Screw piles have been around a long time.
First used on wooden port lighthouses in Georgian and early Victorian times and later for the famous West Pier in Brighton, the screw pile technique languished for many years until it was taken up in the mid-West US early last century.
'Albert Bishop Chance began using them, mostly as tension anchors, to support power line pylons weighed down with ice in the plains winter, ' says Rich Zinser from the modern-day AB Chance company. But research in the 1970s and 1980s developed their use as small piles 'with literally hundreds of applications', he says. They are particularly useful for small building extensions and underpinning and for applications where spoil removal would be a problem.
Currently, contractors and suppliers report a surge in use with growth up to 50% annually - as telephone transmission masts are built in ever larger numbers, and other uses expand too, such as railway signalling gantries for line upgrades.
'They are also good for contaminated ground where you might not want to disturb the horizon, ' says Stephen Reid, business development manager at contractor Van Elle.
Van Elle has specialised in the technique, using both the Chance system and a larger cylindrical system developed by specialist contractor Screwfast. This won a recent Queen's Award for Enterprise.
'They are also good for temporary buildings because they can be removed afterwards with almost no ground disturbance, ' says Peter Dunn at Screwfast. 'And premanufactured housing systems fit well with the piles.' The piles are made of almost circular plates welded to a central steel spine at wide spacings - usually about three diameters of the plate - and sustain tension and compression loads essentially by ground bearing pressure. Loads are proportionate to the area of the plate and the type of ground; very large plates are used in weak ground and tiny ones in soft rock.
The piles are screw piles because the plates are in fact a single 360 0 part of a shallow helical screw thread, which allows them to be rotated into the ground, the pitch on each plate following the line into the ground. A torsion motor on a normal excavator is the main tool required and it is very quick.
Typically the piles are used in applications requiring speed of installation, and where lateral load is important - from wind on towers for example, or cantilevered roadsigns.
Screwfast says they excellent for railways works as they can reduce possession times substantially.