3) The number of borings and the depths to which bore holes should be advanced are dependent upon the subsurface geological conditions, the project site areas,types of projects and structural features. Where rock mass conditions are known to be massive and of excellentquality, the number and depth of boring can be minimal.Where the foundation rock is suspected to be highly variable and weak, such as karstic limestone or sedimentary rock containing weak and compressible seams, one or more boring for each major load bearing foundation element may be required. In cases where structural loads may cause excessive deformation, at least one of the boreholes should be extended to a depth equivalent to an elevation where the structure imposed stress acting within the foundation material is no more than 10 percent of the maximum stress applied by the foundation. Techniques for estimating structure induced stresses with depth are discussed in Chapter 5 of this manual.