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Fin tuning

'It's a bit of a camel,' says Kvaerner chief engineer Ken Fleming, referring to his company's latest development. He chuckles as he waits to deliver the punch line. 'It was designed by a committee.'

Such comments would normally represent a damning indictment on a new product. But Fleming is extremely proud of his camel. In fact it has just received a Millennium Product award for innovation from the Design Council.

'Innovations in piling usually come from tweaking the existing technology,' he explains. 'But for this one we went right back to the drawing board. We asked 'what do we want?' and came up with a completely new system for piling.' The company claims it is the first radically different piling technique to be developed in the last 25 years.

Fleming heads a Kvaerner Cementation Foundations research and development team that set out 14 months ago to design an environmentally friendly piling system. Conventionally driven piles are no longer practical in urban areas because of their high noise and vibration levels. Bored piles, however, produce large quantities of displaced soil which takes time and money to get off site, particularly if the material is contaminated.

Fleming's goal was to come up with a quiet, vibration-free system for driving enlarged base piles that did not produce large quantities of surplus soil. 'Increasingly, engineers need to be efficient with materials,' says Fleming. 'They need to minimise the amount of material that needs to be carted away from site and also the quantities of energy-intensive materials such as concrete used on site.'

First, the team had to define what it wanted. 'We were looking to produce piles that could collect the most end bearing possible,' explains Fleming. 'At the same time we wanted a thin shaft to keep concrete costs down.'

The resulting pile looks fairly standard. High end bearing on the ground bearing strata is achieved with the enlarged conical base, a smaller diameter shaft stressed to near its design limits ensures maximum efficiency of concrete use. However, installing the piles is very different.

A hollow cylindrical steel shaft sealed at the lower end by a one-way valve and fitted with triangular steel fins is pressed into the ground by a hydraulic ram. There are no vibrations.

Displaced soil is compacted in front and around the shaft. Once it reaches a suitably resistant stratum the shaft is rotated. The triangular fins either side of its leading edge carve out a conical base cavity.

At the same time concrete is pumped down the centre of the shaft and out through the one-way valve. The rotation of the fins is calculated so that as soil is pushed away from the pile base it is simultaneously replaced by in-flowing concrete.

Rates of push, rotation and concrete injection are all controlled by an onboard computer. Torque on the shaft is also measured by the computer. When torque levels reach a constant low value the base is formed.

To minimise withdrawal forces in the soft soils the pile is designed for, rotation is stopped with the fins in exactly the same location as they started. This means that as the tool is withdrawn to form the shaft, the fins pass through soil already displaced.

An enlarged pile head may be produced by the same process at ground level. Kvaerner claims the system can install a typical pile in 12 minutes.

Around 50,000 to 70,000 of development later, the system is now being used in anger. The company has been monitoring piles installed using the system and are pleased with the results. A typical 6m long pile with an 800mm diameter base and 350mm shaft founded on moderately dense gravel beneath soft overlying soils can achieve an ultimate capacity of over 200t.

'This system represents a threat to certain parts of the market,' claims deputy managing director Robin Wood. 'The piles are suitable for embankments, hard standing supports and floor slabs, particularly in estuarine conditions where you have a soft silty layer over a gravel strata.' RT

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