Geologic Techniques in Civil Engineering 



1131 



of rocks, either when applied independently or as a means of selecting 

 those quantitative tests necessary to measure specific properties. The 

 latter function is the more important. Most tests to measure properties 

 of rock materials are expensive and time consuming, and they usually 

 require carefully selected samples so protected that at the time of testing 

 they truly represent the character of the rocks and materials in place, 

 especially with regard to moisture content and fractures. Consequently, 



Figure 594. Rock stresses in walls of Prospect tunnel are determined by cementing 

 one-inch strain-gage rosettes on rock and then relieving stress by drilling around 

 gages with six-inch-diameter core drill. This tunnel forms a part of the Colorado- 

 Big Thompson project. 



it is wise to determine the necessity for certain tests before they are 

 requested. For example, testing the quality of an aggregate by performing 

 tests on concrete containing the aggregate usually can be avoided by the 

 application of physical and chemical tests and petrographic examination 

 of the aggregate. Tests of concrete need be performed only when the 

 results of the physical and chemical tests of the aggregates are anomalous, 

 or if the petrographic examination indicates adverse properties not evalu- 

 ated by the aggregate tests. Determination of volume increase of rock 

 materials with wetting is significant only if clay minerals of the mont- 

 morillonite type (bentonitic) are present. The presence and abundance 



