Some useful information on required depth of footings/foundations

The footings should be provided underneath the top (organic) soil, miscellaneous fill, deserted foundation, debris or muck. If the top loose soil or fill is adequately intense, two alternatives should be applied based on the relative economy and accessible time:

(i) Extracting the top soil directly underneath the footing and displacing it with lean concrete.

(ii) Extracting the top soil in an area greater than the footing and displacing it with compacted sand and gravel fill. The area of the compacted sand and gravel fill should be adequately bigger to disperse the load of the footing load.

In both cases, it is required to attain the level of the strata that contains the necessary bearing strength allowed for the design of footing.

Sometimes, the top soil is good and compact, and may contain sufficient bearing strength. Under such circumstance, it will be effective to retain the minimum depth of foundation as per Rankines formula.

Note: From the formula, it is found that q denotes the actual load intensity and not the safe bearing strength of soil. Some times, the actual load intensity remains under the safe bearing strength of soil, necessitating minor minimum depth.

If the footings rest on very stiff soil with very high safe bearing strength, the minimum depth of foundation measured depending on the safe bearing strength would become very large.

Under such soils, the width of the footing (available from other considerations) would be in excess of the one required from the bearing strength considerations, as a result the actual soil pressure is reduced as compared to the safe bearing strength.

The depth of footing should also be maintained in such a manner that the rate or angle of spread of the load from the wall base to the exterior edge of the ground bearing does not go beyond the allowable value.