The depth to which foundation should be taken depends on the following principal factors:
(a) Securing of adequate allowable bearing capacity,
(b) In the case of clayey soils, penetration into the soil has to be below the zone where shrinkage and swelling due to seasonal weather changes and due to trees and shrubs are likely to cause appreciable movements,
(c) In fine sands and silts penetration has to be below the zone in which trouble may be expected from frost,
(d) The maximum depth of scour whenever relevant (say, in bridge piers) should also be considered and the foundation should be located sufficiently below this depth, and
(e) It should be below the top soil, miscellaneous fill, tree roots etc.
All foundations should be taken down to a minimum depth of 0.5 m below natural ground level. In filled-up ground it may be necessary to go beyond the depth of fill or take special precautions. In such cases, it may be necessary for economic considerations to have the foundation at a higher level, and get the difference in level between the base of foundation and the level of excavation filled up with either:
(a) Concrete of allowable compressive strength not less than the allowable bearing pressure on the soil, or
(b) Incompressible fill material, for example sand, gravel etc. in which case the width of fill should be more than the width of foundation for dispersion of load on either side of the base of foundation.
In sloping grounds, the horizontal distance from the bottom edge of the footing to the ground surface shall be at least 60 cm for rock, and 90 cm for soil. A line drawn at an angle of 30″ to the base from the outer edge should not intersect the sloping surface (Figure 1).
Fig.1: Foundation on sloping ground
Foundation near Existing Building
The minimum horizontal distance between existing and new footings shall be at least equal to the width of the wider footing. In important cases, analysis of bearing capacity and settlement shall be carried out.
Foundation at Different Levels
In the case of footings on granular soil, the distance between the footings should be such that a line drawn between the lower adjacent edges of the footings shall not have a slope steeper than one vertical to two horizontal as shown in Figure 2.
Fig: 2 Foundation on granular soils
In clayey soils, a line drawn between the lower adjacent edge of the upper footing and the upper adjacent edge of the lower footing shall not have a slope steeper than one vertical to two horizontal as shown in Figure 3.
Fig:3 Foundation on clayey soils