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 bentonite slurry.
Upon reaching the required depth, concrete is pumped down the hollow stem as the auger is steadily withdrawn. Reinforcement is placed immediately after withdrawal of the auger into the fluid concrete.
The bearing capacity and settlement behavior of CFA piles is to a large extent influenced by the equipment used. For optimum pile performance the more powerful the rig the better. Over-flighting can occur when using under-powered rigs.
Casting of pile base and shaft
When the auger has reached the required depth, the temporary plug (usually of plastic or steel caps), 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. A precharged volume of concrete is pumped to the base of the pile before extraction of the auger begins.
When the concrete head is established, extraction is commenced at a rate, consistent with concrete supply. Positive rotation of the auger is necessary to retain the drilling spoil. The concrete oversupply must be maintained and the auger must be extracted at a rate consistent with concrete oversupply and ground conditions. The onboard computer gives real time information to the rig driver to assure correct construction of the pile shaft.
Placing of reinforcement
The reinforcement is installed after the auger has been withdrawn and while the concrete is still fluid. The placement of a long reinforcement cage into an uncased hole requires care so that no contamination of the concrete shaft is caused with eroded soil. Therefore, it is important that the cage is stiff and properly welded/ tied in order to overcome this problem.
Advantages
The CFA pile is essentially a non-displacement bored type pile. Hence, there is little 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 are installed with little vibration 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.
The use of onboard computers provides the piling contractor and the client with excellent information on the strength of the soil bored and the pile construction ( refer typical computer log attached ). These logs give great assurance to clients that the CFA piling works have been carried out in accordance with best current practice.
There is no need for drill casings or changing of drill tools during boring of pile. Therefore there is no manual or crane handling required or the need for vibrators to install or extract casings.
Most importantly CFA piling is very fast and therefore the most economic way to install bored piles today. Due to developments in powerful dedicated rigs and computer aided construction CFA piling has become the most popular bored piling system in the UK and Ireland and has replaced Rotary piling on most large building and civil engineering projects in this region.
