The depth of exploration at the start of the work or the significant depth, for pile foundation, should be 10 to 30 meters, or more, at least one and a half times the width of the structure.

Penetration and soundings help in estimating the relative density and strength properties of coarse-grained soils and also to locate the weaker strata and lenses.

Site exploration should yield precise information about the following: i) the order of occurrence and extent of soil and rock strata ii) the nature and engineering properties of soil, and iii) the location of ground water.

Exploratory borings, shallow test pits, subsurface penetration, soundings and geophysical methods are used for general exploration.

The methods of site investigation are largely dependent upon the nature of the engineering project and the site.

Exploration in general, should be carried out to a depth up to which the increase in pressure due to structural loading is likely to cause perceptible failure or shear failure. Such a depth is known as the significant depth.

Depending upon the subsoil conditions the number and disposition of bore holes are varied, usually in the valley floor, on the abutment etc.

The general exploration is able to give information about the following:

Depending upon the details or the precision of the sub-soil data to be obtained, the site exploration may be classified as general (or preliminary) and detailed.

On going over the site, a study of the following features may be useful: local topography, excavations, cuttings, escarpments, flood marks and evidence of erosion.