What is the minimum number of boreholes required for a compact building?

At least three boreholes, arranged not in a straight line, to establish the dip of the geological strata.

How deep should a borehole be for an isolated spread footing?

1.5 times the width of the footing (1.5B) below the proposed foundation level.

How is the depth of exploration determined for pile foundations?

It should extend 1.5 times the width of the pile group, measured from the lower third point of the pile group.

When can exploration be terminated before reaching the 1.5B depth?

When rock or incompressible strata are encountered at a shallower depth, provided the rock is penetrated sufficiently to confirm it is not a boulder.

IS 1892

: 1993

Code of Practice for Subsurface Investigation for Foundations

CurrentEssentialCode of PracticeBIMGeotechnical · Geosynthetics and Ground Improvement

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This code provides comprehensive guidelines for subsurface investigations required for the safe design and construction of foundations. It specifies the necessary number, spacing, and depth of boreholes, alongside appropriate methods for soil sampling and groundwater monitoring. Proper adherence ensures an accurate understanding of the subsoil profile and minimizes geotechnical risks during construction.

Lays down the procedure for subsurface investigation for various types of foundations.

Quick Reference Values

minimum boreholes for building3 (not in a straight line)

depth of exploration isolated footing1.5 times the width of the footing (1.5B)

depth of exploration raft foundation1.5 times the width of the raft

depth of exploration pile group1.5 times the width of the pile group from lower third point

depth of exploration retaining wall1.5 times the base width or 1.5 times the height

stress influence limit10% of applied stress (exploration should reach this stratum)

Key Formulas

D = 1.5 B — General rule for depth of exploration below foundation level

Practical Notes

Always ensure boreholes are not placed in a single straight line to accurately capture the 3D subsoil profile and dip of strata.

If bedrock is encountered, drilling must penetrate at least 3 meters into the rock to ensure it is not merely a boulder.

Continuous monitoring of the groundwater table is crucial, taking seasonal variations and potential artesian pressures into account.