Ch. 8] SUBSURFACE INVESTIGATIONS 151 



and the variations in rock types over the short distances between the 

 borings. He is aware that there are variations in dips of the beds 

 and possibly displacements along faults with resulting breccia and 

 gouge. And what constitutes a representative sample of such a mass? 

 Is it the almost unobtainable sample of the gouge or the easily re- 

 coverable, smooth-sided, unbroken length of solid core from the 

 massive rock? If the properties of the massive rock are assumed to 

 be typical of the foundation, one would feel safe in recommending 

 high unit loads. However, if one were to recover a sample of the 

 gouge, it would have to be subjected to the laboratory techniques of 

 soil mechanics to obtain an idea of the highest limit of allowable unit 

 loads. With these unit loads the logical assumption would follow that 

 the site should be abandoned for a normal design or that a radical 

 departure from normal design would be required with consequent in- 

 crease in cost. All that is desired here is to emphasize that it would be 

 most unusual to find a foundation of sedimentary rocks so homogene- 

 ous that the physical properties of representative samples thereof could 

 be extrapolated to the mass foundation. 



The third method recognizes that, in sedimentary rocks, foundations 

 are composed of heterogeneous materials, the physical properties of 

 which may be determined within limits, although in the final analysis 

 the physical properties of the rock mass are generally indeterminate. 

 Indications may be developed, however, which will permit limiting, 

 rational assumptions as to the safe, allowable loadings and the reason- 

 ably anticipated rates of flow of water. Under this approach it is 

 assumed that the presence and orientation of geologic flaws, such as 

 planes of stratification, joints, faults, random fractures, and solution 

 channels in distinction from inherently differing lithologic character- 

 istics, will be compensated in the equations by safety factors. Or it 

 may be assumed that the geologic flaws will be minimized to the ex- 

 tent that they may be disregarded through inclination of base of the 

 structure, localized design to offset their effect on the mass of the 

 structure, or specialized treatment to raise their properties to accept- 

 able minima. 



SUBSUKFACE INVESTIGATIONS 



All three of these approaches require that the character of the 

 foundation rocks be known, but in varying degrees. In the first case, 

 in which the designer will rely on past experience and general practice 

 to set the allowable loadings, there may be made little or no attempt 

 to determine the details of geologic conditions other than to ascertain 

 from the general geology of the area the probable rock types to be en- 



