In the case of saturation, however, the appropriate relationship describing 

 stability is : 



Co ^b 



— v— n- - cos z i (tan i - tan <|>) . 



r t H Yt 



where now y - 118 p.c.f. and y^- 55 p.c.f. Assuming 4> = 35°, a cohesive strength of 



c = y H cos z i (tan i - tan <f>) = 148 p.s.f. 



t Y t 



= 1.0 p.s. i. 



is required for stability. Even if <j) = 38°, a cohesive strength of 133 p.s.f., or 

 approximately 0.9 p.s.i., would be needed. It is reasonable, therefore, to assume 

 that failure might have occurred without road construction. 



An additional factor to consider is that all timber had been removed from site 

 917H-3. Yet, adjacent to this site is equally steep terrain also intersected by the 

 road cut, but this terrain has not been denuded and remains stable. There is evidence 

 throughout the Zena Creek and adjacent watersheds that timber harvesting on steep 

 slopes, with subsequent destruction of the stabilizing effects of live root systems, may 

 contribute to the occurrence of shallow landslides. 



Finally, as mentioned above, this slide occurred in a moderate natural swale. The 

 existence of the swale indicates some form of structural weakness in the subsurface, 

 which ultimately might have led to deeper dissection. Thus, the failure at this site, 

 though undoubtedly hastened by construction and timber harvesting activities, might 

 easily be considered to be merely one of a series of inevitable geomorphic events lead- 

 ing to a deeper gully. 



Conclusions From Failure Investigations 



For the failures investigated, it must be emphasized that no single cause can be 

 isolated. Indeed, each individual failure was probably a result of a combination of 

 circumstances . 



Perhaps the natural instability of the terrain in the Zena Creek and similar 

 drainages is of greatest importance. In the absence of human activities, a climatic 

 event such as that which occurred in 1965 could be expected to result in landslides 

 even on undisturbed terrain, though the frequency of such occurrences is undoubtedly 

 increased manyfold as a result of road construction and timber harvesting operations. 

 The bedrock and soils are extremely frangible, and landslides are undoubtedly very 

 important as landforming processes in this region. Thus, it should be recognized that 

 soil is continuously undergoing displacement in the relatively rapid natural dissection 

 of the terrain. This recognition, however, should not provide rationalization for 

 the accelerated soil movement resulting from human exploitation. Instead, it should 

 supply an impetus for considerably greater care in the use of these lands. 



In every road-associated failure, slope steepness was an important factor contrib- 

 uting to instability. As a consequence of the laboratory tests, it appears that no 

 unretained slopes should be allowed in excess of 35°, or 70 percent, on road fills. 

 Although both natural and artificial slopes in excess of 35° can be found in many loca- 

 tions, it is believed that most of the necessary additional strength required to main- 

 tain these slopes is from more or less temporary sources. For example, live tree roots 

 extending into the deeper fractured zones undoubtedly provide some structural reinforce- 

 ment to the upper soil layers. Also, large fragmentary boulders embedded in newly 



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