1144 Subsurface Geologic Methods 



made in the map, new cross sections constructed or old ones revised, and 

 any other new data recorded for use, in that further questions may arise 

 during construction, operation, or maintenance, or subsequent repairs or 

 alterations. The construction phase is a geologist's proof of predictions 

 made during the previous stages. The geologist should usually be on 

 hand during the construction phase in order to check the interpretations 

 made in the light of the evidence uncovered, in order that revisions may be 

 made in the design if the need should become apparent. Frequently 

 economy can result from interpretations that may prove the foundation 

 area better than originally anticipated. 



Examples 



The following are only a few examples that point up the need for, 

 and the application of, various laboratory and investigation techniques. 

 Up to the present time, a large number of the problems solved by engi- 

 neering geology have been related to failures or impending failures of 

 existing structures. The present engineering geologist has gained con- 

 siderable knowledge from these past failures and has a better idea of 

 what to search for in the investigational stages of new projects. 



Davis Dam 



Investigations disclosed a large fault zone in the area of the pro- 

 posed spillway and powerhouse foundation for Davis dam. The fault 

 consisted of a zone of gouge and brecciated granite-gneiss rock, which was 

 an average of forty feet in thickness, with dips from 12 to 37 degrees. 



The design problems were resolved into the strength of the founda- 

 tion rock, the amount of the fault zone that would require remedial 

 treatment or removal, and the most appropriate location for the required 

 structures. 



An exploration program was laid out and further amplified or modi- 

 fied as the work progressed. This exploration included NX core holes, 

 six-inch core holes, test pits or shafts, and 36-inch calyx holes. Field bear- 

 ing tests, laboratory bearing tests, and petrographic investigations were 

 all employed. A geologic peg model was constructed and maintained up to 

 date in order to correlate the data derived from the exploration program. 

 (See fig. 600.) 



The general outlines of the proposed structures were etched on 

 plastic. Exploration holes were represented by metal pegs, which were 

 cut to length so that the top of the peg represented the top of the hole. 

 Diflferent rock types were represented by means of colors on the pegs, 

 with white representing the difference between the bottom of the hole 

 and the chosen datum. The portions of the hole showing fault breccia 

 were tied together with string. White siring represented the top and 

 bottom of the major fault zone. Dark string represented a minor fault 



