placed fills provide an interlocking and keying effect that permits creation of steeper 

 slopes. However, the rapid disintegration of the larger stones, and the death and rot- 

 ting of tree roots can result in significant stability reductions. 



Aside from mass stability requirements, erosion of finer particles from the sur- 

 face of steep slopes is considerably enhanced when the slopes are at, or near, the angle 

 of internal friction. Further, since revegetation of these slopes appears to be a dif- 

 ficult and slow process, such steep slopes are serious contributors of sediment for 

 extended periods. 



Water was another important factor contributing to instability. As demonstrated 

 in several of the stability analyses, complete saturation with equipotential lines as- 

 sumed normal to the ground surface results in appreciable reductions of safety factors. 

 Although seepage conditions are probably never quite so severe as assumed for some of 

 these analyses, neither are the safety factors in the assumed dry conditions sufficiently 

 large to compensate for even moderately adverse water conditions. Furthermore, there 

 is good evidence that with the passage of time after construction and considering the 

 weathering and compaction, the seepage conditions may worsen from the standpoint of 

 stability. Undoubtedly many of the road-associated failures could have been prevented 

 if surface water had been better handled. 



Minimal compactive effort in the road fills was evidenced by the low in-place 

 densities. The problem of liquefaction, a result of high void ratios in combination 

 with saturation, or near saturation, in these soils is apparent throughout the Zena 

 Creek area. There is little doubt that liquefaction occurred at practically every large 

 failure, as evidenced by the long transport distances (often several hundred yards) and 

 "mud" splattered on tree trunks up to heights of several feet. This is not just a prob- 

 lem in the road fills, however, as the in-place densities at shallow depths in the 

 natural materials were equally low. Nevertheless, even nominal compaction of the fills 

 during construction should have prevented some failures, and certainly should have re- 

 duced both the size of some of the failures and the ease of transport to the streams. 



Aside from the liquefaction problem, significantly lower strain energy is required 

 for failure in these loose, uncompacted materials. In denser materials, shear strain 

 must be accompanied by a corresponding volumetric increase. But in loose materials, 

 either at or above the critical void ratio, shearing can occur without volume change, 

 and volume decreases usually accompany failure. Thus, lack of compaction considerably 

 increases the chances for failure even in the dry condition. 



Besides the apparent lack of compaction control, other examples of poor construc- 

 tion practices were in evidence in the Zena Creek road system. At one site, a large 

 log was found embedded in the fill. The inclusions of logs and stumps in embankments 

 and fills should never be permitted. Loose surface materials should be compacted and 

 keyed if clean, or removed if not. It is doubtful that adequate clearing and grubbing 

 were performed in most locations. 



Cyclic displacement resulting from sonic booms is another factor which may be of 

 significance, but one for which very little information is presently available. During 

 the field investigation the authors experienced several unnerving shocks caused by the 

 supersonic aircraft that passed over the study area, usually two or three times each 

 day. It is suspected that these shocks may provide sufficient ground motion to initiate 

 liquefaction in some of the more critical slopes. The authors believe that this de- 

 serves some additional study; however, they do not believe that sonic booms would be 

 of much consequence to the behavior of adequately designed and compacted slopes. 



Finally, as discussed in connection with sites 917-3 and 917H-3, live trees prob- 

 ably are important factors influencing the stability of both natural slopes and road 

 fills. It is suspected that the live tree root systems exert a significant stabilizing 



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