Dimensions of Foundation

Guidelines for minimum dimensions are given below:
(a) Depth of Foundation: For all types of foundations minimum depth required is calculated using Rankine’s Formula:

where
p = safe bearing capacity of soil
w = unit weight of soil
ϕ = angle of repose of soil.
However in any case it is not less than 0.9 m. Finding safe bearing of the soil is an expert’s job, and it is found after conducting tests in field or in Laboratories. However general values for common soils are listed in Table 7.1.

(b) Width of Foundation: Width of wall foundations or size of column footing is determined by first calculating the expected load and then dividing that with SBC. Thus,

Width of wall foundation = Load per unit length of wall / S.B.C. of soil

Area of column footing = Load carried by column / S.B.C. of soil