Determine the Depth of Exploration

The depth of exploration depends on the following factors:

i) Size, type of the structure and loading intensity.

ii) Design considerations such as safety against foundation failure, seepage, earth pressure, excessive settlement.

iii) Character and sequence of sub-soil conditions.

The bore hole/boring must penetrate through all the deposits which are not suitable for foundation. In other words, the bore hole should extend upto strata, which is of adequate bearing capacity. You can use the geophysical profile to estimate the depth of boring.

In the absence of geophysical profile the depth of boring can also be estimated based on stress distribution criteria. This must include all the strata which are likely to undergo settlements due to load of the structure. This depth is called significant depth. De Beer recommends that the increase in stresses at the needed minimum depth of exploration should not exceed by 10% of the original vertical stress or less than 5% of the effective stress in the soil at this depth.

You must always keep in the mind that this significant depth will not account for coefficient of compressibility of the strata.