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DIY inspired

Geotechnics - A new type of pile is inspired by garden fence posts

Anyone who has erected a garden fence recently will recognise Screwfast Foundations' new Star Pile: It bears a close resemblance to the steel spike hammered into the ground by thousands of home owners to support the stakes to which fence panels are attached.

It comprises four tapered steel flanges, arranged in a cruciform shape and capped with a mounting. The pile can be are vibrated into the ground with the aid of a pecker.

Screwfast chairman Peter Dunn says the design 'is the result of a sequence of thoughts relating to the support of tall, laterally loaded structures'.

The Star Pile has evolved from work on multi-pile foundations designed for the swift erection of tall structures in poor ground conditions. Often at present, piles which are Y-shaped in plan are vibrated into loose railway embankment material, for example, and linked with a bolted steel grillage to support gantries.

'In many situations, where the ground was reasonably competent and the structures not too massive, a single pile could accommodate the overturning moments, ' says Dunn.

'This was provided that the width of the pile gave sufficient lateral area in contact with the ground.' Screwfast responded by adapting its screwed steel pile foundations, encasing the head of the pile in concrete, and later installing 'angel wings' over the shaft at the head of the pile.

But for small structures such as lighting poles, road and rail signs and fence supports, the cost was too great.

Dunn says that inspiration for the Star Pile came at B&Q.

'The steel stakes are easily driven into the ground so that they offer a wide face near to the surface yet at the same time give a reasonable depth and point of rotation. Their advantage over a driven pointed wooden post is the speed and ease of driving, ' he explains.

'But what they do not do is consolidate the ground around the post.' Redesigning the pile, Screwfast scaled it up to 1.2m long and added triangular plates at the top of the cruciform - they add volume to the pile. As the upper part of the pile enters the ground it therefore displaces and compacts material.

'The plates enhance the lateral strength of the soil in the same way that a driven pointed stake does, ' Dunn explains.

Trials have confirmed the theory.

Research relating lateral and vertical capacity to the power of the driving hammer and amount of penetration is the next task, says Dunn.

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