cut-and-fill-slope failures. In addition, a sliding block or wedge analysis was used 

 to analyze some of the cut- and fill-slope failures wherein the failure surfaces could 

 reasonably be considered planar. 



To satisfy requirements of completeness and brevity, only three stability analyses 

 are presented and discussed in this paper. Those included were selected primarily for 

 their illustrative value. Complete descriptions of all the sites and analyses may be 

 found in the original report of this investigation. 1 



The reliability of any analysis of the types conducted herein depends upon five 

 fundamental factors : 



1. Accurate description of the geometry (i.e., topography and 

 cross section); 



2. accurate knowledge of the soil properties (i.e., shear strength and 

 unit weight); 



3. correct definition of external loads, if any (e.g., vehicle or structural 

 foundation loads); 



4. correct description of ground water or seepage conditions (i.e., pore 

 water pressures); and, 



5. correct method of analysis. 



As was mentioned previously, there had been significant alterations of the topography 

 during the period between time of failure and time of investigation. Undoubtedly, 

 subsurface conditions had also undergone some changes during this interim. Neverthe- 

 less, reasonable confidence is placed in the accuracy of the topography and subsurface 

 measurements . 



The tests for soil properties in the Zena Creek study area have previously been 

 described, and their results discussed. Admittedly, there are questions about the 

 accuracy of the measured soil properties. The reader is cautioned not to extrapolate, 

 or place excess confidence in these results for purposes of decisionmaking in other 

 regions of the Idaho Batholith. Nor should any of these results be considered abso- 

 lutely correct within the Zena Creek study area. Often in studies of this type, there 

 is a strong tendency to question the method of analysis when discrepancies between the 

 "prototype" and "model" are discovered. However, it must always be remembered that the 

 assumed strength properties have been determined under laboratory conditions which never 

 perfectly duplicate the actual field conditions. Further, it must be realized that the 

 field conditions are continually changing due to geologic and climatic variations, and 

 thus the soil properties are also continually changing. Therefore, it will be noted 

 that the analyses have been conducted to account for variations in the soil properties. 



External loads were not considered in the analyses because of poor definition. 

 However, it is believed that the "rain on snow" so often referred to by many observers 

 constituted a significant load on the road fills and denuded slopes. Most references to 

 this condition emphasize the soil saturation aspects and neglect the importance of its 

 weight. 



One other factor, which is discussed briefly in the conclusions of the analyses, is 

 the effect of sonic booms. There is a strong suspicion that these shock sources might 

 also have played an important role as external loads leading to failure. 



The description of seepage conditions in this region is undoubtedly the most 

 questionable and complex factor detracting from the reliability of the analyses. There 



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