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THE BEAT GOES ON

Cantilevered ground beams are helping a housing developer to squeeze maximum value from a former nightclub site in Torquay, reports Mike Walter.

Drunken revellers in the English Riviera resort of Torquay have long since stopped falling over outside Munroes nightclub in Victoria Road. All that remains of the recently demolished club is its white brick facade.

Behind this, a team from foundation contractor Roger Bulli ant is carrying out a complex ground engineering package, including cantilevered ground beams, for a block of 18 apartments.

The cantilevered beams are designed to butt up against the 8m high nightclub wall.

They will allow the new building - which will vary in height from two to four storeys - to be built as close to the wall as possible.

The beams provide not just a solid base for the flats but also a degree of lateral support to the original wall, a small part of which has been underpinned by Bullivant.

Contracts manager Derek Murphy explains: 'Cantilevered beams enable new walls to be made as close as we can to the nightclub wall. Planning restrictions mean the wall cannot be demolished and it was not possible for a rig to install piles within 1m of it.' He adds that the cantilevered option is ideally suited to situations where a beam cannot be supported by piles at both ends. The beams are designed to accommodate brick and block construction in the same way as conventional beams.

The cantilevered beams stand proud of a standard grid of precast concrete ground beams by about 1m. Each is 450mm square and supported by a large cast insitu pile cap, measuring up to 2m.

These pile caps span a trio of piles which are designed to work in both tension and compression. A 'tiedown' pile at one end of the cantilever provides the necessary balance to the load on the other end.

The beams are formed from rectangular cages of reinforced steel and are secured to rebar which protrudes from each pile. Once in position the cages are fi lled using a C35 grade structural concrete.

Each cantilevered beam is tied in to the grid of rectangular precast beams and stitched together with cast insitu concrete. Precast beams are supported by single pile caps or double insitu caps where loadings are greater.

The cantilevered beams have allowed a flight of steps to a footpath to remain open throughout the contract.

Had they not been used, the steps would have had to be demolished to allow a piling rig access and the footpath reinstated at a later date.

The beams are supported by 81 piles which Bullivant installed in two days using a 300mm diameter CFA rig. It built the piles through the underlying clay and silt bands and they found in shallow sockets in the mudstone beneath.

Piles, up to 10m long, are designed to accommodate loads of between 125kN and 400kN.

'Augered piling was chosen because the hard rock could cause precast piles to break, ' Murphy says.

'Use of the augered technique also meant that residents and customers of a barber's in an adjacent building suffered very little disruption.' It was not all plain sailing. A 500mm diameter stormwater culvert running the length of the site led to the redesign of some of the pile positions.

The culvert was found to run in a curve between two manhole covers.

Property developer AH Gadd carried out a computerised axial tomography scan of the area to give an indication of the culvert's position.

The ground was then probed and small quantities of soil carefully excavated to verify the exact posi ion of the culvert. With new positions decided, piles were installed either side of the drain and bridged with a cast insitu pile cap.

Ground beams across the site vary in level by up to 300mm and this difference is accommodated using a series of concrete spacers known as 'cheeses'. These are 75mm and 150mm thick and have a 40N crushing strength equivalent to the beams.

Beams towards the back of the site are supported at one end on 1.2m high reinforced concrete columns that accommodate a step down to a lower level.

A total of 200m of both cantilevered and precast beams will have been built at the site by the middle of this month.

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