Continuous flight auger piles
Contents |
[edit] Introduction
Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics. Very broadly, foundations can be categorised as shallow foundations or deep foundations.
Pile foundations are deep foundations. They are formed by long, slender, columnar elements typically made from steel, reinforced concrete or sometimes timber. A foundation is described as piled when its depth is more than three times its breadth.
Continuous flight auger (CFA) is a cast in-situ method of piling that was first used in the UK in the 1960s and is now one of the most common. Due to its low level of vibration, the CFA process is particularly suited to environmentally sensitive sites and soft and/or water-bearing strata where deep casings would otherwise be necessary. They can be constructed as single piles or installed as part of a pile group similar to driven pile foundations, typically for bridge construction or large structural foundations.
[edit] Installation
The construction process begins with a hollow stemmed auger being screwed into the ground by the piling rig with minimal vibration and using a constant penetration rate. Upon reaching the design depth, concrete is pumped through the hollow stem of the auger whilst it is slowly extracted. During the controlled extraction, the auger is rotated so as to remove the ground material.
Pressure and volume is controlled by instrumentation in the piling rig so as to ensure positive pressure in the concrete being pumped into the ground is maintained throughout the placement as this prevents the hole from collapse. Extracted material brought to the surface is removed and the shaft is left full of concrete into which steel reinforcement can be placed.
The range of pile diameters can be from 450-1,200 mm and depths are usually in the range of 15-30 m depending on the ground conditions. Load capacities of 7,500 kN can be achieved.
Sophisticated CFA piling rigs will be equipped with a computer monitor that displays the boring and concreting parameters, speed of rotation and penetration rate during the drilling phase. This data can later be analysed and used as a further check on the integrity of the pile.
[edit] Advantages
There are several advantages to using the CFA piling method. These include:
- Very low levels of sound and vibration.
- Compared to conventional bored piles they can be installed very quickly and economically.
- They have high load-bearing, shear and moment capacities.
- They are suitable for a wide range of challenging ground conditions.
- Piling rigs can be adapted to operate in conditions with low headroom and confined space.
[edit] Related articles on Designing Buildings
- Bored piles.
- Driven piles.
- Footings.
- Foundations.
- Geothermal pile foundations.
- Ground anchor.
- Groundworks.
- Micropiles.
- Pad foundation.
- Pile cap.
- Pile foundations.
- Piling equipment.
- Piling mat.
- Raft foundation.
- Retaining walls.
- Screw piles.
- Sheet piles.
- Types of pile foundation.
[edit] External references
- Balfour Beatty - CFA piling
- Colets Piling
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Continuous Flight Auger (CFA) piles, also known as auger cast piles or augered piles, are a type of deep foundation system used in construction to provide support for structures. CFA piles are formed by drilling a continuous flight auger into the ground and then injecting concrete through the hollow center of the auger as it is slowly withdrawn. Here are some key features and benefits of CFA piles:
1. Construction Process: The CFA pile construction process involves the following steps:
a. A hollow-stem continuous flight auger is drilled into the ground to the desired depth using a rotary drilling rig.
b. During the drilling process, the auger removes the soil, creating a borehole.
c. Once the desired depth is reached, the auger is partially withdrawn while concrete is pumped through the auger's hollow stem, filling the void left by the auger's removal.
d. As concrete is pumped, the auger is gradually extracted, and the concrete displaces the soil, forming a pile shaft.
e. Reinforcement, such as steel rebar, may be inserted into the wet concrete to enhance pile strength.
f. The top of the pile is typically finished with a concrete pile cap or a connection to the structure being supported.
2. Advantages of CFA Piles:
a. Efficiency: CFA piles can be installed rapidly, which helps reduce construction time and costs compared to other deep foundation methods.
b. Noise and Vibration: The construction process of CFA piles generates less noise and vibration compared to other methods like driven piles, making them suitable for urban and sensitive environments.
c. Ground Displacement: CFA piles create minimal ground displacement during installation, minimizing potential disruptions to surrounding structures and minimizing the risk of settlement.
d. Load Capacity: CFA piles have high load-carrying capacity and can support heavy structural loads.
e. Installation in Various Soils: CFA piles can be installed in a variety of soil conditions, including cohesive soils, non-cohesive soils, and mixed ground conditions.
f. Self-Supporting during Installation: The auger acts as a temporary casing during the drilling process, providing stability to the borehole walls and preventing cave-ins.
3. Limitations and Considerations:
a. Soil Conditions: CFA piles may not be suitable for highly dense or hard soils, as the auger may encounter difficulties during drilling.
b. Pile Length: The length of CFA piles may be limited depending on soil conditions, reinforcement requirements, and equipment capabilities.
c. Quality Control: Proper quality control measures are crucial during CFA pile installation to ensure the integrity and strength of the piles. Monitoring of drilling parameters, concrete consistency, and reinforcement placement is essential.
CFA piles are commonly used in a variety of construction projects, including high-rise buildings, bridges, industrial structures, and infrastructure development. They offer a cost-effective and efficient deep foundation solution with minimal disruption to the surrounding environment.