(a) Fill slope 



(b) Cut slope 



(c) Embankment, 

 or through 

 fill 



Figure 5. — Failure types. 



A preliminary reconnaissance of the Zena Creek study area was made in May of 1967 

 to identify the extent and types of failures that occurred during the previously dis- 

 cussed 1965 storm event. As a convenience for analysis and discussion, four general 

 failure types were identified, as shown in figure 5. 



Based upon the preliminary reconnaissance, eight sites representing the three 

 failure types shown in figure 5(a), (b), and (d) were selected for intensive study 

 (i.e., fill-, cut-, and natural-slope failures, respectively). None of the embankment 

 failures (c) were deemed amenable to analysis, but one stable embankment was selected 

 for sampling and analysis. In addition, four other sampling sites were chosen for 

 analysis of material properties representative of alluvial and bank slough deposits. 

 All 13 sites are identified and briefly described in figure 4 and table 1. 



During the months of July and August of 1967, sampling, field testing, and mapping 

 were conducted at the selected sites. 



Seismic subsurface exploration was conducted at each of the failure sites and also 

 at the site of the stable embankment. In addition, some verification work was performed 

 by trenching with a tractor-mounted backhoe. Representative, disturbed samples were 

 obtained by manual excavation, some with the aid of the backhoe. Depths, sizes, and 

 other descriptions of these samples are given in table 2. In-place density tests were 

 performed with a balloon volumeter; these results are also shown in table 2. Finally, 

 the geometry at each of the sites was mapped and photographs were taken. 



Laboratory tests were performed to determine grain-size distribution, liquid limits, 

 and plastic limits for classification purposes. Results of these tests are given in 

 table 3. For the material passing the 3/8" sieve, the gradations of all samples, ex- 

 cluding samples which were obtained from alluvial or slough deposits, were within the 

 range shown by dashed lines in figure 6. Further, the extremes represented by the 

 dashed lines represent two samples obtained only 20 feet apart in natural soil. The 

 gradations of all other samples, representing both fills and natural deposits and re- 

 presenting a wide range of other conditions, were within the range shown by the solid 

 lines in figure 6. Because these materials are easily broken down, and thus the sieve 

 analyses are subject to error simply from variations in abrasive action during prepara- 

 tion and testing, the variabilities in grain-size distributions for samples passing the 

 3/8" sieve were judged negligible. 



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