190 



MIDDLEBROOKS. EARTH DAMS 



[Ch. 10 



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tion is one with a narrow central core 

 with large sand and gravel shells. Re- 

 cent large-scale triaxial tests have 

 shown that the angle of internal fric- 

 tion for well-graded sand and gravel is 

 about 45°. This high value of shear- 

 ing resistance cannot be fully utilized, 

 as it is not practical to construct on 

 slopes steeper than 1 on iy 2 for angular 

 gravels and about 1 on 2 for smooth, 

 rounded gravels. The Mariposa Dam 

 (Fig. 8) furnishes a good example of 

 the economical use of such materials. 

 The slopes are protected both upstream 

 and downstream with large stones 

 screened out of the main embankment 

 fill. 



Soils from required excavation can in 

 most cases be employed in the embank- 

 ment to economic advantage even 

 though some processing is necessary, 

 such as the crushing and mixing of the 

 spillway excavation at the Youghio- 

 gheny Dam (Philippe, 1948). At the 

 Fort Randall Dam the excess chalk 

 rock excavating from the spillway and 

 portal cuts was used in berms to reduce 

 the main compacted dam section, and 

 a heavy blanket of chalk was used on 

 the upstream slope in place of costly 

 riprap. A thick blanket of weathered 

 granite, stripped from the aggregate 

 quarry, was also used to replace rip- 

 rap on the Clark Hill Dam. 



Riprap for upstream slope protection 

 is one of the most expensive features 

 of earth-dam construction. Extensive 

 field investigations are justified to lo- 

 cate the nearest source of satisfactory 

 material. In a recent report, the Corps 

 of Engineers (1948) gives results of a 

 survey of riprap on more than thirty 



