Uxbridge-based piling contractor Westpile has just taken delivery of its new Soilmec R825 rig after final testing at the Italian manufacturer's factory at Cesena, south east of Bologna, in Northern Italy. The latest addition to the company's fleet boasts the Cased Flight Auger Pile (CFP) system, capable of installing CFA piles with temporary casing up to 1000mm diameter, which both companies claim offers a variety of advantages over rotary bored and conventional CFA techniques.
Large diameter CFA piling has become increasingly popular in the UK over the last few years, but although the technique offers many advantages over rotary bored methods, it is reputedly less accurate.
However, proponents of the technique claim advances in CFA technology, in particular in instrumentation for controlling and measuring installation, which has led to greatly improved accuracy and integrity of piles. This has meant more contractors are specifying the method, since it offers a rapid and cost-effective way of installing both large diameter individual piles and secant pile walls.
Westpile contracts director Pat Deighan says the UK market is becoming increasingly CFA dominated, with around 90% of the company's work being CFA rather than rotary. He admits there is still a need for rotary for very large diameter piles (up to 2000mm) to depths of 40m to 50m in ground that stays open. But as CFA is now able to go deeper and with larger diameter, the method is taking a larger chunk of the rotary market.
'Our Soilmec R622 hasn't done any rotary work since we got it six months ago,' he says. The rig has recently finished work on a large diameter CFA secant pile wall for the Gillingham northern relief road in Kent (GE January 1998).
Soilmec (UK) director Robin North agrees. He says that over the last few years the proportion of the market that is exclusively rotary has shrunk. He reckons that 'within two years the market will be half rotary and half CFA', predicting that in the near future the method will have the capacity to form piles up to 1500mm diameter and to depths of 35m.
The CFP system has been around for a few years but has only recently been available in the UK, with Westpile and Kvaerner Cementation Foundations now having CFP equipment. It comprises two independent rotary heads, one to drive the auger and the other - rotating in the opposite direction to reduce the torque transferred to the mast - turning the casing. Holes can be formed with the auger ahead or behind the casing depending on ground conditions. The casing shoe is fitted with cutting teeth that, added to the extra torque from the second head, can speed up production. If the pile has to go deeper than the full casing depth, the casing head can be stopped and the auger progressed to the full depth, using the extension kelly if needed.
And the casing provides stiffness and good control of drilling direction, preventing drift caused by flexing of the auger. Spoil is removed from the auger using a cleaner on the top of the casing head and can be cleared from the pile position before the casing is pulled, which greatly reduces the chances of spoil falling onto the top of the pile.
Concrete is placed, as for conventional CFA work, through the hollow stem of the auger, with the auger withdrawn slightly ahead of the casing to ensure a head of concrete that will fill the annulus left when the casing is pulled. This should be small, as the clearance between the auger and the casing is just 10mm.
Where piles are required with cut-off below ground level, the casing can be disconnected using a quick-release mechanism and left in the ground long enough to ensure that when it is removed the concrete will not slump below the specified level.
Once concrete has been placed, the auger is removed and cleaned and reinserted into the casing to rebore the top of the concrete to ensure that no spoil is left on the pile cap. It is then withdrawn again and the rig pulled off position to allow the reinforcement to be placed. The casing can be reattached and reamed out.
Deighan adds that longer reinforcement can be installed with a greater chance of success and accuracy, making it possible to form stiffer piles than conventional CFA. The casing can be left in while reinforcement is placed in the wet concrete and the auger reattached to continue concreting to completely fill the bore and the casing. In general, he says that the loading capability of CFP piles is only 'minimally' smaller than CFA and rotary, as the tops of the piles are slightly larger.
One application of the system that may prove crucial to its success is in the construction of hard/hard secant pile walls. Ensuring the verticality and integrity of the piles is essential to make sure that the structure is watertight. CFP not only allows the piles to be installed with a greater degree of accuracy and to higher tolerances, but the temporary casing gives a cleaner cut of the female piles. Deighan says this removes the need for reduced strength gain in the concrete.
The 80t, completely self erecting R825 is fitted with fully diagnostic systems that the operator can access in the cab. Torque, depth, concrete pressure and volume are all monitored to ensure that the piles are correctly installed, this data being held for future reference.
Another advance is the ability to vary the hydraulic power to the heads, allowing load sensing operation so power can be varied to go through difficult ground.
The rig is fitted with a single 16.5m length of casing. It is able to auger up to 1200mm diameter down to 30m in CFA mode and 1000mm diameter down to 25.5m using the CFP system. But the rig is multifunctional, able to carry out a variety of piling operations including rotary bored, cast insitu and diaphragm walling. This, says Deighan, means Westpile can keep it working when there are no suitable CFA contracts available.