The CFA process is virtually vibration free and one of the quietest forms of piling, making it ideal for environmentally sensitive areas. This method enables piles to be formed in water-bearing strata without the need for casing or bentonite.
- Very low levels of sound and vibration – practically vibration-free operation
- Installed very quickly and economically (compared to conventional bored piles)
- High load-bearing, shear and moment capacities.
- Suitable for a wide range of challenging ground conditions.
- Able to operate in conditions with low headroom and confined space.
- Higher output rates indicate that piling is commercially viable.
- A broad range of auger dimensions (300mm – 1200mm diameter) ensures the most efficient use of construction materials.
- Depth up to 30 meters implies that Continuous Flight Auger piling is efficient for low to mid-range loading and is therefore ideal for most business and residential projects.
When to use CFA Piling
CFA piles are easy to mount and provide an effective solution for lightly loaded structures.
Continuous CFA piles are ideal for most building projects and for a wide variety of soil conditions ranging from medium dense sands, gravel to rigid clays to even poor-quality rocks, however this technique is not suggested for very soft clays or silts or very loose sand or gravel.
CFA piling method provides the perfect solution for urban developments because it prevents vibration and disruptions in adjacent structures and decreases noise pollution.
CFA rigs enable contractors to offer the safest and most cost-effective solution for clients with the added benefit of on-board technology to track the quality and efficiency of the work in real-time. Jean Lutz and NDT are some examples of the processers currently on the market.
Bored pile walls can be constructed using the CFA piling technique. They can be designed to function as a simple or a propped cantilever and can also support considerable vertical loadings. The three main types of bored pile walls which are commonly constructed are: Contiguous, Interlocking and Secant. The selection of a particular type of bored pile will depend on the soil and loading conditions.
What is a Kelly bar?
Kelly bars are key components in the execution of boreholes by hydraulic rotary drilling rigs. Telescopic Kelly bars transfer torque and crowd force from the rotary drive to the drilling tool.
The Kelly is the polygonal tubing and the kelly bushing is the mechanical device that turns the kelly when rotated by the rotary table. Together they are referred to as a kelly drive. The upper end of the kelly is screwed into the swivel, using a left-hand thread to preclude loosening from the right-hand torque applied below.
The two types of Kelly Bar are..
- Friction – defined by their frictional contact between rail of each element.
- Interlocking – provided with drive ribs which are welded with lock devise in order to transfer the maximum torque to each scope.
A standard kelly is approx. 3m (10’) longer than the drill pipe segments thus leaving a portion of newly drilled hole open below the bit after a new length of pipe has been added (“making a connection”) and the drill string has been lowered until the kelly bushing engages again in the rotary head. A rotary bushing fits around the flat sides to provide the torque needed to turn the kelly and the drill string. Rollers in the bushing permit the kelly free movement vertically while rotating. Since kelly threads would be difficult to replace, normally the lower end of the kelly has saver sub — or a short piece of pipe — that can be refurbished more cheaply than the entire kelly.
Usually, a ball valve called the lower kelly cock is positioned between the kelly and the kelly saver sub. This valve is used for well control if the surface pressure becomes too high for the rotary hose or surface conditions.
The kelly hose is the flexible, high-pressure hose connected from the standpipe to a gooseneck pipe on a swivel above the kelly and allows the free vertical movement of the kelly while facilitating the flow of the drilling fluid down the drill string. It generally is of steel-reinforced rubber construction but also assemblies of Chiksan steel pipe and swivels are used.
How CFA Piling works
CFA piles are formed by drilling to the required depth using a hollow stem continuous flight auger. After reaching the desired depth, a high slump concrete is then pumped through the hollow stem. While the concrete is being pumped the auger is withdrawn at a controlled rate, removing the soil and forming a shaft of fluid concrete extending to ground level. A reinforcing cage is then inserted into the fluid concrete. Reinforcing cages with lengths up to 12 meters are common; greater lengths can be installed with the assistance of cage vibrators.
This technique does not need external ground support, such as encasing or drilling fluids, since the bore itself is self-supporting. The auger is wheeled to the ground and the concrete holds the bore after excavation.