CFA piling is created 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 the temporary casing or bentonite slurry. Upon reaching the required depth, sand-cement grout or concrete is pumped down the auger flight filling the pile, whilst the auger is steadily and slowly withdrawn. Once filled reinforcement is immediately placed into the pile.
The bearing capacity and settlement behaviour of CFA piles is to a large extent influenced by the equipment used and the experience of the operator. The significance of the two aspects is often underestimated or overlooked at the design stage, but does in fact play an important role within the performance of the CFA pile.
Open bore piling
This piling method is ideal for use in cohesive soils. The rotating auger drives into the strata reaming out the soil as it goes. Once the required or desired depth has been met the steel cage is dropped in prior to being filled with concrete.
These highly mobile rigs are economic to use on large open sites with cohesive ground, and on chalk or sand, which have good access. Smaller models are suitable to internal use. As a rotary auger causes minimal vibration it is ideal for use next to buildings or underground services. Tremie techniques can be used if water is present.
Contiguous and secant piling
Contiguous walls are commonly and mainly used for holding back soil so an area can be excavated. They are constructed with small spaces between adjacent piles where the size of this space is determined by the nature and condition of the soil itself. Secant pile walls are an innovative way to build retaining walls. They are formed by a series of interlocking drilled shafts and are used primarily where there is a high water table or unsuitable ground conditions. Secant pile walls are attractive near buildings, roads and other sensitive structures because of the lack of noise and vibration during the construction. Secant walls can be used to form a continuous watertight wall – an added benefit on environmental projects.
Secant piling is used to build concrete retaining walls often utilised for basement construction and flood defences. There are various difference methods including hard/soft secant piling and hard/firm secant piling. With eco foundations, we generally only install hard secant walls as this method is the only truly waterproof system available in today marketplace.
Driven cased piles
There are various clear advantages to driven cased piles. Firstly driven cased piles are simple and economical to drive with an internal drop hammer, there is less noise pollution and thin light, manageable casings can be used. They provide flexibility as piles can easily be cut or lengthened as casings can be supplied in any length and in a wide range of diameters and thickness. Casings are light but are rigid enough to be transported and handled without the precautions needed with pre cast concrete piles.
Driven cased piles are commonly used on sites where soft soils overlay more competent strata, permanently steel cased piles are driven with an internal drop hammer to achieve a solid set depth. Suitable reinforcement is then added and the casing filled with concrete.