Continuous flight auger pile (CFA)

The “Continuous Flight Auger” or CFA pile is an auger bored ‘replacement’ pile with soil extraction and  pumping of concrete under pressure. The pile is formed by means of a helical screw (auger) with a hollow stem bored in to the ground to design depth. Concreting is achieved by pumping fluid concrete through the hollow stem of the auger, delivering concrete to the bottom of the pile bore. Pile arisings are brought to the surface during drilling and concreting phase.

Construction Sequence

1. Positioning of the CFA rig over proposed pile position.

2. Commence augering into ground to design depth. The continuous flight auger and soil held within the flights support the sides of the pile bore during this phase. An expendable cap or flap on the bottom of the lead-flight prevent soil from entering the hollow stem of the auger.

3. Design depth reached.

4. Concrete is pumped to the base of the pile bore through the CFA system and through the hollow stem of the flights. As the auger is extracted, the pumped concrete maintains stability of the pile bore. No voids are created during this process. Computerised rig telemetry  is used in both drilling phase and concreting phase to assist the rig operator in forming the pile. 

5. Once concrete is placed and arisings removed from the pile position, the pile cage is plunged lowered into the wet concrete. 

Computerised rig telemetry provides continuous data during the drilling phase and concreting phase in order to maintain a level of quality and workmanship.

Fields of application

  • Able to construct piles into highly stratified layers of soil comprising very resistant layers
  • As a single pile or as an element of a piled wall (contiguous and  secant piled walls)  
  • As an alternative for large diameter bored piles
  • Infrastructure and industrial projects

Environmental impact

  • In vibration sensitive areas: no vibration is induced as the pile is formed by a hydraulic boring technique as opposed to a percussive element. 
  • In a noise-sensitive environment: low noise level generated compared to other methods of piling.
  • The concrete used to form the pile has a high energy input to make the cement, but this can be off-set by the use of ground blast furnace slag as a cement replacement (which would otherwise be a waste product). 
  • Removal of soil from the job site (CFA piling is a ‘replacement’ technique and creates pile spoil which will require disposal)
  • A dry stable, and level working platform is required.

Technical Specifications

  • Possibility to screw through and anchor the pile in very resistant layers of soil (gravel or sandy layers with cone resistance qc > 15 MPa).
  • Wide range of diameters: from 300 mm up to 900 mm typically, and up to 1200 mm with specialist plant.
  • Depth: up to 25 m typically and 35 m with specialist plant.
  • Inclined piles can be installed with specialist plant.
  • Reinforcement is placed after the concreting phase which can limit the pile cage specification (with respect to depth)
  • Allowable bearing capacity up to 3500 kN (depending on ground and pile diameter)
  • High lateral load capacities enable contiguous and secant piled walls to be constructed

Related projects for this technique

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