Where the bearing capacity of the soil is poor or imposed loads are very heavy, pile foundations are required. There are two main types of pile; end bearing piles, where the pile acts as a column carrying the load down to the bearing strata, and friction piles, where the load is gradually transferred along the length of the pile. There are two basic construction methods; bored piles, in which the concrete is cast in place, and precast concrete piles that are driven into the ground.
With cast in place piles, the hole is bored into the ground, a reinforcement cage installed and the hole gradually filled with concrete. (See also Contiguous bored piles/secant piles). Precast concrete piles may be square, octagonal or other shape in cross-section. They may be cast to the full length required (depending on the ability to transport and handle them) or units may be joined to form longer lengths.
Precast piles need heavier reinforcement than cast in place piles as it is required to carry the stresses due to transportation and installation as well as the final loads from the structure. Alternatively they may be prestressed. Depending on the nature of the ground, the piles may be supplied with a cast iron or steel shoe to aid driving. As a combination of the two methods of construction, precast concrete shells may be driven into the ground and the interior filled with concrete.
Generally the load from a column will be carried by a number of closely-spaced piles, the load being distributed by a pile cap, the latter being basically a thick, reinforced concrete slab.