Since their introduction some fifty years ago in North America, continuous flight auger (CFA) piles, also known as augercast piles, have become increasingly popular, as they can be considerably cheaper than alternative pile types. With proper planning and design, efficient equipment and experienced personnel, high production rates can be achieved.
Piles are available in sizes 0.3, 0.45, 0.6, and 0.75 m diameter, and exceptionally 0.9 m diameter. They can be constructed to depths in excess of 30 meters.
CFA Equipment Tools
The equipment normally comprises of:
a base machine with a fixed, hanging or swinging lead
an auger drive unit (usually hydraulic) with sufficient power to advance the continuous flight auger to the required depth with minimum decompression of the surrounding soil
continuous flight augers of the required length and diameter (small or large stem), equipped with auger cutting heads suitable for the material to be penetrated Installation process
CFA piles are formed by drilling a continuous flight auger into the ground. The sides of the hole are supported at all times by the soil-filled auger, eliminating the need for temporary casing or betonies slurry. Upon reaching the required depth, sand-cement grout or concrete is pumped down the hollow stem as the auger is steadily withdrawn. Reinforcement is placed immediately after withdrawal of the auger.
The bearing capacity and settlement behavior of CFA piles is to a large extent influenced by the equipment used and the experience of the operator. The significance of these two aspects is often underestimate or overlooked at the design stage, but plays an important role for the performance of CFA piles.
Casting of pile base and shaft
When the auger has reached the required depth, the temporary plug (usually of wood or cork), which prevents soil from entering the hollow stem, has to be ejected. This is accomplished by slightly lifting the auger and injection of the concrete. During this phase soil decompression is almost unavoidable and it is common practice to reinsert the rotating auger while concrete is injected. This method minimizes to some degree decompression at the bottom of the pile but the quantitative effect is difficult to assess.
When the grout head is established, extraction is commenced at a rate, consistent with grout supply. Positive rotation of the auger is necessary to retain the drilling spoil and to ensure that the grout fills the entire pile cross section. The grout pressure must be sufficiently high and the auger must not be extracted too fast. Otherwise, drilling spoil can enter into the freshly placed grout, resulting in a soil-contaminated pile shaft.
Placing of reinforcement
In the case of a small stem diameter, which is most commonly used, the reinforcement must be installed after the auger has been withdrawn and while the grout or concrete is still fluid. The placement of a long reinforcement cage into an uncased hole can pose considerable practical problems and contaminate the concrete shaft with eroded soil. Therefore, it is important that the cage is stiff and properly welded In order to overcome the problem of proper placement of the reinforcement, augers with a larger stem are used and the reinforcement is installed inside before extraction of the auger.
The CFA pile is essentially a non-displacement type pile. Hence, there is limited risk of damage to adjacent foundations or underground utilities from ground displacement or densification of loose sands, as can occur with displacement piles. Another advantage is that CFA piles can be installed with little vibrations or noise. Should problems occur during pile construction, it is relatively simple to re–drill and install the pile at the same location, thereby eliminating the need to redesign the pile group or the pile caps.
An important feature of present day CFA piles is the use of a reliable flow meter to monitor and record the whole process of construction of this type of pile. The items recorded are usually penetration / uplift per revolution, auger depth, concrete supply per increment of auger uplift during placing, and injection pressure at the auger head.
The most severe disadvantage is poor and/or inconsistent quality and load carrying capacity. The advantages of the CFA pile are many times outweighed by the limitations, which are inherent to this pile type. Initially, the method of CFA pile construction was crude and did not always ensure a pile of high structural and geotechnical quality. Major advances have been made with respect to the design and execution of CFA piles. However, the most severe limitation of the CFA technique is still its sensitivity to operator performance,
which can lead to a pile of poor or inconsistent quality and reduced load carrying capacity. Thus, it is vitally important that experienced personnel install the piles. Great attention must be given to every phase of the field installation procedure, including the drilling of the hole, the casting of the shaft, the extraction of the auger and the placement of the reinforcement.
In contrast to driven piles, the drilling resistance usually does not provide a direct indication of the soil stiffness and the strength of the bearing stratum, except when a very dense soil layer or rock is encountered. In spite of its apparent simplicity, the installation process of a conventional auger pile involves the use of several pieces of equipment, which need to be operated in a specialized manner.
Soil decompression during auguring
The installation procedure of CFA piles influences the stress conditions in the ground. The magnitude of the pile friction is determined by the lateral earth pressure after completion of the pile shaft. Decompression of the surrounding soil can adversely affect the capacity of the pile, and is likely to vary with ground conditions, equipment and operator.
The term soil “decompression” refers to the reduction in soil stiffness and strength brought about by the pumping action of the continuous flight auger. Increasing the diameter of the hollow stem can to some extent reduce the risk of decompression. However, the larger the hollow stem, the greater the power needed to advance the auger. Decompression results in an increased grout volume, which can be between 15 and 60 % in excess of the theoretical pile volume. The cost of additional, unexpected concrete consumption can have severe economical consequences for a piling project.