Foundations - Topic Home



The four paragraphs below provide a brief introduction to the main foundation types in the UK. The strip foundation is by far the most common and, in most cases, the cheapest. It has been used, in one form or another, for hundreds of years and, for low rise housing, is suitable for the majority of ground conditions likely to be found. The remaining three foundation types are more likely to be used where sites and ground conditions are more complex.  
The strip foundation is basically a strip, or ribbon, of insitu concrete running under all the loadbearing walls. This 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 the Building Regulations. Trenchfill is an alternative type of strip foundation. It is usually 500mm or more thick and avoids the need for bricklayers to work in the trenches.
Rafts are an expensive form of construction, probably the most expensive of the four, 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.   
Piled foundations can be of various types. They can be used to transmit the loads from the foundations 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. 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 building.
Pier foundations are quite rare and are similar in function to end-bearing piles, in other words the loads are transmitted through end-bearing to a deeper strata. Unlike piles, which are almost always installed by special piling 'rigs', the pier bases are normally excavated by digger. They are often used where steeply sloping ground precludes the use of piling.
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