Continuous Flight Auger (CFA) Piles are a popular foundation construction method that is also known as auger cast piles, augered cast-in-place piles (ACIP), auger-pressure grout piles, drilled displacement piles, and screw piles. This method is particularly suitable for medium to highly loaded structures and offers a quick, low-noise, and vibration-free solution for constructing piles.
The CFA pile construction system is advantageous as it eliminates the need for support fluid or casings in various soil types, including cohesive soils, non-cohesive soils, and water-bearing soils. Additionally, it is a feasible option for environmentally sensitive sites and construction projects near existing structures.
This foundation construction method involves drilling the pile to its final depth in a continuous process using a continuous flight auger. During the drilling process, the flights of the auger are filled with soil, which offers lateral support and maintains the stability of the hole.
As the augers are removed from the hole, concrete or grout is placed by pumping the concrete/grout mixture through the hollow center of the auger pipe to the base. This process ensures that the hole is never left open or unsupported. After the grout/concrete mix is placed, reinforcement is inserted into the fluid-filled hole to complete the pile.
Construction Procedure
The auger used for drilling has a digging head that has an expandable cap. It is screwed into the ground to reach the desired depth. Once the pile toe level is reached, concrete is pumped through the hollow stem of the auger to fill the cavity as the auger is pulled out. This simultaneous pumping of concrete and withdrawal of the auger ensures continuous support of the hole. After the concreting is completed, the steel reinforcement cage is immediately installed into the fluid concrete right after the auger is withdrawn. Continuous flight auger piles are advantageous as they eliminate splices and cutoffs, providing a seamless and efficient construction process.
The insertion of reinforcement into a bore filled with concrete distinguishes it from cast in situ piles. Typically, reinforcement cages with lengths of up to 12 meters are commonly used, although longer lengths can be installed with the aid of cage vibrators.
Pile Size and Depth
The diameter of the object in question ranges from 300mm to 1500mm. The depth at which the object is situated varies depending on the ground conditions and can fall within the range of 18 meters to 32 meters.
Advantages
Piles can be constructed without causing excessive noise or vibration. CFA piles are known for their quick installation compared to other types of piles. In unstable soils, temporary casing is not necessary for the installation of these piles. The use of powerful rigs allows for a significant reduction in construction time for foundations. When implemented on a large scale, the process becomes more cost-effective. These piles are well-suited to a variety of difficult ground conditions. Additionally, piling rigs can be modified to operate in conditions with limited headroom and confined spaces.
Disadvantages
Constructing low-scale buildings can often prove to be uneconomical due to the high cost of construction involved. Additionally, soil mining, if not carried out with the appropriate auger rotation and penetration rate, can lead to settlement and cause damage to nearby structures. Furthermore, the installation of piles generates spoils, which can be problematic. This method of construction is limited to weak rock and soil and is not recommended for alternating hard and soft layers. Another challenge is that this process requires a specialty subcontractor, which is a critical aspect of the project. Finally, extensive testing and monitoring are necessary to ensure the quality control and structural integrity of piles. These tests may include 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 (CFA) piles are commonly used in areas with unstable soil conditions and a high groundwater table. These piles can be used in a variety of soils, including clays, granular soils, soft rock, and hard load-bearing strata, either as bearing or friction piles, due to their continuous operation. CFA piles are ideal for constructing multi-story buildings or in areas where excessive noise and vibration from pile installation are not permitted. This is because CFA piles produce less noise and vibration during installation than other pile types.
CFA piles are a suitable option for projects where installation speed, batter piles, a large number of piles, low headroom, tangent pile walls up to 10 meters in exposed wall height, and pile-supported embankments are required. Pile-supported embankments are a common application for CFA piles. A diagram of a pile-supported embankment is shown in Figure 6.
In addition, CFA piles can be used to construct secant pile walls for various projects, including light rail systems. Figure 7 shows a diagram of a secant CFA pile wall constructed for a light rail system.
