The footings should be carried below the top (organic) soli, miscellaneous fill, abandoned foundation, debris or muck. If the top loose soil or fill is too deep, two alternatives may be used depending upon the relative economy and the time available :
(i) Removing the top soil directly below the footing and replacing it with lean concrete [Fig. 3.1 (a)]
(ii) Removing the top soil in an area larger than the footing and replacing it with compacted sand and gravel fill. The area of the compacted sand and gravel fill should be sufficiently large to distribute the footing load, as shown in Fig. 3.1 (b).
In either case, it is essential to reach the level of the strata which has the required bearing capacity adopted for the design of footing.
Sometimes, the top soil may be good and compact, and may have adequate bearing capacity. In that case it is desirable to keep the minimum depth of foundation given by Rankines formula: (Eq. 2.8)
Where Ø is the angle of repose, the values of which may be taken from Table 3.1.
It is to be noted that q is the actual load intensity and not the safe bearing capacity of soil. Some times, the actual load intensity may be less than the safe bearing capacity of soil, requiring lesser mínimum depth. When footings are supported on very stiff soil, having very high safe bearing capacity, the minimum depth of foundation computed on the basis of safe bearing capacity would come out to be vcry large which is ridiculous. In such soils, the width of the footing (found from other considerations) would be larger than the one required from the bearing capacity considerations, thus giving rise to actual soil pressure lesser than the safe bearing capacity.
TABLE 3.1. VALUES OF UNIT WEIGHT AND ANGLE OF REPOSE.