When choosing the depth of footing for a given structure, the following points should be taken into strict consideration.
Depth of Top Soil
The footing should be located below top soil that consists of organic matters which decays. In most cases, it is more ideal to remove all top soil before commencing the digging of foundation such that the top of the soil after removal of the organic top soil becomes the reference to the foundation depth.
Depth of Frost Heave
The footing should be placed at appreciable depth below frost depth which may be 1 m or more in cold climates. This would help to prevent frost penetration. If located at insufficient depth, the footing would be subjected to frost damage due to formation of ice lenses and consequent frost heave.
Zone of Volume Change
Volume changes in soil are greater at the surface and decrease with depth. If a soil falls into the expansive soil class, they tend to swell when wet and shrink when dry, causing cracks in the foundation and superstructure. It is necessary where a foundation is expansive to take the foundation to a depth below the expansive soil layer and to also provide protection for the foundation against expansive soil.
Adjacent Footings and Property Lines
If a new footing is closer to existing footing and at a lower depth, it has potential to damage the existing footing. To prevent this from occurring, it is more ideal to establish the new footing at same depth as the existing footing. In a situation where the new footing should be deeper than the existing footing, the slope of the line joining the two footings should not be steeper than two (2) horizontal to one (1) vertical.
If a footing is located adjacent to a sloping ground, the minimum distance from the lower edge of the footing to the sloping ground surface should be 900 mm.
High water table around a footing prior to construction reduces the bearing capacity of the soil, during construction, it affects the construction activities while after construction, it causes dampness to structures if appropriate damp protection is not applied. Hence, these undesirable effects should be avoided especially for shallow foundations by locating them as far as practicably possible above the water table.
If the footings are located at places prone to scour such as streams, rivers, water fronts etc, they should be located below the potential scour depth.
The footing should be located safe from natural underground defects such as faults, caves and mines and man-made underground bores such as service lines.
Root holes and cavities in soils weaken the soils and reduces the bearing capacities of such soils. In this case, the footing should be located below such depth to avoid damage.
Different codes of practice have specifications for minimum depth of foundation and these should also be followed when deciding the depth of footings.