Geo-Technical Contracting & Engineering

Continuous Flight Auger (CFA)

This piling system is suited to saturated ground conditions where collapse and groundwater are a problem. There is little noise with no vibration and high production levels are possible. The piles are installed by means of a hollow stem, continuous flight auger, which is suspended from the boom of a crane, and guided by rails attached to a pile leader, is drilled into the ground by means of a powerful hydraulic motor mounted on top of the auger.

When the desired founding level is reached in soil or rock, cement grout is pumped into the hollow stem of the auger, and is pressurised, such as to open a flap valve at the bottom of the auger. As there are no voids around the bottom of the auger, no remoulded soil or rock chips can fall, nor remain beneath the grout. The spoil rising up the auger ensures that soft material cannot fall into the void at the bottom of the rock socket. Any small chips of rock which are not picked up the rolling edge of the auger becomes thoroughly mixed with the grout, when it enters the void under pressure, with considerable velocity. This also flushes the base of the pile ensuring a clean contact with the bedrock.

The auger is then slowly withdrawn from the ground, bringing the soil held between its flights with it. While grout is simultaneously filling the shaft. Soil cannot fall from the sides of the grout-filled shaft due to the stabilising pressure of the liquid grout.

Once the auger has been removed from the grout-filled pile shaft, a prepared steel reinforcement cage, fitted with cover blocks, is immediately lowered into the still-liquid grout. Once the grout has hardened, the portion above the soffit of the pile cap is removed, to expose the reinforcement, which is then lapped with the pile cap reinforcement.
Pile sizes range from 200 mm up to 1 200 mm diameter and depths of 22 m are obtainable.

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