Clays

These are cohesive along with low shear strength and plastic in nature, prone to shrinkage and swelling depending on water content. Particle size is below 2 microns.

Silt

These are excellent grained soil of particle size among 75 microns to 2 microns along with little or no plasticity, and are compressible. Within the presence of clay, it exhibits plasticity.

Sandy and gravely strata do not generally pose foundation problems although much more care is needed in the case of clays and silts.

The first step within deciding about the foundation of a building is to have a site investigation carried out. Site investigation would cover all the methods and enquiries to gather feature information regarding the site, involving study of maps, reconnaissance surveys, topographic surveys, soil investigations, climatic data etc. As conclusion of such investigations, subsequent information should be available:

(1)  Topography of site, contour maps, along with details of streams, ponds, high transmission electric lines, wells, trees, rock outcrops, ditches and etc.

(2)   Locations of underground sewers, waterlines and cables,

(3)   Liability to flooding,

(4)   Ground water details,

(5)  Details of any structures existing at the site or nearby with details of their foundations, including information regarding failures, cracks and etc.,

(6)  a detailed soil investigation report involving bore charts, results of laboratory tests on soil samples and recommendations regarding depth and bearing capacity for foundation design, and

(7)  Meteorological data.

The detailed soil report would form the primary for the choice and design of foundation. Although for small single storied buildings, it might be enough to have a few trials pits, for larger structures or while there are doubts regarding the substrata, borings at site taken to enough depth would be needed to collect and analyse the soil samples. The number of borings  and the depth would depend on the category of structure and the soil conditions.