A novel - and environmentally friendly - pile developed by Bachy Soletanche reduces the volume of concrete and produces no spoil.
A novel soil displacement screw pile developed by geotechnical contractor Bachy Soletanche combines the flexibility and control of continuous flight auger piling with the advantage of no surface spoil.
Recently completed trials on several sites have shown the Screwsol pile to be as technically efficient - yet 10% more cost effective - as an equivalent CFA pile. To achieve the same capacity a CFA pile is typically 40% larger.
The bottom line advantages are the need for less concrete to create what is really a 500mm diameter pile with a shaft only 350mm wide, and less spoil removal, which is particularly significant on contaminated sites. Furthermore claims Bachy, depending on soil conditions, pile length can be reduced by 20%, compared to conventional CFA alternatives.
'The pile uses a standard CFA rig but works its way into the ground like a wood screw, needing no pull-down force, ' explains Bachy Soletanche design manager Nick Wharmby. 'A lower cutting fin creates a broad helical thread in the soil around the pile shaft which is then immediately filled with concrete. This increases the pile's effective width by over a third.'
Trials have been based on a 275mm diameter smooth shaft auger stem, with only the lower 1.5m cutting head equipped with auger flights. The head tapers to 150mm at its tip, with flights increasing shaft diameter to 350mm.
As the rig screws the auger head into the ground, the soil is displaced and compacted, enhancing friction capacity. The thin, angled cutting fin is attached only to the lower flight, and protruding 75mm either side, effectively increases the pile bore diameter by 150mm.
As the auger is screwed to depth, this cutting tool creates only a narrow thread in the soil around the pile string. During extraction, the auger continues to rotate clockwise but more slowly and the cutting fin opens up a 75mm wide wedge shaped thread around the pile casing, tapering to 50mm thick. This slot is immediately filled with concrete, forming an integral extension to the pile shaft.
The computerised controls of CFA rigs - monitoring precise rotation, extraction rates and concrete feed - allow the Screwsol pile to be constructed with different sized and pitched threads, depending on ground conditions. The continued clockwise rotation of the auger ensures no soil contamination of pile concrete, and Wharmby claims similar production rates and accuracy control as with conventional CFA piles.
He maintains that Screwsol piles can be used in wide ranging ground conditions, with granular material benefiting from the enhanced friction capacity of the displaced compacted soil. In clay soils, friction resistance is also increased, relying this time largely on a high bond, soil to soil interface between the thread.
Later this month Screwsol piles will be installed commercially for the first time on the company's BAE factory contract at Broughton, Cheshire and Bachy Soletanche is optimistic of further projects soon.