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Last edited 30 Sep 2020
Pile foundations are a type of deep foundation. They are formed by long, slender, columnar elements typically made from steel or reinforced concrete, or sometimes timber. A foundation is described as 'piled' when its depth is more than three times its breadth.
Pile foundations are principally used to transfer loads from superstructures, through weak, compressible strata or water onto stronger, more compact, less compressible and stiffer soil or rock at depth. They are typically used for large structures, and in situations where soil is not suitable to prevent excessive settlement.
Pile caps can also be linked together with reinforced concrete to create a capping beam. At least three capped piles are needed to ensure stability against lateral forces (with the exception of caisson piles). Capping beams are also suitable for distributing the weight of a load-bearing wall, or of close-centred columns to a line of piles.
Piles may be staggered along the beam to allow for any eccentricities that may occur in loaded conditions. If light loading is expected to result in only minor eccentricities, then the piles can be driven in a line beneath the capping beam’s centre.
The capping beam should be kept clear of the ground where the purpose of the piles is to overcome the problem of the subsoil swelling and shrinkage. This can be done by casting the capping beam on polystyrene or other compressive material, thereby allowing an upward ground movement without damage to the beam.
An economical construction method that can be adopted is to construct concrete block walls to the exact sizes of the cap. Having first backfilled the wall with soil the space can then be filled with concrete. This cuts down on the need for formwork and the extra excavation required to position it.
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