influence due to both mechanical reinforcement and moisture depletion. To avoid future 

 distress, all stumps and large roots should be removed before placing road fills. It 

 is believed, however, that barriers of live trees should remain undisturbed immediately 

 below the toe of the fill slope and above the cut slope. In addition, it is suspected 

 that weathering of the subsurface materials may be accelerated after removing the 

 timber since daily temperature extremes are undoubtedly more severe after timber harvest. 

 Thus, it is not sufficient to consider only surface disturbances in the planning of 

 har zesting operations but, in addition, long range mass stability must also be an 

 important consideration. 



Summary and Recommendations 



An investigation of several slope failures in the Zena Creek watershed, located in 

 the Idaho Batholith, successfully explained the causes of failure and provided an 

 opportunity for a more critical evaluation of current road construction and harvesting 

 practices . 



A natural instability exists in the terrain at Zena Creek and in similar drain- 

 ages. In the absence of human activities, a climatic event such as occurred in 1965 

 could be expected to result in landslides even on undisturbed terrain, though the 

 frequency of such occurrences is increased manyfold as a result of certain harvesting 

 and road construction practices. It is this accelerated damage that must be greatly 

 minimized . 



In every road-associated failure, the steepness of both natural ground slope and 

 fill slope was an important factor contributing to failure. As a result of the labora- 

 tory tests and subsequent stability analyses, it can be concluded that no man-made 

 slopes exceeding 35° should be constructed without special provisions to assure stability. 



Aside from the mass stability problems, erosion of finer particles from surface 

 slopes is considerably enhanced for cohesionless materials when slopes are at or near 

 the angle of internal friction. 



The problems of liquefaction, a result of high void ratios in combination with 

 saturation, were common. The long transport distances of failed fill materials and the 

 presence of "mud" splashed high on tree trunks were irrefutable evidence that liquefac- 

 tion had occurred in conjunction with most failures. High void ratios were not just a 

 problem found in road fills, but were common at shallow depths in natural material. 

 Nevertheless, even nominal compaction of the fills could have prevented some of the 

 failures . 



In addition to the low in-place densities and inadequate control of surface and 

 subsurface flows, the presence of logs and other debris in the fills may have contrib- 

 uted to many of the road failures. 



Live roots and trees are probably important factors influencing the stability of 

 natural and artificial slopes. In addition, it is suspected that disintegration of the 

 subsurface materials may be accelerated by exposure to greater daily temperature ex- 

 tremes as a result of timber harvesting. 



The type of investigation conducted and reported upon herein probably cannot be 

 adopted as a routine procedure for all situations. Certainly one may find stable 

 natural and man-made slopes in the Zena Creek and similar drainages which are greater 

 than 70 percent. Similarly, failures of slopes less than 70 percent are possible. It 

 should be understood that the tests and analyses conducted in this type of investigation 

 are based upon extremely simplified models and assumptions of uniformity and homogeneity. 



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