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UNDER-REAMING - Tony Suckling explains the reasons behind Stent's decision to re-enter the under-reaming market and describes its newly designed equipment.

After an absence of nearly 10 years, Stent has returned to the under-reaming sector with a recently completed contract in central London.

The company says that predicted sector growth over the coming years, combined with engineers struggling to fi nd foundation solutions for an increasing number of projects which have been previously piled, has led it to believe that the time is right to re-establish its market presence.

'Over the past few years we have seen steady growth in the number of London projects where the removal of pile foundations has been needed to redevelop important pieces of real estate, ' says technical development manager, Tony Suckling.

'When seeking new foundation solutions, where piles are present but cannot be used, we have noticed that engineers are turning increasingly towards a combination of removal and avoidance of previous piles.

'Under-reamed piles offer the possibility of avoiding existing piles, while simultaneously providing single points of highly concentrated load carrying capacity. Although we have had a strong presence in the pile-removal market over the past few years, we felt the time was right to include under-reamed piled solutions.' Suckling says that after so long away from the sector, the contractor decided to make an effort to improve upon one important factor in under-reaming: the time taken.

With ever-greater building loads being specifi d, Stent tailored its development strategy to consider the call from consulting engineers to speed up the process of forming under-reams - perhaps allowing larger under-reams to be formed in limited working shift periods.

During 18 months of development, the firm claims to have built and tested two innovative underreaming tools at the BRE testing site in Chattenden, north Kent.

Suckling says development of tools initially centred on a radical departure from conventional configurations. 'After trialling a half-sized version of our design that incorporated all our ideas, we learned that some worked well and others, while contributing to significant reductions in the time taken to form the under-ream, presented further problems.

'Reviewing trial results, we realised that we could make all of the improvements by applying just a few of our ideas to the commonly used basic-tool configuration. We also wanted to take advantage of the increase in power and torque available from modern piling machines and designed our full-size tool to make best use of them.

'Our experience suggests that we are able to dig significantly faster than before and ably deal with clay stones, a traditional hazard for under-reaming. Our tool simply smashes them to pieces.' The design centres on two things.

First, Stent claims it has made a significant increase in the efficiency by which vertical and rotational forces applied to the tool by the piling rig are translated into lateral forces needed to excavate. Second, it covers the need to see what is going on down the hole as work progresses.

To achieve these two goals, the company has built a bucket with a two-stage pushing arm mechanism.

This applies a three to four-fold increase in the pressure applied to the clay, in the important early stages of forming an under-ream, where, Stent says, traditional tool geometry works least efficiently.

Alongside this an on-board camera system fitted to the tool enables the rig driver to see what is going on down the hole, on screens fitted inside the cab. This, Stent says, has helped with digging efficiency and the base cleaning protocol.

Suckling says: 'In one way, we have been very fortunate as we have a number of site personnel who recall digging under-reams with underpowered rigs and cleaning them by descending the borehole with rakes and shovels to remove loose material.

'This proved invaluable during the development stages and the knowledge gained years ago by these men helped us understand what can go wrong, and to design tools and working protocols accordingly.'

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