Every construction project faces its own unique challenges in terms of soil types, environmental issues, access and surrounding buildings, and these challenges can make groundworks and foundations very complex. One of the most widely used methods of constructing piled foundations is CFA piling. In this article, we take a look at exactly what is involved with CFA piling, why it might be used, and its advantages and disadvantages, to provide a full overview of this cost-effective and reliable technique.
What is CFA Piling?
CFA piling is also known as continuous flight augering, or auger cast piling, and it is used to create deep piled foundations. CFA piling has been widely used in the UK since the 1960s, but has only been adopted in the US and other regions relatively recently.
In CFA piling, the auger used to drill the foundation hole has a hollow stem. Once the required depth has been reached, an end cap on that hollow stem is blown off under pressure, and concrete is poured into the hole through the stem, at the same time as the auger is withdrawn. Sophisticated monitoring equipment in the CFA rig measures the rate of concrete flow and the speed of the auger withdrawal, to ensure the integrity of the concrete and to prevent collapse of the hole. CFA piled foundations can be up to 30 metres in depth, and can have a diameter from 450mm up to 1200mm.
Once the concrete has been poured into the hole and the auger removed, any remaining spoil is cleared from the head of the pile, and then a prefabricated steel reinforcement frame is pushed down into the concrete, using a specially-designed vibrator tool. These reinforcement frames are typically 6 metres long, but they can be linked together for greater depths, if the foundation design requires this.
Common Uses for CFA Piling
CFA piling is widely used in the UK, for both residential and commercial construction projects, for energy plant sites and for infrastructure projects such as bridges.
CFA piles can also be used in the construction of a bored pile wall, which may be a temporary or a permanent structure. Bored pile walls can be used as a cantilever or to support vertical loads. The three main types of bored pile walls are secant, interlocking and contiguous. A contiguous pile wall involves piles that are spaced apart at regular intervals. Interlocking bored pile walls start out the same as contiguous piled walls, but the gaps between the piles are filled with a weak concrete that is not reinforced. Secant bored pile walls involve interlocking piles, which are all hard piles, rather than alternating hard and soft piles. In order for the secant piles to interlock, the secondary pile is cut using specialist auger cutters, so that the primary pile can then be positioned correctly.
Why Use CFA Piling?
CFA piling offers a fast and cost-effective means of producing deep foundations. It is suitable for a wide variety of soil types, including clay, silt, sand, chalk and gravel. It is also effective for locations where there is a very high water table, or where conditions vary across the site. Since it involves very little noise or vibration, it is ideal for sites that are particularly environmentally sensitive.
Advantages and Disadvantages of Continuous Flight Auger Piling
There are a number of advantages to the CFA piling method, which make it very attractive across a broad range of construction projects. CFA piling is almost free of vibration and produces very little noise, which means it is ideal for sites that are sandwiched between existing buildings, such as city-centre towers, for example. Specially-adapted CFA rigs can also be used where there is reduced headroom available, and these are able to work with as little as 8m of cleared headroom.
CFA piling is also a very cost-effective method of producing piled foundations. Since the diameter of the piled hole can be varied between 450mm and 1200mm, it allows for precise quantities of construction materials to be used, depending on the design specification and loading requirements. CFA piling is also a fast method of production, since the same rig is used for drilling the piled hole and filling it.
Another advantage for this type of piling is that it does not require any form of temporary casing of the piled hole prior to filling with concrete, since the auger remains in the hole as the concrete is pumped in under pressure.
Whilst these factors may suggest that CFA piling is the answer to every piled foundation issue, the technique does have some drawbacks. Firstly, the costs involved in CFA piling may mean that it is not economically viable for smaller construction projects. CFA piling is a specialist technique which requires careful operation and rigorous monitoring to prevent damage to neighbouring structures and guarantee the structural integrity of each pile.
The Differences Between CFA Piling and SFA Piling
Where CFA piling is not suitable, for example where it would not be cost-effective due to the smaller size of the project, or where access or headroom would not permit a CFA rig to operate, an alternative method is often used, and this is called SFA piling. SFA stands for Sectional Flight Auger, and this method of pile construction differs from CFA piling in that sectional lengths of hollow stem auger are used, instead of one continuous flight auger. As one section of the SFA auger enters the ground, another is attached to the top, thereby reducing the amount of cleared headroom that is required. Once the required depth is reached, the auger is withdrawn and concrete poured into the hole under pressure in the same way as for CFA piles. SFA piles are sometimes referred to as ‘mini piles’. The rigs used for SFA piling are incredibly compact, making them suitable for even the most restricted of sites. Again, they have minimal vibration levels and produce very little noise, making them suitable for sites that are surrounded by other buildings.
As outlined in this article, CFA piling offers a reliable, cost-effective solution for producing deep foundations in complex and challenging environments, especially in built-up areas with limited access and noise restrictions. These advantages make it a preferred choice on many construction projects across the UK and overseas.