182 middlebrooks. EARTH DAMS [Ch. 10 



study. The detailed site study should develop the nature of the 

 overburden and rock foundation in the river valley and abutments. 

 Special attention should be given to foundation conditions at all pos- 

 sible spillway locations in order that the most economical spillway 

 that will perform the specified functions may be designed. Founda- 

 tions for outlet works should be thoroughly explored to determine 

 whether a "cut and cover" type of conduit or a tunnel is preferable. 

 Selection of the type of dam, whether concrete, rock-fill, or earth, 

 has been adequately covered in other publications (Creager, Justin, 

 and Hinds, 1945; Middlebrooks and Bertram, 1948). 



FOUNDATION STUDIES 



A study of foundation features of an earth-dam site may be divided 

 into three general classifications: (1) investigation of foundations for 

 the spillway structure; (2) investigations of foundation for the earth 

 dam; and (3) investigations of foundations for the outlet structure. 

 After the general geological and soil study of the site, a detailed 

 study should be made for each of the three structures enumerated. 



Spillway Structure 



In order that sufficient geological information may be furnished 

 the designers, the geologist and the soils engineer should have a gen- 

 eral knowledge of different types of spillways and the foundation re- 

 quirements for each. In Figs. 1 through 6 are shown a few examples 

 of various types of spillways and typical foundations upon which 

 they are used. The spillway type should be varied to fit not only 

 the foundation but also the frequency of use of the spillway. At 

 most earth-dam sites the spillway design presents a major problem 

 and represents a high percentage of the total cost of the project. 

 Therefore a careful foundation study of all possible spillway loca- 

 tions should be made. 



A gravity overflow spillway as shown by Figs. 1 and 2 presents 

 fewer operational problems because it discharges into the river chan- 

 nel. However, it is generally more expensive than any of the other 

 types, except possibly some chute spillways on earth. The flip-up 

 bucket is used in this country principally on massive hard-rock 

 foundations. In Europe, this type of energy dissipater is extensively 

 used on practically all rock foundations and on some low-head dams 

 on soil foundations. The practice in this country is more conserva- 

 tive than the European practice relative to the design of energy dissi- 



