Exploration, in general, should be carried out to a depth up to which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure of foundation. This depth is known as depth of exploration, which depends upon the following factors.
Type of structure
Load on structure
Size and shape of foundation
Position of loaded areas
Soil profile and its properties
The depth of exploration at the start of the work may be decided according to the thumb rule given below, which may need modification as exploration proceeds.
Thumb Rule for Deciding Depth of Exploration
The depth of exploration should be one and half times to two times the estimated width (lower dimension) of the footing, single or combined, from base level of the foundation.
In case of weak soils, the exploration should be continued to a depth at which the loads can be carried by the stratum without excessive settlement or shear failure.
Isolated spread footing or a raft: one and a half times the width.
Adjacent footings with clear spacing less than twice the width: one and half times the length
Pile foundation: 10 to 30 meters, or more, or at least one and a half times the width of the structure
Base of the retaining wall: one and a half times the base width or one and a half times the exposed height of face of wall, whichever is greater.
For floating basement the depth of construction should be equal to the depth of construction.
In any case, the depth to which weathering process affect the soil should be regarded as a minimum depth for exploration of sites and this should be taken as 1.5 meters. In case of black cotton soil the minimum depth of exploration is 3.5 meter.