CFA (Continuous Flight Auger) piles are often used for deep foundations or for discontinuous retaining walls.
Piles are defined as structural elements placed in the ground to transfer loads and reduce deformation. Their aspect ratio is unlimited. Pile shafts can be straight, telescopic, wide and with a wide base or prefabricated.
Piles are either isolated or in groups. They can also form a retaining wall in the form of either a mixed curtain wall, adjoining piles, secant piles or a composite curtain wall, such as a Berlin-type wall or similar. Piles are also used as stanchions to be integrated into the structure of a building with basement levels.
These columns are put in place before any earthworks and can be used to bear part of the superstructure load when it is built in parallel with construction of the basement levels. They can be angled according to specifications indicated in the construction standards.
The main types of piles include:
A good production performance in granular soil
Mobilisation of smaller teams
Less vibration disturbance
Continuous Flight Auger (CFA) piles are constructed using a full-length segmental flight auger with hollow stem (or a combination of augers and an extended Kelly bar for deeper piles).
The excavation is stabilised by the Continuous Flight Auger and the spoil generated during the boring process, eliminating the need for temporary casings or stabilising drilling fluid.
Concrete is injected at the base of the excavation via the hollow stem and fills the excavation once the auger is lifted out. The concrete pressure must be sufficiently high and the auger must remain embedded in the freshly cast concrete to avoid any risk of soil collapse which could harm the quality of the pile. Unlike the LDA process, in which cages are suspended in the pile hole, the reinforcement is immersed in the fresh concrete at the end of the pile construction process. Carefully managed concrete rheology is vital, especially in the longest reinforcement cages.
The diameters of bored piles commonly range from 600mm to 1.50m.
This technique makes it possible to reach common depths of around 35m, due to the limited size of Kelly bars. Soletanche Bachy is also equipped with a Fundex F5500 drill rig, allowing it to build CFA piles up to 52m deep.
Soletanche Bachy’s Starsol® (and RodioStar®) piles are a patented development of the traditional CFA technique in relation to the concreting process and the ability to drive into rock with resistances of up to 50 MPa.
Starsol® piles are CFA piles which are concreted using a hydraulically activated dip tube placed inside the stem of the hollow auger. At the end of the excavation, the tube is extended by activating hydraulic cylinders.
Starsol® piles are recognized by a number of standards as providing superior performance, allowing improved design parameters when calculating pile capacity.
End of October 2020, Soletanche Bachy Fondations Spéciales set a new record by the construction of a Starsol® pile, with full-height reinforcement, at a depth of 50m using its Fundex F-5000 rig.