What is Soil Exploration? || What is the depth of exploration for type of foundation? || IS 1892 1979

The object of site exploration is to obtain reliable, specific and detailed information about the underlying soil/rock and groundwater conditions at a site for enabling engineers in the safe and economic design and execution of engineering works.

To meet this objective investigation are carried out to the required depth and horizontal extent in the region likely to be affected by the proposed constructions.

Purpose of soil exploration

The soil exploration should yield precise information about:

·        Order of occurrence and extent of soil/rock strata
·        Nature and engineering properties of the soil/rock strata
·        Location of groundwater table and its fluctuation

Depth of investigation, in general, is decided based on the intensity of structured loading and the type of foundation contemplated.

This depth up to which the increase in stress due to structural loading causes shear failure or excessive settlement of foundation is known as significant depth.

This depth of investigation is generally taken as the depth of pressure bulb of intensity 0.1 q where ‘q’ is the intensity of loading at the base of foundation.

IS 1892 provides the following guidelines for depth of exploration for different types of foundations.

Sl no	Type of foundation	Depth of exploration
1.	Isolated spread footings or raft or adjacent footings with clear spacing equal or greater than four times the width	One and half times the width
2.	Adjacent footings with clear spacing less than twice the width	One and half times the length
3.	Adjacent rows of footings i. With clear spacing between rows less than twice the width ii. With clear spacing between rows greater than twice the width iii. With clear spacing between rows greater than four times the width	Four and half times the width Three times the width One and half times the width
4.	Pile and well foundations	One and half times the width of structure from bearing level (toe of pile or bottom of well)
5.	Road cuts	Equal to the bottom width of the cut
6.	Fill	Two meters below the ground level or equal to the height of the fill, whichever is greater

The member and spacing of boring/test pits depend on the type and size of foundations and extent of variation in soil conditions.

IS 1892 makes the following recommendations:

·        For a compact building site covering an area of about 0.4 hectare, one bore hole or trial pit in each corner and one in the centre should be adequate
·        For smaller and less important buildings even one bore hole or trail pit in the centre will suffice
·        For very large areas covering industrial and residential colonies, the geotechnical nature of the terrain will help in deciding the number of bore holes or trail pits
·        Cone penetration tests may be performed at every 50 m by dividing the area in a grid pattern and number of bore holes or trail pits decided by examining the variation in penetration curves. The cone penetration tests may not be possible at sites having gravelly or boulders strata. In such cases geophysical methods may be suitable. 

•