Continuous Flight Auger (CFA) Piles
CFA piles should be considered where speed of installation is important, a large number of piles are required, and access is unrestricted. Piles are most commonly installed in groups , similar to driven piles, and tied into a pile cap. Each pile is typically 12 to 18 inches in diameter and capable of carrying loads ranging from 10 to 150 tons. Depth of installation can be up to 100 ft.
Piles are constructed by rotating a hollow stem auger into the ground in a single continuous motion. As the auger advances to design depth, the flights on the auger are filled with soil which provides lateral support and maintains hole stability. Once design depth is reached, pumping of high strength concrete through the hollow stem of the auger begins immediately. The auger is initially lifted a short distance to allow the hollow stem cap to be expelled by the pumping pressure and continuous grout flow to begin. The auger is then lowered again to design depth and controlled pumping continues as the auger is simultaneously slowly withdrawn. Since the process is continuous throughout auger advance and withdrawal, providing the required wall stability for the full length of hole, no casing is necessary. If required, reinforcing steel can be added while the grout is still fluid.
- New building foundations
- Retaining walls
- Tank foundations
- Sewer foundations
Advantages and limitations
- Speed of installation
- Minimal noise and vibration
- Very economic where large numbers of piles are required
- Casing not required
- Collection and disposal of soils can be an issue on contaminated sites or where there is limited handling room.
- Soils containing boulders, underground obstructions, or highly variable ground conditions can be problematic.
Drilled Displacement (DDP) Piles
Drilled displacement (DD) piles offer similar installation, scheduling and economic advantages to CFA piles. However, since CFA piling is a replacement technique, spoil is generated. The drilled displacement method, as the name implies, pushes the soil laterally rather than raising it to the surface therefore spoil generation is limited and disposal is not a major consideration. This can be advantageous where contaminated soils are present. Lateral displacement also has the effect of providing a degree of ground improvement around the piles which can improve bearing capacity. Drilled displacement piles are most effective in sandy soils where ground improvement can be realized, where limiting spoil removal is desirable, or where vibratory pile driving techniques are precluded.
If you have any questions, or would like to discuss partnering with Moretrench on your next deep foundation project, please fill out our contact form, and we will be happy to get back to you as soon as possible.