188 



middlebrooks. EARTH DAMS 



[Ch. 10 



i« 



a reasonably equal distribution of im- 

 pervious and pervious material, the im- 

 pervious material might form the upstream 

 half of the dam as in Cottage Grove (Fig. 

 9) and Great Salt Plains (Fig. 10) dams. 

 These dams represent the more general 

 cases in which the "impervious" zone con- 

 sists of impervious or semi-impervious ma- 

 terial as classified above. However, there 

 are flood-control dams in which the shell 

 materials are so coarse that semi-pervious 

 material is used in the core. If loss of water 

 ! | | o is not a problem for consideration, the main 



5 2 5 criterion to be satisfied is that the core be 

 I p3 tight enough to insure a low saturation line 



.2 in the downstream portion of the dam. 

 -2 This is usually accomplished in a satisfac- 

 g 1i tory manner if the downstream shell is 100 

 g ^ times more pervious than the core. In cases 

 g _oj where the entire embankment is composed 

 £ .■§ of impervious material, or where a satis- 

 ■E . factory ratio of permeability cannot be ob- 



% tained, drainage is used to lower the satura- 

 \ tion line. 



\ J 12 Pervious material is a most valuable as- 



" 1 m set at any earth-dam site. Extensive ex- 

 .1 $. 5=3 plorations are justified in locating possible 

 o. S .2 sources for borrow areas. All types of 

 . % % pervious material from fine sand to gravel 

 s Q ^ can be employed advantageously in the 

 embankment section. Even sands generally 

 ^ classified as semi-pervious (silty or clayey 

 sand) are relatively free-draining when 

 compared with very impervious core ma- 

 terials and will, when properly placed in 

 the dam, assure a low saturation line. 

 Locally available coarse sands and gravels 

 are usually economical for use as filter 

 blankets and drains even when screening is 

 necessary to meet a required gradation. 

 Where foundation stability is not a prob- 

 lem, the most economical embankment sec- 



fe 



