SEISMIC METHODS 925 



and the results of special pumping tests indicated that the channel was 

 filled with rather pervious material consisting of volcanic ash, sand, gravel, 

 and some clay. It was feared that the buried channel, after crossing the 

 axis from the west or upstream direction, might turn sharply to the south 

 and follow along the downstream toe of the proposed dam, thus creating 

 a potentially serious condition by providing a short leakage path for water 

 from the reservoir. 



Twenty seismic spreads were completed in 13 days, and about 90 bed- 

 rock depth measurements were made. An area approximately 3,500 feet long 

 by 2,500 feet wide was covered in sufficient detail that the shale bedrock and 

 the buried channel could be shown by contours. The desired information 

 was obtained more quickly and at less expense than would have been 

 possible by drilling. A seismic bedrock depth point costs about $30, as 

 compared to an estimated cost of over $3 per foot for core drilling; the 

 average depth of the core holes would be over 75 feet. 



The depth to bedrock from existing core holes was used as control for 

 the interpretation of the geophysical measurements. Where seismic depth 

 points were in the vicinity of these drill holes, it was found that the calcu- 

 lated depth to the shale checked with the depth obtained from drilling 

 within about 5 per cent. 



As the work progressed, it became apparent that the supposed old 

 channel was simply an erosion feature on bedrock, or a buried drainage, 

 and that its preglacial origin was uncertain. The bedrock contours showed 

 that the course of this drainage was in a northeast-southwest direction 

 where it crossed the proposed axis and that it drained into the Smoky Hill 

 River from the northeast. The seismic studies indicated definitely that the 

 channel does not have an outlet near the downstream toe of the dam. 

 Therefore it could not provide a leakage path for water from the reservoir. 



The map of the shale bedrock surface, contoured on an interval of 10 

 feet, was comparable in degree of detail to the usual surface topographic 

 map. It gave a subsurface topographic picture of bedrock. This suggests 

 that with some drill control the seismic-refraction method can be used to 

 provide information on the configuration of bedrock at dam sites, so that 

 the proposed axes can be located in relation to bedrock topography in much 

 the same manner as they are fitted to surface topography. 



Investigations of Soil Dynamics 



Analysis of the dynamic constants of soil furnishes useful information 

 for determining load-bearing capacity. In the past, foundation and load- 

 bearing studies had been principally conducted by static load methods, 

 which are relatively expensive and oftentimes inadequate or misleading in 

 their results. The dynamic methods usually can be applied more economi- 

 cally to obtain data on the load-bearing capacity of the soil as well as its 

 behavior under vibrations such as are produced by earthquakes. 



To determine dynamic constants, the site under investigation is sub- 



