Again, it is noted that either the friction angle (<}>) or cohesion (c) has been under- 

 estimated, since safety factors less than 1.0 were calculated. In these analyses, an 

 estimated cohesive strength of 1.0 p.s.i., or 144 p.s.f., would have resulted in a 

 minimum computed safety factor of approximately 1.4. 



To evaluate the influence of error in the estimate of <j>, a series of analyses, again 

 assuming no seepage, were conducted with 4> = 38°, c = 0, and a = 36.3°. The resulting 

 safety factors are shown in figure 20. Thus, only moderate increases in either 4> or c , 

 or in both <}> and c, result in computed safety factors indicating stability. 



Finally, an extreme seepage condition wherein the top flow line is coincident with 

 the fill slope was analyzed by assuming the validity of the infinite slope stability 

 theory. Ordinarily, an arbitrary requirement that the length of failure be in excess 

 of 20 times its depth is made. Noting that the fill slope is 80 feet long, a depth to 

 failure of 4 feet is assumed. From the relationship: 



0^ = Y t H cos 2 i (tan i 



tan <}> d ), 



wh er e 



y = saturated, or total, unit weight 



H = vertical depth to failure surface 

 i = ground slope 



= buoyant unit weight 



<t>^ = friction angle 

 and letting 



Y = 134 p.c.f . , i = 36.9°, 

 Y, = 71.6 p.c.f. 



= 35' 



we compute a developed cohesion of 129.3 p.s.f., or 

 c , = 0.9 p.s.i. 



Figure 21. — Evidence of 

 incipient failure at 

 site 917-2 3 cracking 

 and settlement of 

 fill. 



