Depth and Spacing of Trial Pits or Bore Holes
The location and the spacing of trial pits or bore holes for a particular site require special consideration. The pits or the bore holes should be so located so as to give adequate information in respect of changes in properties of the underlying strata with depth. The number and spacing of the test pits or bore holes to be adopted for a site will depend upon the area of the plot as well as the type of structure to he built. For a plot of 0.4 hectare we may have one trial pit or bore hole near each corner and one in the middle. For smaller size of plot and for ordinary structure, one trial pit or bore hole near the centre should be sufficient. The depth of the pit or bore hole will depend upon the characteristics of the soil as well as the type of structure, its shape, size and loading condition. As a thumb rule, its depth should be one and a half times the probable width of the footing or 1.5 m whichever is more. In case of weak soils, however, the test pits or the bore holes should he taken to a depth at which the loads can be carried by the soil without undesirable settlement.