Geologic Techniques in Civil Engineering 1121 



factors of the geological situation and have interpreted their meaning in the 

 light of the engineering problems. 



The first step in any problem of engineering geology is to elucidate 

 all the geologic conditions that may pertain to the engineering problem. 

 The engineering geologist, like any other geologist, studies faults, folds, 

 joints, stratigraphy, petrography, geomorphology, and ground water. But 

 the elucidation of the geologic conditions is only the first step for the 

 engineering geologist. The next step is to interpret the significance of 

 these conditions in terms that are intelligible and useful to the engineer. 

 The objective of engineering geology is to translate the geologic condi- 

 tions into answers to such questions as these: Will this canal leak, and 

 how much? Will this rock dissolve or break down through the agencies 

 of weathering, and how rapidly? Will this material stand on steep slopes, 

 and how steep can they be? Can this material be used in construction, 

 and how can it be used to best advantage? Will this foundation settle, 

 how much, and how rapidly? How will these soils react to a pile foun- 

 dation? 



The writer wishes to thank the members of the geology section of 

 the United States Bureau of Reclamation for their assistance and com- 

 ments on various parts of this paper. Roger Rhoades, assistant head, 

 Research and Geology Division, Bureau of Reclamation, has ofi"ered valu- 

 able criticism during the preparation of this chapter. 



General Geologic Techniques 



The area to be investigated for most construction sites is frequently 

 rather restricted. The engineering geologic study of a site may best be 

 considered as two separate phases. The first phase should be a study of 

 the geologic conditions in the immediate construction area, more com- 

 monly called "site geology." The second phase should be a consideration 

 of the geologic conditions on a regional scale, from which conclusions 

 may be drawn to answer or explain the geologic conditions at the con- 

 struction site. 



One of the first responsibilities of an engineering geologist should 

 be thje preparation of a geologic surface map. This geologic map should 

 include all available information on the attitude of the exposed forma- 

 tions, structural relationships, stratigraphy, and lithology, as well as topo- 

 graphic conditions. Information on the geologic map should be further 

 amplified by means of geologic cross sections or stratigraphic columns 

 so that the subsurface conditions in the area concerned may be clearly 

 defined. Where bedrock exposures at the construction site are meager, it 

 may be necessary to make interpretations based on observations of geo- 

 logic conditions some distance from the site. The farther the distance 



