The definition and functions of foundation and broad guidelines for fixing the dimensions of foundation are given and different types of foundations are explained.

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:

H= (p/w)*(1-sinϕ/1+sinϕ)^2

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 below,


Type of the soil and SBC in Kn/m2

1.Igneous rocks (granite, troy etc.) -3300
2. Sedimentary rocks (sand, stone etc.)- 1650
3. Residual deposits, hard shale, cemented materials -900
4. Soft rock, coarse sand- 450
5. Medium sand- 250
6. Fine sand- 150
7. Loose gravel or sand gravel- 250
8. Soft shale, hard clay- 450
9. Medium clay, readily indented with thumb nail -250
10. Moist clay, clay and sand mixture -150
11. Soft clay -100
12. Black cotton soil, peat and made up of ground- nill

(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)