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Overhead power lines, water mains and a railway line complicated piling for a new retirement complex in Somerset. Mike Walter reports.

High voltage power cables crossing the site of a sheltered housing development in south west England led to use of a reduced height rig to install 18m long precast concrete piles last month.

The three-storey block in Westonsuper-Mare, due to open in April 2007, will include 53 flats, a roof terrace and a central atrium. The developer and main contractor is Castleoak Care Partnerships.

Construction will begin in earnest following completion of foundation contractor Roger Bullivant's piling and installation of pile caps, ground beams and flooring.

The electrical cables cross the site at a height of 16m, and an exclusion zone for metal objects was set at a 6.7m radius from them.

Under these plant restrictions, Bullivant chose to use one of its TR1 rigs with a mast height of only 8m to install 307 precast piles on the site.

'Working beneath the power cables [also] precluded use of larger sections of precast pile and suited our segmental sections which, on this contract, came in 3m and 4m lengths, ' says Bullivant contracts manager Derek Murphy.

The 250mm square pile sections were connected using a 'Collet' mechanism - a reinforcing bar from one section of pile is locked into a socket in the other. Pile sections were driven into soft ground containing peat using a 5t hammer.

A second hazard also had to be taken into account. A high pressure water main had earlier been diverted around the site by a local water company, but workers had to ensure they did not damage two new thrust blocks buried underground.

This meant Bullivant installing piles for the building perimeter to within 1m of one of the thrust blocks.

Use of the reduced height rig also ruled out any risk to train services on the Bristol to Exeter railway line which borders the site.

Although piles were being driven as close as 10m from the railway boundary, the rig's short mast meant that, in the unlikely event it were to topple, it would not fall on the line.

Pile loadings vary from 125kN up to 400kN and dynamic load testing of selected piles revealed that each pile could take between two and three times design load. Six piles were tested to ensure their load capacity had been achieved.

Before the rig left site each pile position was verified by Bullivant site engineer Simon Wakelam.

'We have a tolerance of 50mm in either direction to keep to, ' he says.

'More than that and a pile would be deemed out of position and would have to be repositioned, which has not been necessary.' Eight days of piling finished in February. Work included installation of four piles to support a base for a tower crane, which will be lifting the three-storey building's timber frames over coming months, and a further 12 piles to support two internal lift shafts.

Piles caps are mixture of 250mm square and 700mm circular precast caps and cast insitu concrete caps.

Only three of 307 pile positions were changed because of obstructions. In each case, two identical piles were driven either side of an obstructed pile and bridged with a pile cap.

Bullivant is now placing about 1km of precast concrete beams on site and will follow up with precast beam and block flooring.

Changes in floor level of up to 300mm will be accommodated using a series of concrete spacers known as 'cheeses', with the same 40N strength as the precast beams.

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