is no doubt that better and more rigorous assumptions regarding the subsurface flow 

 might have been made, or that more exact and detailed field tests might have been con- 

 ducted to improve the reliability of the analyses. However, neither time nor funds 

 would permit these desirable refinements, nor were such refinements necessary to 

 demonstrate the probability and predictability of the occurrences of failure in the 

 Zena Creek study area. The matter of ground water flow characteristics in the Idaho 

 Batholith is deserving of perhaps the greatest immediate research endeavor if develop- 

 ment of this region is to be continued and accelerated. 



Finally, the reliability or exactness depends upon the correctness of the analysis 

 procedures. All of the techniques used in this investigation are widely used and 

 accepted. However, as mentioned in connection with soil properties and seepage condi- 

 tions, many assumptions and idealizations are made which, to greater or lesser degrees, 

 depart from the actual field behavior. It is important to know whether these assump- 

 tions are made for convenience (ease of computation) or out of ignorance; but this 

 knowledge is not nearly so important as the fact that these assumptions are made and 

 that, therefore, another source of error must be recognized. It is beyond the scope of 

 this report to elaborate upon this matter except to note that, in general, the safety 

 factors obtained by the ordinary method of slices are conservative. That is, for design 

 purposes, this method usually will yield a lower computed safety factor than will most 

 other more refined methods. More important, the actual factor of safety will usually 

 be higher than the computed factor. Therefore, in analyses of the type performed in 

 this study, a computed safety factor less than 1.0 may, or may not, mean that failure 

 actually would occur. 



It must also be understood that the methods used in these analyses are based upon 

 the assumption that the soil mass behaves, or moves, as one or more discrete, solid 

 units. In fact, most soils behave plastically. Further, it is strongly suspected that 

 the dominant mode of failure in most of the cases discussed was liquefaction, wherein 

 the soil mass behaves neither elastically nor plastically, but rather as a heavy fluid. 

 No analytic techniques are yet available for completely describing this phenomenon, 

 although the conditions leading to liquefaction are reasonably well understood and 

 avoidable. 



Before proceeding with the analyses, it is appropriate to briefly discuss the 

 typical subsurface conditions found in the study area and, in particular, the assump- 

 tions regarding ground water conditions. A typical subsurface profile in the natural, 

 undisturbed state, is shown in figure 10. 



Referring to figure 10, the major portion of the flow that affects the stability 

 of slopes occurs above the competent rock, although this deeper bedrock is also 

 weathered and fractured and undoubtedly conducts considerable flow. What is referred 

 to as the weathered zone is actually a transition zone composed of highly fractured, 

 weathered material near its top and increasing in content, angularity, and size of 

 stones, and also in unit weight, from the top to the bottom of the layer. The topsoil 

 is almost without stones. Under certain climatic conditions and during certain times 

 of the year, it is probable that all of the subsurface materials are saturated, with 

 the top flow line coincident with the ground surface. This seepage condition would 

 be the most critical in, and was assumed in the analysis of, natural slopes. 



When the natural slope is interrupted by a road prism, the subsurface profile 

 appears typically as shown in figure 11. In this situation, part or all of the relevant 

 flow is interrupted. Above the cut slope, under the most critical conditions, the top 

 flow line is distorted as shown in figure 11. This distortion, or drawdown, is not so 

 critical as would be the case if the top flow line could be maintained coincident with 

 the surface of the slope. Hence, in analyses of the cut slopes, a conservative assump- 

 tion that no drawdown occurs was used. 



i 1 



