As the load on a structure is ultimately transmitted to the soil, it is essential that the soil where the foundation would be lead is properly investigated. Sometimes for small structures we may rely on investigation reports of adjacent areas having similar soil but for important structures it is essential to have proper soil investigation.
Soil grains consists of inert rock materials (boulders, gravel, sand often combined with significant amount (say more than 5 per cent) of clay. The behaviour of the foundation soil mostly depends on the size and shape of grains, gradation of grains, mineralogical composition of grains, arrangement of soil grains in relation to each other, the inter particle forces and some other factors. Soil investigation at the laboratory as well as the field reveals all these properties of the foundation soil and helps us to decide about the most useful type of foundation and also the safe bearing capacity.
A soil may have a high ultimate bearing capacity based on shear strength but the expected settlement may be more than the permissible settlement for the type of structure to be constructed. In such cases the safe bearing capacity of the foundation soil would be decided by the maximum settlement that can be decided by the maximum settlement that can be allowed for the structure. In such cases the safe bearing capacity would be less than the maximum bearing capacity based on shear strength.
The foundation affects not only the soil immediately below it but also affects the soil 'at a lower depth depending on the size of the foundation and the load corning over it. It would be desirable that soil investigation is carried out by experts before the foundation is decided for all important works.
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