GEOLOGIC AND ECONOMIC BACKGROUND 57 



engineering concerned with excavation of material or selection of sites 

 for dams and other earth works. The geophysical methods and pro- 

 cedures employed, and the nature of information desired, are similar 

 to those described in the preceding section. Literature dealing with 

 engineering applications of geophysics is fairly extensive, t Electrical 

 methods have been employed almost to the exclusion of the other methods, 

 although application of seismic methods is increasing and magnetic 

 methods are often useful for special structural problems. 



Examination and Location of Dam Sites. — Excellent discussions 

 of the relationship between geology and engineering for dams and res- 

 ervoirs have been given in various publications, t 



In evaluating the location of a dam, five geological factors are of 

 primary importance : ( 1 ) the depth of overburden or fill materials which 

 must be removed, (2) the presence of faults or other structural defects 

 which might constitute a failure or leakage hazard, (3) the elevation or 

 height of the water table in the abutments and the reservoir rims, (4) 

 the strength of the foundation rocks, and (5) the perviousness of the foun- 

 dation rocks. 



Ordinarily, surface geological work will not provide sufficient detailed 

 information along these lines and in such cases proper geophysical work 

 combined with a small amount of confirmatory core drilling will usually 

 provide the needed information. This is particularly true of the first three 

 factors enumerated above. Bedrock depth determinations by geophysical 

 methods are relatively simple and inexpensive. The accuracy obtain- 

 able is well within the necessary limits, especially when occasional drill- 

 holes are used for control. 



The location of major faults in the vicinity of the dam site is of 

 great importance, not only from the standpoint of possible movement 

 along them which might endanger the dam, but also from the standpoint 

 of leakage through the reservoir rocks. The location of faults from surface 

 evidence is a well-known geological technique. However, important faults 

 may be covered by alluvial material, and they must therefore be identified 

 by geophysical methods. 



Ground water conditions at dam sites are important chiefly from the 

 standpoint of leakage. The depth to the water table in the dam abutments 



t Irving B. Crosby and E. G. Leonardon, "Electrical Prospecting Applied to Foundation Prob- 

 lems," A.I.M.E. Geophysical Prospecting, Tech. Pub., No. 131 (1928). 



E. G. Leonardon and Sherwin F. Kelly, "Some Applications of Potential Methods to Struc- 

 tural Studies," AI.M.E. Geophysical Prospecting, Tech. Pub. No. 115 (1928). 



S. F. Kelly, "Engineering Uses for Geophysics," Civil Engineering, October, 1932. 



G. G. .Stipe and Sherwin F. Kelly, "Geophysical Methods Aid Construction Work," Civil 

 Engineering, April, 1937. 



Karl S. Kurtenacker, "Some Practical Applications of Resistivity Measurements to Highway 

 Problems," A.I.M.E. Geophysical Prospecting, 1934. 



E. G. Leonardon, "Electrical Exploration Applied to Geophysical Problems in Civil Engineer- 

 ing," A.I.M.E. Geophysical Prospecting, 1932. 



t "Geology and Engineering for Dams and Reservoirs," A.I.M.E. Geophysical Prospecting, 

 Tech. Pub. 215. (Papers and discussions presented at the New York Meeting, February, 1929. )_ 



Douglas Clark, "Application of Geology to Civil Engineering," California Journal of Mines 

 and Geology, Vol. 29, January and April, 1933, pp. 161-173. 



