Design of Shallow Foundations

Following guidelines may be taken while designing shallow foundation other than rafts and mats.

1. In case of wall footing the width of foundation should be computed based on the allowable soil pressure.
2. In case no footings are to be provided to the walls the width of foundation should be equal to three times the width of the wall.
3. In case of piers the width of foundation is equal to square roots of total load of the pier divided by the allowable soil pressure.
4. For unreinforced strip footings the thickness should not be less than the projection from the base of the wall. It should not be less than 150 mm where the foundations are laid at more than one level.
5. For unreinforced column footing the spread of footing may be 1 vertical to one horizontal.
6. As a general rule, the shallow foundation should be taken down to a depth where the allowable bearing capacity is adequate.
   1. As for as possible the foundation should be kept above the ground water table.
   2. In order to safeguard a against minor soil erosion, a minimum depth of 500 mm is provided for strip or column foundation
   3. The depth of foundation can be also determined by plotting the pressure distribution lines (Fig. 15.13).h₁, h₂ = Depth of footing, Depth of base concreteh = Depth of foundationThen h = Figure 15.13
   4. Minimum depth of foundation for loose soils may be obtained from Rankine’s formula, i.e.,Where h = Minimum depth of foundation in mw = Weight of soil in kg/m³ϕ = Angle of reposep = Load in soil kg/m²The depth of concrete block is given as  in cm.where a = offset of concrete in cmf = safe modulus of rupture in kg/m²