1122 Subsurface Geologic Methods 



from the actual site, the less valuable the subsurface geologic predictions 

 become. An important purpose of the regional geologic study is to de- 

 termine the uniformity or variability of a particular stratum or series of 

 strata throughout the area. By knowing the foregoing, exploration pro- 

 grams can be designed to obtain the maximum amount of subsurface 

 geologic information with a minimum amount of exploration. 



Engineering geology is concerned not only with the technical geologic 

 description of the material at a particular site but with the physical prop- 

 erties of the material as well. The designation of a geologic formation as 

 sandstone, shale, or some other type of lithology is not sufficient for engi- 

 neering purposes. It is important to know the load-carrying capacity in 

 pounds per square foot, the coefficient of friction, cohesion values, the 

 modulus of elasticity, and the permeability. To obtain this information on 

 physical properties, the engineering geologist must make use of other 

 chemical and physical sciences. In this way he can obtain actual labora- 

 tory data on the particular materials in question in order to obtain the 

 answer to these pertinent problems. 



All of the more common geologic techniques are employed in engi- 

 neering geology. The lithology, stratigraphy, geomorphology, structure, 

 and ground water are all integral parts of an engineering geologic report. 

 A complete understanding of the basic geologic information must be the 

 basis for a sound interpretation of the engineering implications of a par- 

 ticular site. 



In listing the requirements of the engineering geologist, Berkey ^ has 

 said: 



He must have the principles of geology so well in hand and feel so sure 

 of them in their application to the actual ground as it is that he is not in the 

 least disturbed at finding everything of a geologic nature belonging to a par- 

 ticular project materially different from anything he has ever seen. 



Lithology 



The lithologic description of a geologic formation should contain all 

 pertinent data regarding the grain size, the mineralogy of the individual 

 grains, the type and amount of cementation material, as well as a sum- 

 mary term such as "shale" or "sandstone," to complete the description. 

 The actual formation names as given in geologic literature are of minor 

 importance to the engineer. He is more interested in an accurate word 

 description of the material or geologic formation as found at the location 

 where he must build his structure. 



To the engineer "shale" implies a hard, durable rock having consid- 

 erable strength. The term "shale" does not adequately describe a number 

 of formations that are composed of poorly indurated clays and silts. 

 Such additional descriptions as soft or hard, and fissile or nonfissile; the 



2 Berkey, C. P., Responsibilities of the Geologist in Engineering Projects: Am. Inst. Min. Met, Eng. 

 Tech. Pub. 215, p. 5. 1929. 



