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Prefab sprout

PREFABRICATED FOUNDATIONS

Prefabricated foundations are ensuring that classrooms are being built at speed in the shadow of Salisbury Cathedral. Mike Walter reports.

Children at the Bishop Wordsworth school near Salisbury Cathedral resume lessons this month in new classrooms built in record time thanks to use of precast foundations.

Both the buildings and foundations were largely fabricated off site to accommodate a fast track construction programme, part of which took place during term time. Use of precast units also helped minimise the number of lorries carrying heavy materials to the site and hence disruption for pupils and teachers.

The school lies within the historic Salisbury Cathedral Close.

Its location meant extra care had to be taken during piling works and delivery of foundation beams.

Main contractor George & Harding Construction was responsible for a programme of works totalling nearly £2M, which involved construction of eight new classrooms and an art and drama studio.

G&H asked specialist contractor Roger Bullivant to design, supply and install a foundation system that would save time and minimise disruption.

Bullivant's precast foundation system consists of a combination of precast concrete segmental piles, pile caps and beams manufactured at its Burton upon Trent facility. For the Salisbury project, a bespoke foundation system was designed.

Contracts manager Nick Calvert from Bullivant's house foundations division says the precast foundation system was well suited to the fasttrack nature of the works and the city centre site.

'The fabrication of beams off site allowed us to achieve greater speed and accuracy on site, ' Calvert says.

'If we had chosen to cast beams insitu, we could have been constructing them for a long time, especially this summer's wet weather.

'In our opinion, insitu construction on this site would have caused unnecessary disruption to the school.' Programming constraints meant Bullivant's work had to be carried out during July before the school broke up for the summer.

Foundation works began with the driving of precast 200mm square segmental sections, into ground which consisted largely of sand and gravel overlying chalk.

A custom built 17T KT rig with a reduced mast height was brought in to drive 1.5m, 3m or 4m sections of pile to depths between 3m and 5m with a working load ranging from 200kN to 350kN. Crushed stone and geogrid matting was used to provide a solid bed for the piling rig to work on.

Site manager Alan Crabb says the school head was pleasantly surprised that the piling system chosen for the contract did not cause excessive noise and vibration during lessons.

A grade I listed medieval wall which borders the site was monitored for vibration throughout the piling, but the works were later found to have caused no adverse effects. Because of the site's proximity to historic landmarks, a team of archaeologists kept a watching brief on the works but they found little more than a 16th century token buried in the ground.

Preliminary site preparation plans had specified the use of vibro compaction. But after consultation with the project engineer and historians from English Heritage and the Salisbury Cathedral Committee, it was decided that segmented piling would reduce the possibility of interference to the wall and to a nearby chapel used by the school.

When piling was complete a combination of square and circular precast and insitu pile caps were installed.

Where the design stated that greater loads were likely be exerted from the classroom units to the piles, two piles were driven close together and a single cap installed over the pair.

Three different varieties of precast foundation beams designed by Bullivant were then brought to site and lowered into position.

Inverted T-beams were specified to support the external skin of the prefabricated buildings, as well as internal walls and the flooring. Both L-shaped and rectangular beams were used for internal support. Beam placement took two days. The beams were then stitched together using steel and concrete to form the joints.

Accurate setting out was aided by a geodimeter, providing accurate pile positions, cut off heights and positions of beams.

Crabb says: 'The geodimeter uses pre-programmed co-ordinates downloaded from AutoCAD to allow positions to be checked before and after piling, at the capping stage and during and after beams have been placed. The geodimeter certainly helped us complete works accurately and to schedule.' Careful planning was essential before works could begin. Access for works traffic was shared with vehicles and pupils entering the school grounds.

Crabb says: 'We agreed with the school not to take delivery of materials at the beginning and the end of the school day or at break times, so as to minimise disruption to pupils and teachers entering and leaving the school.

'The use of prefabricated classrooms and precast beams on the job helped avoid undue deliveries of materials to site during term time and allowed us to complete construction works in time for the school to open its doors to pupils again on 3 September.'

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