The foundation that a building requires in order to support the structure and resist subsidence depends on a number of factors. The quality of the ground is integral in dictating the type of foundation required, as is the size of the structure. Suitable and accurate calculations by a qualified structural engineer should always be carried out at the design stage to ensure an adequate foundation is constructed.
A strip foundation--also known as a spread footing-- is most commonly used for smaller structures such as houses. It is characterised by a shallow concrete footing about 17 inches wide laid at the bottom of a trench about 36 inches deep. In some cases, the concrete is reinforced by one or two layers of steel mesh, which is submerged in the concrete. Blockwork can then be built on this foundation, forming what is, in essence, an upside-down "T."
Trench Fill Foundation
A trench fill foundation comprises an excavation of about 6 feet deep, filled with about 5 feet of concrete, leaving space to build blockwork on top. Trench fill foundations are used in softer ground or ground more susceptible to movement, such as clay, where the load of the building needs to be transferred to stronger sub-layers of soil.
A pile foundation is also used in areas of soft ground, where the load needs to be directed toward bedrock or stronger substrata of soil. A pile foundation, the deepest of all foundations, consists of a bored hole of a depth specified by the engineer (usually, in skyscrapers, the piles go down to meet bedrock) that is filled with a structural material. There are different methods of piling, including filling the bored hole with compacted stone (vibropiling) or wet concrete with steel reinforcement (continuous-flight auger piling). Piles also can be precast and then driven into the soil to a specified depth.
Raft foundations are useful in areas of very soft soil where other foundations will not suffice and pile foundations are too expensive or impractical. With a raft foundation, the load of the building is spread over a very wide area, usually the size of the building itself, consisting of a large concrete slab often reinforced by a steel mesh or reinforced steel bars.
Base-isolating foundations can protect large buildings in locations that are at risk of earthquakes. While a foundation rigidly attached to a building may give the impression of strength, during an earthquake this monolithic quality can have an adverse effect as the building moves and loses structural integrity. Base-isolators are essentially large shock absorbers between the foundation and the building superstructure that prevent the building from moving violently during an earthquake.