The depth and spacing of trial pits or exploratory borings should be such as to give a true picture of the underlying soil, with regard to any major changes in thickness, depth or properties of the strata over the base area of the structure and in its immediate vicinity. Only a preliminary estimate of spacing is made first. This may be decreased or increased depending upon the information revealed by borings.
In any case the boring should be carried down to hard strata below which softer material is not expected.
When the bed rock is met, the surface should be plotted by a number of borings or surroundings in order to locate any depressions. For this purpose, a few borings and sub-surface soundings are first made at large intervals and intermediate values, if needed, can be normally obtained by soundings, or by further borings in case of unpredictable profile nature.
According to Standard
The following specifications are laid down in this regard
For a compact building site covering an area of about 0.4 hectare (i.e. 4000 m2), one bore hole or trial pit in each corner and one in the centre (i.e. 5 boreholes in all) should be adequate.
For smaller areas and less important buildings even one bore hole or trial pit in the centre should be sufficient.
For very large areas covering industrial and residential colonies, the geological nature of the terrain will help in deciding the number of bore holes or trial pits. Dynamic or static cone penetration tests may be performed at every 100 metre by dividing the area in a grid pattern and numbers of bore holes or trial pits are decided by examining the variation in the penetration curves.
For exploration purposes, the minimum depth is taken as 1.5 m because the weathering processes affect the soil up to this depth. But where industrial processes affect the soil characteristics this depth may be more.
Normally the depth of exploration should be taken 1.5 to 2 times the estimated width of the largest footings (single or combined), from the base level foundation; but in weak soils, the exploration should be continued to a depth at which the loads can be carried safely (generally up to the hard stratum).
According to Thumb Rule
The following values are suggested
For preliminary estimate, the boring spacing may be adopted as follows:
Types of Buildings
Very Uniform Soil
30 – 60 m
15 – 30 m
Single storey building
60 – 90 m
30 – 60 m
A single rule about the depth of exploration which is found to be satisfactory for most conditions is 2.5 m to 3.5 m of borings depth for each floor in the building or its equivalent in loading.
The depth of boring depends upon the type of proposed structure, its total weight and the type of sub-soil encountered. Though normally the depth of boring may be taken as 1.5 times the width of largest footing, but at least on boring should be made deep enough to ascertain the existence of any exceptionally compressible stratum, preferably up to rock.
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