Deep-foundation elements comprise piles and piers and are like slender columns buried in the ground. They are prevented from buckling because they are confines by the soil. Piles and piers transfer the load either to bedrock or to soil of high bearing capacity while passing through unsuitable soil.
As previously stated, deep foundation are used where shallow foundation cannot be used because the topsoil is expansive or has a low bearing capacity or where the frost line is deep. They are also used in buildings that house items or instrumentation sensitive to settlement of shallow foundations.
Piles are generally driven into the ground, except concrete piles, which can also be site-cast in predrilled holes (boring method). Site-cast piles are called piers, or caissons.
Steel, reinforced concrete, prestressed concrete, and wood are commonly used as pile materials. Steel and concrete piles are generally used under heavier loads. Wood piles are limited in their load capacity because of the nature of the material and the limitations on the cross-sectional are of tree trunks.
Steel piles consist of H-shape, or shallow, pipes. Hollow steel piles are filled with concrete after being driven. Steel piles are susceptible to corrosion in some environments.
Precast concrete piles are generally solid, but hollow precast concrete sections have been used. Concrete piles are subject to attack by sulfur present in some soil, requiring the use of sulfate-resistant Portland cement. Wood piles are treated with creosote or CCA as a preservative.
The selection of piles for a building is a function of several factors, such as availability, cost, below-grade environment, load capacity of piles, and the equipment required to drive them.
Piles that transfer the load through friction created between the surface of the pile and the sold are called friction piles. They are generally tapered to a narrow cross section at the bottom to facilitate driving and to increase friction. Piles that transfer most of the load to the bottom strata, with very little through friction, are called end-bearing piles.
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