There are three basic foundation types:
• Strip foundations
• Piled foundations
• Raft foundations
Strip foundations are by far the most common. The foundation is basically a strip, or ribbon, of insituconcrete running under all the loadbearing walls. These will normally include all the external walls and possibly some, or all, of the internal walls. The depth and width of the strip depends on the building load and the nature of the ground. In many cases these foundations do not need specialist design; the foundation size can be determined by referring to tables in the Building Regulations.
Piled foundations can be of various types. They transmit the loads from the building through weak, compressible, or unstable strata, to firmer ground beneath (end bearing piles). In clay and other cohesive soils piles can be used to distribute the loads into the ground through the friction forces along the length of the pile sides (friction piles). Piles are usually made from insitu or precast concrete but can also be steel and timber. In housing built from loadbearing brickwork, a reinforced concrete beam bridges the piles and directly supports the walls.
Rafts are an expensive form of construction, probably the most expensive of the three, and are used where only a very low load can be applied, for example, on soft or variable ground. They are also used where differential settlement is likely or where there is a risk of subsidence (they are common in mining areas). The raft is a rigid slab of concrete, reinforced with steel, which spreads the building load over the whole ground floor area.
Prior to the end of the 19th century it was not uncommon to build external walls on a rubble foundation...