The object of site exploration is to provide reliable, specific and detailed information about the soil and ground water condition of the site which may be required for a safe and economic design and execution of the engineering work.
Dr. B.C. Punima
Generally soil exploration should be advanced to a depth up to which the increase in pressure due to structural loading will have no damaging effect (such as settlement & shear failure) on the structure. In other words, the depth at which soil does not contribute settlement of foundation. This depth is termed as significant depth.
Various factors affecting significant depth are as follow.
Type of structure
Weight of structure
Dimension of structure
Disposition of the loaded area
Soil profile and layer properties
The following 3 thumb rules can be used to know the significant depth.
It can be that depth where net increase in vertical pressure becomes less than 10% of the initial overburden pressure.
The maximum depth reached by the pressure bulb or isobar diagram drawn with an intensity of pressure varying from 1/5th or 1/10th of the surface loading intensity (i.e. 0.2Q to 0.1Q). (Where Q = Initial loading intensity).
It may be equal to one and half to two times the width or smaller lateral dimension of the loaded area.
Thumb Rules to Decide Depth of Soil Exploration
The following rules (Table-1) can be used as a guide to decide the depth of soil exploration to commence the exploration work.
Table-1 (Thumb Rules to Predict Depth of Exploration)
Type of Foundation
Depth of Exploration
Isolated spread footing
One and a half times the
Adjacent footing with
clear spacing less than
twice the width
One and a half times, the length of the footing
Pile and well foundations
10 to 30 meters or more, or to a depth of one and a half
times the width of structure
from the bearing level
(toe of pile or bottom of
Base of retaining wall
One and half times the base width or one and half times the exposed height of face of wall, whichever is greater
Depth of construction
1. One-half the bottom width of earth dams
2. Twice the height from stream bed to crest for concrete dams, for dams less than 30 m high
3. Upto bed rock, or else, though all soft, unstable and permeable strata of overburden
1. One metre where little cut or fill is required
2. In cut sections, one metre below formation level
3. In deep cuts, equal to the bottom width or depth
of the cut
Two metre below ground
level or equal to the height
of the fill whichever is
Convenience of excavation and thickness of available material
Note: The above values may be modified depending upon the type of soil encountered on site.
IS: 1892 – 1979 – Subsurface Investigation for Foundations
Soil Mechanics & Foundations by Dr. B.C. Punima, Ashok Kumar Jain, Arun Kumar Jain