FEW SITES ARE entirely obstruction-free, so designers need foundation solutions that are flexible enough to accommodate different layouts, depending on what is found in the ground.
Normally, with pile and beam methods, there is little scope for changing pile locations to avoid obstructions, as the piles tend to be regularly spaced in straight lines beneath the ground beams. While a site investigation may locate some obstructions, once piling starts, with pile and beam methods there is little ' exibility for contractors to change piles or redesign the foundations.
On some sites it is not possible to use beams at all, particularly if the ground is known to contain tree roots. 'Beams are typically between 400mm and 600mm deep, so they would cut across lots of roots, ' says Abbey Pynford managing director Paul Kiss. 'This will disturb the trees and, potentially, kill them.' At a Golders Green site in north London, a maze of tree roots had hampered development of the King Alfred School, until engineers decided on Pynford's Housedeck system. This will provide contractor Vascroft with a flat, level deck slab from which brickwork or a timber frame can be erected.
Project architect Geoff Strange says: 'We chose Housedeck as it solved the main issue on site of working around the trees while building so close to them.' The system consists of a reinforced concrete slab 225mm deep, which covers the entire footprint of the building and is supported on piles between 140mm and 350mm in diameter. The optimum slab and pile design for every site can be changed using different design parameters, including slab reinforcement, concrete strength and pile location - if circumstances prevent the first design from being practicable.
The work will provide the school with a new craft and design technology building, housing two new workshops and computer studios.
Kiss says: 'We set the piles out on the ideal locations and then dug down to identify any roots. Any we found meant moving to the next preferred pile location, until we found the best available position for every pile. Once we'd done that, we could phone the positions through to the design department and they could redesign the slab straight away.' Site workers installed 35 CFA piles - all 250mm in diameter - to an 18.5m depth through fill and London Clay. Although six piles were relocated as a result of 'nding roots, there was little delay to construction as new pile locations were established before the rigs arrived on site.
Pynford began work on its £62,000 contract in October and finished last month. The deck was due to be poured at the end of November.