Site Investigations

The surface crust of the earth consists broadly of rocks and soils. Their engineering properties are largely dependent on the particle size, shape, texture, the bedding, chemical composition etc. 

Rocks are classified as follows : 

1. igneous (such as granite) formed by cooling and solidification of molten lava material, 
2. sedimentary (like sand stone) formed by deposition of particles in layers, and 
3. metamorphic (like slates) formed by recrystallisation of existing igneous or sedimentary rocks under the action of great pressure and for high temperature. 

Generally, a rocky strata is a good base for foundation unless it is disintegrated or has faults, fissures etc. 

Soil can be classified into the following categories : 

Sands and Gravels 

These are non-cohesive materials with good shear strength. The structural properties depend on density and particle size distribution. Size of particles of gravels range from 75 mm to 4.75 mm while that of sand from 4.75 mm to 75 microns. 

Clays 

These are cohesive 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 fine grained soil of particle size between 75 microns to 2 microns with little or no plasticity, and are compressible. In the presence of clay, it exhibits plasticity. 

Sandy and gravely strata do not normally pose foundation problems while much more care is required in the case of clays and silts. 

The first step in deciding about the foundation of a building is to have a site investigation carried out. Site investigation would cover all the techniques and enquirers to gather detailed information regarding the site, including study of maps, topographic surveys, reconnaissance surveys, soil investigations, climatic data etc. As a result of such investigations, following infodon should be available : 

1. topography of site, contour maps, with details of streams, ditches, ponds, wells, trees, rock outcrops, high transmission electric lines etc., 
2. locations of underground sewers, waterlines and cables, 
3. liability to flooding, 
4. ground water details, 
5. details of my structures existing at the site or nearby with details of their foundations, including information regarding failures, cracks etc., 
6. a detailed soil investigation report including 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 basis for the choice and design of foundation. While for small single storeyed buildings, it may be sufficient to have a few trial pits, for larger structures or where there are doubts regarding the substrata, boring at site taken to sufficient depth would be required to collect and analyse the soil samples. The depth and the number of boring would depend on the type of structure and the soil conditions.