Piled Foundations
Continuous Flight Auger (CFA)
CFA piles are constructed by drilling a continuous flight hollow stem auger into the ground to a specified depth. The fact that the hollow stem auger provides support to the excavated shaft at all times during the drilling process precludes the need for temporary casings or drilling fluids. Once the pile toe level is achieved, concrete is pumped through the hollow stem to then fill the cavity as the auger is extracted. At the completion of the concreting, the steel reinforcement cage is installed into the fluid concrete. While initially developed to cater for sites with weak and sandy materials beneath the water table, today CFA piles are a very popular foundation type due to their ease of installation, low vibration nature and depth/torque capability. Installation method is shown below.
Advantages
- Minimal levels of vibration
- Lower noise levels generated by piling rig
- Faster installation
- More suitable for tension loads at stability elements depending on ability to install cage prior to concrete drying out
- Much higher load capacities on larger diameter piles
Disadvantages
- Longer piles required than driven (not driven to a predetermined set)
- Removal and disposal of spoil material generated from the pile
- Drilling through large obstructions/boulders cannot be undertaken
- A strict quality control and thorough supervision need to be done during the pile installation
Driven Cast-In-Situ ~ Franki Piles (DCIS)
The concrete is then rammed into position by a hammer as the casing is withdrawn ensuring firm contact with the soil and the compaction of concrete. Care must be taken to see that the concrete is not over-rammed or the casing withdrawn too quickly. There is a danger that as the liner tube is withdrawn it will lift the upper portion of the in-situ concrete, thus leaving a void or necking in the upper portion of the pile. This can be avoided by good quality control of the concrete and slow withdrawal of the casing.
Advantages
- Good axial and tension load capacity
- Pile lengths are easily adjustable
- Reinforcement is not determined by the effects of handling or driving stresses
- An enlarged base can be formed which may increase the relative density of a granular founding material leading to a higher end bearing capacity
Disadvantages
- Heave of neighbouring ground surface, which could lead to re consolidation and the development of negative skin friction forces on piles
- Displacement of nearby retaining walls / building. Lifting of previously driven piles, where the penetration at the toe have been sufficient to resist upward movements
- Tensile damage to unreinforced piles or piles consisting of green concrete, where forces at the toe have been sufficient to resist upward movements
- Damage piles consisting of uncased or thinly cased green concrete due to the latent lateral forces set up in the soil, for example, necking or waisting
- Limitation in length owing to lifting forces required to withdraw casing, nose vibration and ground displacement may be a nuisance or may damage adjacent structures
- Relatively expensive
- Very noisy installation
- Drilling through large obstructions/boulders cannot be undertaken
- Load capacity is capped to the maximum size of the pile where pile caps are very often required