1.4 



1.2 — 



.2 — 



Q I III I I I I l I I I I ''III I I I I I I I ' l I I I I I I I I 



1 10 10 : 10 3 10 J 



PERMEABILITY, K ; „ (CM/SEC. X 10" ), AT WATER TEMPERATURE OF 20° C. 



Figure 9. — Permeability test results. 



DISCUSSION OF TEST RESULTS 



In any investigation of this type, the major objective of the laboratory testing 

 program should be to simulate the actual field conditions as closely as possible. In- 

 sofar as available equipment and time permitted, this was done. However, as has been 

 previously indicated, a period of over two years had elapsed between the rain-on-snow 

 event that caused failures at Zena Creek and the beginning of investigation. Erosion 

 and growth of new foliage had significantly altered both the geometry and soil conditions 

 at the failure sites in the interim, and in most cases hardly any traces of the original 

 material remained at or near the sites. Furthermore, fabrication of samples of this 

 material in the laboratory at densities as low as are found in the study area was an 

 extremely difficult task and often met with failure. Thus, in regard to the tests and 

 their results, considerably more work must be done to fully describe these materials and 

 to establish adequate criteria for the design of stable roads and safe harvesting 

 practices in the Zena Creek and similar areas. 



For design purposes in this area, it appears that reasonable values are 35° and 

 for the angle of internal friction (4>) and cohesive strength (c), respectively. These 

 values are based upon the results of the triaxial compression tests, some of which are 

 shown in figure 8. Although the strength envelopes indicated slight cohesive strength 

 in most of the tests on saturated specimens, it is doubtful that any permanent cohesion 

 can be assumed for long-term design purposes. Further, in most cases, it is believed 

 that some of the apparent cohesion measured in the triaxial tests on saturated specimens 

 could be accounted for by corrections for membrane resistance; no corrections were made 

 for this factor. 



The value of 35° for the friction angle is undoubtedly conservative, but not exces- 

 sively so. For analyses of the type conducted here, the plane strain test is more suit- 

 able for determining strength parameters and for simulating conditions along the failure 

 plane, but the apparatus necessary for conducting such tests was not available. It is 



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