60 EXPLORATION GEOPHYSICS 



important in selecting suitable limestone deposits for quarrying, and such 

 defects are readily located by electrical methods in many cases. 



Magnetic and electrical work can be advantageously used prior to 

 excavation to locate faults, changes in formation, degree of weathering, 

 etc., along pipe line and aqueduct routes. If the work requires tunnel 

 construction, determination of thickness of overburden can be made in con- 

 nection with the other geophysical studies.* 



The problems of cut and fill in highway construction involve classifica- 

 tion of material to be excavated. Electrical methods are of aid in this 

 classification. Electrical surveys have a valuable application in problems 

 of highway grade settlement in swamps. It is often possible to determine 

 the depth to firm material at the bottom of the swamp and in some cases 

 whether or not newly constructed fill is properly settled on a firm 

 foundation.! 



Application of electrical work in bridge construction problems is 

 similar to that for dam sites : namely, the determination of depth to bed- 

 rock or to a material of proper characteristics to support piling. 



Seismic studies have also proved valuable in work of this nature and 

 increasing applications of these methods for determining the bearing 

 capacity of soils X and the natural period of vibration of foundations and 

 dams may be expected. 



* Extensive electrical work of this kind was employed by the Metropolitan Water 

 District of Southern California in planning aqueduct routes from the Colorado River 

 to the city of Los Angeles. 



t Karl S. Kurtenacker, loc. cit. 



t R. K. Bernhard, "Geophysical Study of Soil Dynamics," A.I.M.E. Geophysical Prospecting, 

 Tech. Pub. 834, 1938. See also chapter on Seismic Methods. 



