Depth of Soil Exploration

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.

Significant depth
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.

  1. It can be that depth where net increase in vertical pressure becomes less than 10% of the initial overburden pressure.
  2. 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).
  3. 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)
Sl. No. Type of Foundation Depth of Exploration
1 Isolated spread footing

or raft

One and a half times the


2 Adjacent footing with

clear spacing less than

twice the width

One and a half times, the length of the footing
3 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


4 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
5 Floating basement Depth of construction
6 Dams 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


7 Road cuts 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

8 Road Fill Two metre below ground

level or equal to the height

of the fill whichever is


9 Borrow areas 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


Add a Comment

Your email address will not be published. Required fields are marked *