Figure 17. — Assumed conditions for L. Casagrande solution at site 917-1. 



Site 917-1 was included in the investigation primarily for its comparative value 

 and also to serve as a check on the stability analysis techniques and measured soil- 

 strength parameters. Because a safety factor of unity separates stability from 

 instability, theoretically no hypothetical failure surface with a safety factor less 

 than 1.0 should exist in this embankment. 



Based upon the data available, a representative profile of this site is shown in 

 figure 16. 



For an initial analysis, an in-place dry unit weight of 115 pounds per cubic foot 

 (p.c.f.) is assumed (noting from table 2 that the dry unit weights found at sampling 

 sites 917-1-4 and 917-1-5 were 119 p.c.f. and 106.3 p.c.f - ., respectively). The corre- 

 sponding void ratio, e, would be 0.44, and saturated unit weight, y , would be 134 p.c.f. 



A conservative analysis can be performed by assuming a plugged culvert and subsur- 

 face flow as shown in figure 12. In this analysis, the L. Casagrande solution for the 

 top flow line through a triangular dam is used 2 (see Taylor, p. 181) . It is arbitrarily 

 assumed that the upstream and downstream slopes of the dam are equal. Figure 17 depicts 

 the assumed cross section. 



A series of analyses were conducted, assuming circular failure arcs and using the 

 method of slices. In all cases, failure arcs were assumed to pass through the toe of 

 the fill slope. The material above the top flow line was assumed to weigh the same as 

 that below the top flow line, or 134 p.c.f. The results of these analyses, assuming 

 that c = and <}> = 35°, are shown in figure 18 and, alternatively, in figure 19, 

 wherein the parameters R and a are as shown in figure 17. Safety factors significantly 

 lower than 1.0 are noted in figures 18 and 19. Three major conclusions can be drawn 

 from these analyses : 



(1) The assumed friction angle of 35° may be too low. 



(2) The material actually possesses a minimum cohesive strength, 

 even when saturated. 



(3) The assumed seepage conditions are significantly more severe than 

 would ever arise naturally. 



20 



