GUIDING RULES TO DECIDE DEPTH OF SOIL EXPLORATION
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.
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 1/5^{th} or 1/10^{th} 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 (Table1) can be used as a guide to decide the depth of soil exploration to commence the exploration work.
Table1 (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
width 
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 well) 
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. Onehalf 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 greater 
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.
References
IS: 1892 – 1979 – Subsurface Investigation for Foundations
Soil Mechanics & Foundations by Dr. B.C. Punima, Ashok Kumar Jain, Arun Kumar Jain
Thanks for providing this rule of thumb but my concern that how we shall calculate the zone of influence if you have formula so pl mail.
The thumb rules are quite educative
Thanks for your precise summary. The thumb rules table given above is really helpful. My request it to prepare excel sheets for computation of SBC of soil from cf soil , from SPT values and also from settlement of foundation .If you have developed pl mail to me
Thanks for your precise summary. The thumb rules table given above is really helpful. My concern is to know the approximate depth of exploration for the reconstruction of an existing bridge.