Ch. 10] EMBANKMENT DESIGN AND CONSTRUCTION 191 



dams. The major findings of the survey are: a wide variety of rock, 

 including some very soft formation, have proved successful; thinner 

 riprap layers and correspondingly small stone can be used than have 

 generally been employed for fetches not to exceed 10 miles ; well-graded 

 filter blankets are essential but may be relatively thin (6 to 12 inches). 

 Required rock excavation, even though composed of relatively weak 

 rock, can in most cases be used effectively in the upstream portion of 

 the dam to reduce the quantity of required riprap. 



EMBANKMENT DESIGN AND CONSTRUCTION 



Stability of the embankment and foundation and seepage through 

 the foundation are the principal design and construction problems 

 that must be carefully considered by the soil mechanics engineer and 

 geologist. Only a general summary of these problems will be given 

 in this chapter because they have been adequately covered in other 

 publications (Creager, Justin, and Hinds, 1945; Middlebrooks and 

 Bertram, 1948; Middlebrooks, 1948). 



Factors affecting embankment stability are directly or indirectly 

 related to the shearing strength of the foundation and embankment. 

 The most critical period in the construction of the dam is just before 

 it is brought to grade or shortly thereafter. At this time, pore pres- 

 sures, due to consolidation in the embankment and foundation, are at 

 a maximum. If pore pressures do not develop, there is usually no 

 question concerning the stability of the embankment, since most soils 

 have adequate strength when fully consolidated. 



The effect of rapid drawdown on the upstream slope deserves care- 

 ful investigation. However, owing to the difficulty of accurately 

 determining the degree of saturation that occurs during the filling 

 period and the amount of drainage during the drawdown period, a 

 detailed analysis is not usually justified. The method of analysis 

 most generally employed on flood control dams is the instantaneous- 

 drawdown method. A minimum safety factor of 1.0 is considered 

 satisfactory in checking the slope stability for drawdown from maxi- 

 mum to minimum pool level. Where there is any doubt concerning 

 the possibility of developing pore pressure during construction, pie- 

 zometers should be installed in critical areas. 



In the analysis of both upstream and downstream embankment 

 slopes for stability during construction, a minimum safety factor of 

 1.50 is specified. For the stability of the upstream slope with reser- 

 voir empty and of the downstream slope with steady seepage from 

 full reservoir head, a minimum factor of safety varying from 1.5 



