Need for foundations for structures

The foundation of a building is generally considered to be the part of the structure that transmits the superimposed load of the building to the supporting soil. The foundation shall be provided to ensure that superimposed construction loads do not exceed the bearing capacity of the soil and that differential settlements are kept to a minimum. If the load-bearing characteristics of the soil are good, the depth of the foundation will mainly depend on the design of the building, that is, the depth of the basement and the arrangement of the structural frame. If the soil is unstable, then the foundation must penetrate the poor soil to a more stable stratum.

Typically, soils near the surface are less stable and less compacted than material some distance below the surface. By removing the top layer of unstable soil, the weight of the building, if evenly distributed, can be supported by the more densely compacted material that has been exposed. Where good soil support properties are found near the surface, the foundation can be placed at a minimum depth below grade, usually dictated by the design of the building or by the depth of frost penetration.

A foundation that is a relatively short distance below the final level is called a shallow foundation. Shallow foundations are generally made of cast-in-place reinforced concrete and consist of isolated footings to support column loads, continuous strip footings under load-bearing walls, or a variation of both to ensure proper distribution of loads. However, the term is relative, as there is no hard and fast rule to determine when a shallow foundation becomes a deep foundation. A special type of foundation, known as matte foundation, can be characterized as a shallow foundation or a deep foundation, depending on its placement.

Foundations that must resist large gravity loads and load reversals due to the overturning effects of wind loads require greater foundation depths to maintain superstructure stability. In order for large buildings to develop adequate support for imposed loads, it is not uncommon for portions of their foundations to extend into bedrock or to a depth sufficient to achieve load-resisting capacity through a combination of friction of the skin and final support, particularly when the bedrock is too deep below the surface or the bedrock is not consistent or competent. This type of foundation can be called a deep foundation and is achieved through the use of piles, caissons, deep wall foundations, mats, and in some cases a combination of mats and piles.

The type of foundation used in any particular case depends on the size and distribution of construction loads, the depth from the soil surface to the stable layer of supporting material, the type of material through which the foundation must pass, and the location of the structure.