.The rail blockade from mid-September has allowed immediate excavation of the brick tunnel for conversion into a new underground station.
Original thoughts of completing the bulk of the box piling during the rail blockade would have demanded 11 rigs on site simultaneously. But an early start and a switch from planned secant to contiguous piles saw all but a few end piles installed by early September.
Piling was allowed just 300 mm from the live Thameslink tunnel and within 1 m of large, equally live, gas and water mains, but only after lots of 'owner reassurance', including rigorous structure-condition monitoring, trial pits and location probing.
Wet piles were bored through a hydrocarbon polymer fluid.This was used rather than bentonite, which can cake around pile perimeters, affecting capacity.The polymer's long chemical chain formed a mesh around each pile bore, which is then broken down over time, so as not to affect pile strength.
Among the project's last major piles were a cluster of seven up to 1.8 m in diameter.They were bored just a few days ago, about 39 m deep and passing within 2 m of live Tube lines.This was well within the normal exclusion zone for the Underground.To guarantee no distress to the Tube's aged cast-iron lining, RLE spent nearly three years proving its pile design with detailed testing and finite element analysis.
'With not much change from £1 million, they are the most expensive piles I have seen, ' says Bachy Soletanche's Chris Merridew.'But we have completed them with absolutely no perceivable distress to the Tube tunnels.'
Bachy takes the strain ON A CONTRACT demanding over £50 million of complex geotechnical work, the joint venture claims it is a big advantage to have a specialist foundation contractor on board as a main partner.
On similar major projects, foundation work is often subcontracted to a main contractor's piling division, but on the St Pancras contract the totally independent Bachy Soletanche is a full partner, sharing risk equally with Costain, O'Rourke and Emcor.
'The joint venture has accepted all ground risk, and in foundation work you only get one chance to be right, ' claims Chris Merridew, Bachy's senior manager on site and now the joint venture's civils director.
'Geotechnical engineers spend most of their working lives assessing risk, so hopefully we can make a sizeable contribution on this contract.'
The company is well used to such a role, as nearly half of last year's record £108 million turnover was achieved through similar main contractor roles, several in joint ventures.
At St Pancras, the Bachy team is as fully integrated on site as it is at director level. Even at the height of the piling programme, around twothirds of the company's site engineers were responsible for major civils structures rather than the piling operations.