Continuous flight auger piles
Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics. Very broadly, foundations can be categorised as shallow foundations or deep foundations.
Pile foundations are deep foundations. They are formed by long, slender, columnar elements typically made from steel,reinforced concrete or sometimes timber. A foundation is described as piled when its depth is more than three times its breadth.
Continuous flight auger (CFA) is a cast in-situ method of piling that was first used in the UK in the 1960s and is now one of the most common. Due to its low level of vibration, the CFA process is particularly suited to environmentally sensitive sites and soft and/or water-bearing strata where deep casings would otherwise be necessary. They can be constructed as single piles or installed as part of a pile group similar to driven pile foundations, typically for bridge construction or large structural foundations.
The construction process begins with a hollow stemmed auger being screwed into the ground by the piling rig with minimal vibration and using a constant penetration rate. Upon reaching the design depth, concrete is pumped through the hollow stem of the auger whilst it is slowly extracted. During the controlled extraction, the auger is rotated so as to remove the ground material.
Pressure and volume is controlled by instrumentation in the piling rig so as to ensure positive pressure in the concrete being pumped into the ground is maintained throughout the placement as this prevents the hole from collapse. Extracted material brought to the surface is removed and the shaft is left full of concrete into which steel reinforcement can be placed.
The range of pile diameters can be from 450-1,200 mm and depths are usually in the range of 15-30 m depending on the ground conditions. Load capacities of 7,500 kN can be achieved.
Sophisticated CFA piling rigs will be equipped with a computer monitor that displays the boring and concreting parameters, speed of rotation and penetration rate during the drilling phase. This data can later be analysed and used as a further check on the integrity of the pile.
There are several advantages to using the CFA piling method. These include:
- Very low levels of sound and vibration.
- Compared to conventional bored piles they can be installed very quickly and economically.
- They have high load-bearing, shear and moment capacities.
- They are suitable for a wide range of challenging ground conditions.
- Piling rigs can be adapted to operate in conditions with low headroom and confined space.
 Related articles on Designing Buildings Wiki
- Bored piles.
- Driven piles.
- Geothermal pile foundations.
- Ground anchor.
- Pad foundation.
- Pile cap.
- Pile foundations.
- Piling equipment.
- Raft foundation.
- Retaining walls.
- Screw piles.
- Sheet piles.
 External references
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