Very broadly, foundations can be categorised as shallow foundations or deep foundations. Shallow foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils. Deep foundations are necessary where the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so they need to be transferred to deeper layers with higher bearing capacity.
Strip foundations (or strip footings) are a type of shallow foundation that are used to provide a continuous, level (or sometimes stepped) strip of support to a linear structure such as a wall or closely-spaced rows of columns built centrally above them.
Strip foundations can be used for most subsoils, but are most suitable for soil which is of relatively good bearing capacity. They are particularly suited to light structural loadings such as those found in many low-to-medium rise domestic buildings - where mass concrete strip foundations can be used. In other situations, reinforced concrete may be required.
Older buildings may have brick strip foundations.
Very broadly, the size and position of strip foundations is typically related to the wall’s overall width. The depth a traditional strip foundation is generally equal to or greater than the overall wall width, and the foundation width is generally three times the width of the supported wall. This results in the load being transmitted at 45º from the wall base to the soil.
Typical strip foundation measurements are as follows:
- Single storey building: 450 mm wide and at least 200 mm deep.
- Two storey building: 600 mm wide and 200 mm deep.
Approved document A of Building Regulations defines minimum widths for strip footings based on the type of ground and load-bearing wall, although it is generally advisable to consult a structural engineer when designing foundations.
The underside of strip foundations should be deep enough to avoid frost action; for example, at least 450mm unless they are bearing on rock, and at least 1m on high shrinkage clays.
Deep strip foundations may be necessary where soil with a suitable bearing capacity is deeper.
Wide strip foundations may be required where the soil is soft or of a low bearing capacity, so as to spread the load over a larger area. Wide strip foundations will typically require reinforcement.
Where ground conditions are poor, settlement is likely, or where it may be impractical to create individual strip or pad foundations for a large number of individual loads, raft foundations may be used.
Larger or more complex buildings may involve the use of a number of different types of foundation.
Additional guidance is available in BRE's Simple foundations for low-rise housing: 'rule of thumb' design.
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