This method generally requires piles to be bored in soft to hard clays or well weathered, very weak rock or well graded granular soils overlying a suitable founding strata. The system has undergone many changes since its inception, most important of these is the use of concrete in place of the grout and better purpose built equipment. Ironically although the grouted system is still in wide use in the U.S. they are now re–adopting the method with European equipment using concrete and this is through a largely British influence over there.
Because the ground is supported throughout the piling process, initially by the auger boring the pile out and then by the concrete injected through the hollow central stem as it is extracted there is no need to use casings or drilling fluids to support the excavation. This makes it suitable for use in deep unstable and water bearing strata and this combined with the negligible level of vibration and the low level of noise has contributed to the growth of this piling method to be one of the most common in use in the UK. On the down side the CFA system like driven piles does not allow inspection of the strata although this is compensated for to some extent on larger piles using instrumentation. Also obstructions are not easily overcome despite the great increases that have been made in power and weight of rigs which now allow piles up to a 1200mm in diameter to be constructed down to 25m.
It is recommended that where obstructions are suspected or are thought to be a risk they should be investigated or pre-dug and replaced with a suitably graded material. A variation of this system is the continuous helical displacement pile which displaces the soil using a specially designed auger. This is not a system we are currently able to offer, largely because it requires a high capacity pull down and torque usually to return only a relatively low capacity of pile and is only applicable to certain soils.
CASE STUDY ONE
This site in the centre of Rotherham was formerly a municipal swimming baths that is now being redeveloped as a commercial property with parking and shops below with flats above. Being alongside the river Don, there was a retaining wall and maintaining its integrity was paramount. It was originally intended to pile from the higher level but because it was known that obstructions would be likely we advised the main contractor the area should be reduced to basement level. They also agreed with their design engineer that being flexible with the pile layout would also be wise as these obstructions went below the basement level and may require piles to be moved to span over them. Problems with access to and from the lower area necessitated AJH Piling getting involved with design and location of the ramp. |
