Continuous flight auger (CFA) Piles are also known as auger cast piles, augered cast-in-place piles (ACIP), auger-pressure grout piles, drilled displacement piles and screw piles. It provides an economic, quick, low noise and vibration solution for constructing piles for medium to highly loaded structures.

The continuous flight augur pile construction system eliminates the need to employ support fluid or casings in cohesive soils, non-cohesive or water bearing soils. The process can be used for environmentally sensitive sites and/or working near existing structures.

It is a type of drilled foundation in which the pile is drilled to its final depth in one continuous process using a continuous flight auger. While the auger is drilled into the ground, the flights of the auger are filled with soil, which provides lateral support and maintains the stability of the hole.

At the same time the augers are withdrawn from the hole, concrete or grout is placed by pumping the concrete/grout mix through the hollow center of the auger pipe to the base. The grout/concrete mix is placed as the auger is removed, and the hole is never left open or unsupported. Reinforcement is then placed into the fluid concrete/grout filled hole to complete the pile.

Construction Procedure

  1. The digging head of the auger is fitted with an expandable cap.
  2. The auger is screwed into the ground to the required depth.
  3. Once the pile toe level is achieved, concrete is pumped through the hollow stem to then fill the cavity as the auger is extracted. Simultaneous pumping of concrete and withdrawing of the auger provides continuous support of the hole.
  4. At the completion of the concreting, the steel reinforcement cage is installed into the fluid concrete immediately after withdrawal of the auger. Continuous flight auger piles eliminate splices and cutoffs.
Continuous Flight Auger
Fig. 1: Continuous Flight Auger
Continuous Flight Auger Pile Construction Procedure
Fig. 2: Continuous Flight Auger Pile Construction Procedure
Hole at Base of Auger for Concrete
Fig.3: Hole at Base of Auger for Concrete
Grout at Surface after Auger Withdrawal
Fig. 4: Grout at Surface after Auger Withdrawal
Finishing Pile and Reinforcement Placement
Fig. 5: Finishing Pile and Reinforcement Placement

Insertion of reinforcement in the bore filled with concrete makes it different from the cast in situ piles. Reinforcement cages with lengths up to 12 meters are common, greater lengths can be installed with the assistance of cage vibrators.

Pile Size and Depth

Diameter:  300mm to 1500mm

Depth ranges from 18 meters to 32 meters based on ground conditions.

Advantages

  1. The construction of piles is without significant vibration or excessive noise being produced.
  2. The installation of CFA piles is quick compared to other piles.
  3. This pile does not require temporary casing in unstable soils.
  4. Since rigs used are very powerful, it saves lot of time of foundation construction.
  5. When used in large scale, the process is economical.
  6. They are suitable for a wide range of challenging ground conditions.
  7. Piling rigs can be adapted to operate in conditions with low headroom and confined space.  

Disadvantages

  1. Cost of construction for low scale construction may not be economical.
  2. Soil mining can cause settlement and damage to adjacent structures if the proper auger rotation and penetration rate are not maintained.
  3. The process of this pile installation generates spoils.
  4. Limited to weak rock and soil; not recommended for alternating hard and soft layers.
  5. Requires a specialty subcontractor (critical).
  6. Extensive testing and monitoring are required to ensure quality control and structural integrity of piles such as monitoring during drilling and grouting including auger rotation speed and penetration rate, torque, applied grout pressure and volume factor, load testing, and integrity testing.

Applications

Continuous flight auger piles are used for places with unstable soil conditions and high level of ground water table. These piles can be terminated in clays, granular soils, in soft rock or can be taken to hard load bearing strata i.e. bearing type of piles or friction piles due to their continuous operation.

CFA piles are used for multi-story buildings, or the places where high noise of pile installation is not allowed, as CFA piles produce less vibration and noise during installation.

The continuous flight auger pile is viable option for projects where speed of installation is important, batter piles is required, large numbers of piles are required, low headroom conditions, tangent pile walls up to 10 m of exposed wall height, and pile-supported embankments.

Diagram of Pile-Supported Embankment
Fig. 6: Diagram of Pile-Supported Embankment
Secant CFA Pile Wall for a Light Rail System
Fig. 7: Secant CFA Pile Wall for a Light Rail System