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Precast capping saves time on bored piles

A NEW PILE capping technique for contiguous bored pile walls can reduce time taken for piling and capping beam construction by 25%, claims Kvaerner Cementation Foundations.

The system involves piling through precast concrete units, removing the need to excavate around and break down the piles and incorporating blinding and shuttering for the capping beam.

It was developed by KCF engineer Peter Renouf in 1998, following a contract at the Manchester City Art Gallery. The job involved installing bearing piles and a temporary CFA contiguous bored pile wall with a capping beam. As the wall was temporary, it was decided to use the polystyrene guide wall for maintaining the pile position as a back shutter to the capping beam.This eliminated the need for pile breakdown straight after concreting.

The idea developed when it occurred to Renouf that the sand blinding layer placed in the trench for the polystyrene guide walls could be substituted by an accurately placed 75mm concrete layer that could also serve as blinding for the capping beam.

'We realised we would have achieved what we wanted if we could also make precast concrete units the same size as the capping beam, bore the piles through them and scoop out the concrete down to the blinding level, 'he says.

A chance to use the system came early in 1999, when KCF proposed using the technique on a contract in Bootle.While tendering was under way, shutters and concrete units were made for trial of the system at the Kvaerner's precast yard in Boroughbridge, North Yorkshire .

Results showed the idea was sound and work went ahead on the Bootle contract.

However, the 450mm diameter piles were capped with a beam running at many different levels, which did present some problems.

'Valuable lessons were learnt. But a shuttered, blinded trench for the capping beam was left for the main contractor far more quickly than would have been possible with conventional methods, 'Renouf says.

As well as reducing time and cost, the system produced some unexpected benefits.

It improved the accuracy of pile positioning and, as positions were predetermined, eliminated the problem of lost setting-out pins.

After the Bootle contract KCF applied to patent the technique before using it on a project in Newcastle. Here work involved installing the 750mm diameter piles for the wall, a permanent capping beam, excavation, permanent ground anchors and bearing piles.

New units were cast to deal with the larger diameter piles and the system allowed excavation to start five weeks into the programme rather than the seven or eight weeks expected with conventional methods, KCF says.

The system is now being used on a contract in Birmingham for a curved pile wall with varying levels and capping beam widths.

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