Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics.
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 wall footings) are a type of shallow foundation that is 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 (continuous footings) 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-rises or medium-rise domestic buildings – where mass concrete strip foundations can be used. In other situations, reinforced concrete may be required.
Very broadly, the size and position of strip foundations are 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 supporting wall. This results in the load being transmitted at 45º from the wall base to the soil.
Minimum widths for strip footings based on the type of ground and load-bearing wall
The underside of strip foundations should be deep enough to avoid frost action; for example, at least 450 mm unless they are bearing on the rock, and at least 1 m 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 low bearing capacity, so as to spread the load over a larger area. Wide strip foundations will typically require reinforcement.