58 EXPLORATION GEOPHYSICS 



and reservoir rims usually can be determined satisfactorily by electrical 

 methods. 



In regard to the strength and perviousness of the foundation rocks, 

 geophysical work will often give useful information for correlation with 

 surface evidence, core holes, and laboratory tests. In limestone regions, the 

 foundation rocks may be characterized by solution channels and caves at 

 certain horizons, and such conditions can often be detected by changes 

 in the apparent resistivity or the subsurface current distribution. 



Several examples are cited below to illustrate the need for detailed geological and 

 geophysical work.t 



At the location of the O'Shaughnessy dam on the Tuolumne River at the lower end 

 of the Hetch Hetchy Valley in Tuolumne County, California, the depth of the stream 

 gravels overlying the granite bedrock was found to be excessive, the maximum depth 

 being 101 feet. Owing to this condition a greater amount of excavation was required 

 and a higher dam built than was originally planned, which added greatly to the cost 

 of the project. 



At the site of the St. Francis dam near Saugus, Los Angeles County, mica schist 

 occurs on the east slope of the canyon and conglomerate of the Sespe formation on the 

 west side. The contact between the two formations is a fault which parallels the can- 

 yon on the west slope about 60 feet above its base. Within the fault zone there are 

 numerous seams of clay gouge and fractures filled with gypsum. Furthermore, the 

 conglomerate, where fractured, completely disintegrates when immersed in water. 

 A commission investigating the failure of this dam concluded that it was due to 

 disintegration of the fractured conglomerate. 



During the construction of the Lafayette earth-fill dam in Contra Costa County, 

 California, by the East Bay Municipal Utility District, difficulty was encountered 

 because of the occurrence of plastic clay beneath the dam. The strata underlying the 

 dam site include beds of clays and sands of the Orinda formation, dipping at high 

 angles and overlain by alluvial fill from 71 to 91 feet thick, the bedrock in part being 

 sandy clay. When the dam was completed to within 20 feet of its final height the crest 

 of the dam sank vertically 24 feet, causing a movement of the downstream face. The 

 clay below the dam readily absorbed water and became a soft mud which moved out un- 

 der the weight of the dam. This condition made it necessary to complete the dam with 

 a height of 40 feet less than that originally intended and with greatly flattened slopes. 



The foregoing are typical examples of adverse conditions unexpectedly revealed 

 after the expenditure of large sums had made relocations inexpedient but which would 

 very probably have been disclosed by preliminary geological and geophysical surveys. 



Electrical, seismic, and magnetic methods are now being widely 

 used to secure the geological information outlined at the start of this 

 section. This no doubt will result in less frequent failure of dams, 

 and subsequent discovery of unfavorable geological conditions that 

 might have been disclosed by preliminary geophysical surveys. Geo- 

 physical methods are being utilized by the Bureau of Reclamation, U. S. 

 Army Corps of Engineers, Tennessee Valley Authority, by various 

 Municipal Agencies such as the Metropolitan Water District of Southern 

 California, and by private engineering agencies.* 



t Douglas Clark, loc. cit. 



* Typical examples of geophysical investigation of dam sites are given in the 

 chapter on Electrical Methods. 



