How to Decide The Depth of Soil Exploration

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How to Decide The Depth of Soil Exploration

How to Decide The Depth of Soil Exploration

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.

  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 to0.1Q). (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.

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.

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Table-1 (Thumb Rules to Predict Depth of Exploration)
Sl. No.Type of FoundationDepth of Exploration
1Isolated spread footingor raftOne and a half times thewidth
2Adjacent footing withclear spacing less than

twice the width

One and a half times, the length of the footing
3Pile and well foundations10 to 30 meters or more, or to a depth of one and a halftimes the width of structure

from the bearing level

(toe of pile or bottom of

well)

4Base of retaining wallOne and half times the base width or one and half times the exposed height of face of wall, whichever is greater
5Floating basementDepth of construction
6Dams1.      One-half the bottom width of earth dams2.      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

 

7Road cuts1.      One metre where little cut or fill is required2.      In cut sections, one metre below formation level

3.      In deep cuts, equal to the bottom width or depth

of the cut

8Road FillTwo metre below groundlevel or equal to the height

of the fill whichever is

greater

9Borrow areasConvenience 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.

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