Ch. 5] THE ROLE OF GEOLOGY IN SOIL MECHANICS 107 



process. All applied sciences involve a similar chain of facts and in- 

 ferences. If soil mechanics not infrequently falls short of the ac- 

 curacy that numerical answers imply, so does geology in its more 

 quantitative aspects. Ore is not always found where predicted by the 

 mining geologist on the basis of his geometrical projection of strike, 

 dip, and fault displacement. Geologists recognize that in such cases 

 the error does not lie with the geometry or with the process of analysis, 

 but rather with the fact that, for lack of a thorough understanding of 

 what happens to rock under all conditions, the geologist is forced to 

 rely on simplified and idealized assumptions. 



THE ROLE OF GEOLOGY IN SOIL MECHANICS 



There is little need to emphasize the close relationship between 

 geology and soil mechanics. Soils form the raw material of both 

 sciences. Soil mechanics has, however, limited its interest principally 

 to the behavior of soil, whereas geology has confined its interest to 

 the origin and, to a lesser extent, to the substance of soil. The division 

 between these interests is not always clear-cut; and the origin, sub- 

 stance, and behavior of soils are probably as intimately interrelated as 

 are the human body and the human mind, two man-devised sub- 

 divisions which tradition has somewhat arbitrarily relegated to the 

 physician and the psychiatrist. 



A study of the most recent soil mechanics literature reveals that 

 soil mechanics is looking more and more to geology to explain cer- 

 tain phases of soil behavior which heretofore had been taken some- 

 what for granted. Unfortunately, the soils engineer is rarely a geol- 

 ogist. Indeed, he generally has all he can do to pursue the com- 

 plexities of his own specialty without embarking on those of geology. 

 Geologists are, therefore, needed to cooperate with soils engineers 

 if modern soil studies are to be carried out effectively. There are 

 three ways in which the geologist can help the soils engineer to arrive 

 at a better estimate of behavior: 



(1) The geologist can determine the type and degree of anisotropy 

 (variability) in soils. This is especially important in stability and 

 hydraulic studies. Minute details of Assuring, stratification, and 

 changes in texture, which may not be noticed by an untrained observer, 

 are commonly of great importance to the strength of a soil mass. Such 

 details can be detected more readily by the geologist, partly because 

 of his habitual concern for such detail and partly because of his ability 

 to deduce such detail from considerations of the origin of the deposit. 



The slow deepening and widening of shrinkage fissures in some clays 



