What Are And How To Install Screw Cast In Displacement Piles?

Whilst the installation of this type of pile is effected by means of a type of auger, the process involves compaction rather than removal of the soil and, in this respect; the piles are of a displacement type. In forming the pile, a heavy-duty single-start auger head with a short flight is screwed into the ground to the required depth.

The auger head is carried on a hollow stem which transmits the considerable torque and compressive forces required, and through which the reinforcement cage is inserted after completion of the installation process. The end of the hollow stem is sealed with a disposable tip.

Following placement of the reinforcement, concrete is placed through this tube from a hopper at its head. As concrete filling takes place, the auger is unscrewed and removed, leaving behind a screw-threaded cast-in-place pile.

By virtue of the combined rotation and controlled lifting applied at the extraction stage the ‘threads’ are of robust dimensions. The sequence of pile construction is shown in Figure 3.6.

This method of forming a pile is known as the Atlas Piling System, and is marketed by Cementation Foundations: Skanska Limited in the United Kingdom, in association with N.V. Franki S.A of Belgium. A purpose-designed, track-mounted rig provides hydraulic power for auger rotation and the application of downward force and is fitted with a crane boom for handling reinforcement and concrete skips.

For a given pile size and volume of concrete, pile capacities are greater than for traditionally constructed bored piles, although the restricted diameter of the reinforcement cage may be a disadvantage if the pile is required to resist high bending stresses. The system does however combine many of the advantages of a displacement pile with the low noise and vibration characteristics of a bored pile.

It will operate in most cohesive and granular strata to a maximum depth of 22 m, providing piles ranging in diameter from 360 to 560 mm. To achieve the torque of perhaps 250 to 350kNm required at the auger, power requirements are relatively high.

Whilst the pile is environmentally friendly in that it reduces the amount of potentially contaminated spoil that needs disposal, set against this there may be an increase in atmospheric pollution from the more powerful plant employed compared with a traditional auger pile machine. It has been found that when screw displacement augers are used in dense granular deposits the rate of auger wear is high and in extreme cases this can make the pile uneconomical.

The Omega pile introduces a variation at the concreting and auger removal stage. After screwing the auger into the ground and displacing the soil to the full depth of the pile, the auger is rotated in the same direction on extraction as for installation.

This then leaves a straight-shafted pile of the diameter of the outer auger diameter, but producing a small amount of spoil. Similarly, in the Fundex pile, a flighted casing is pulled vertically out of the bore at the time of concreting, leaving a straight shafted pile. In this pile, however, if required, the casing can be left in place to give protection to the concrete shaft in aggressive ground conditions.

In the patented ‘Screwsol’ piling system by Bachy Soletanche, a fin extends from the main hollow-stem auger to create a helically threaded pile that typically extends the central 350mm diameter shaft to 500 mm diameter across the threads with minimum spoil production.

Concreting of soil-displacement piles is carried out with computerized control of the rate of extraction, rotational speed and concrete feed. The installation sequence for the Cementation Foundations’ type of soil-displacement pile is shown in Figure 3.7.

In a further variation, the continuous helical displacement (CHD) pile produced by Roger Bullivant employs a short flighted ‘bullet’ head with a disposable tip which is screwed into the ground via a hollow auger stem to displace the soil laterally without the production of spoil at the surface. The pile shaft is formed by pumping concrete down the hollow stem as the bullet is back-screwed to produce a simple screw-form with the reinforcement cage added afterwards.

Especially developed for contaminated sites the system has been found to be cost-effective on other sites where the disruption caused by removal of spoil and noise and vibration is to be minimized.

Yet further development of this type of pile has led to a ‘bored displacement pile’ by Rock and Alluvium Ltd in which a short tapered auger is drilled into the ground. Being of smaller diameter, the auger stem that follows the auger reduces the friction against the soil and the auger can be more easily advanced as a result.

At full depth the auger may be rotated out or raised without rotation for a straight shafted pile. In either case, concrete is pumped into the bore via the hollow auger stem to support and form the shaft.

In all these types of pile, comprehensive rig instrumentation is essential to ensure that the pile shafts are suitably formed and where helical flanges are cast on the shaft of the pile, these need to be robust enough to resist pre-mature failure under load.

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