Although the effective friction angle in the field may exceed 35°, it is believed 

 that the angle stated in the first conclusion is a reasonable estimate based upon the 

 consistency of the laboratory results. Further, where the safety factors are as low 

 as 0.7, a friction angle of 45° would be necessary in the absence of cohesion to 

 increase the safety factor to 1.0. It seems inconceivable that the estimate of the 

 friction angle would be this much in error. 



With regard to the second conclusion, it seems possible that this material may 

 possess sufficient cohesive strength, even when saturated, to assure stability at this 

 site. The computed maximum cohesion required for a safety factor of 1.0, among all of 

 the trial failure arcs, was only 175 pounds per square foot (p.s.f.), or about 1.2 

 pounds per square inch (p.s.i.), assuming $ = 35°. Thus, although it has been con- 

 cluded that no cohesive strength should be used for design purposes, it must be recalled 

 that even in the saturated triaxial tests on loose material, a cohesive strength of 

 1 p.s.i. was estimated. Obviously, therefore, some moderate cohesion, combined with 

 a slightly greater estimate for the friction angle, would assure computed safety factors 

 in excess of 1.0 for all the trial failure arcs in this analysis. 



The third conclusion is undoubtedly correct. There is no evidence that the culvert 

 has ever become plugged at site 917-1. Even in the event of such an occurrence, it is 

 doubtful that a stable seepage condition as severe as that assumed in these analyses 

 would arise. However, since the exact flow pattern cannot be predicted, a series of 

 analyses were conducted assuming no seepage. This was done in order to establish a 

 minimum upper limit for the safety factors at this site. The results of these analyses 

 are shown in figure 20, where the parameters R and a are as defined in figure 17. 



1.8 r 



22 



