2018-10-24_PERMIT FILE - M2018055 (6)

Home Browse Search

E Slope Stability Analysis June 29,2018 Page 5 surfaces.Non-circular failure surfaces were better suited to quantify the stability of Material Configurations 2 and 3,both of which contain a discrete weaker layer. 7.0 RESULTS AND DISCUSSION A total of three slope stability analyses were performed. Table 2 summarizes the lowest factor of safety generated by SLIDE associated with each analysis. A factor of safety equal to or greater than 1.5 is often considered satisfactory for permanent slopes. TABLE 2 - Slope Stability Factor of Safety Summary Topographic Material Failure Lowest Profile Configuration Surface Factor of Safety I Circular 1.5 A-A' 2 Non-circular, Block 1.6 3 Non-circular, Block 1.6 The slope stability analyses presented in this report evaluate the global stability of the future 2h:1 v slope at the Vail Valley Ranch potential gravel quarry site based on topographic profile A-A'. Figures A.5 - A.7 show the visual slope stability model outputs for each material configuration and failure surface combina- tion defined in Table 2. Four circular failure surfaces were analyzed to check the global stability of the slope for Material Configuration 1.They produced factors of safety ranging from 1.5 for shallower circular failure surfaces to about 2.2 for deeper failure surfaces. For Material Configurations 2 and 3, a non-circular block search was used in SLIDE to define failure sur- faces. A block search is a user-defined, non-circular option that can be tailored to consider discrete weak layers in the stratigraphic profile. Both block searches produced shallow failure surfaces that pass through the respective weak layers with factors of safety equal to about 1.6. The overburden soil is lower in strength than the underlying gravel/cobble material. Where substantial thicknesses of overburden soil are present at the top of the slope face, stability will be locally affected. Our analyses indicate localized factors of safety in the overburden soils of about 1.3 if excavated at 2h:ly. Recommendations for this condition are provided in the following section. 8.0 CONCLUSION The slope stability analyses results presented above indicate that the proposed 2h:ly excavated slopes in the cobble/gravel material should exhibit adequate factors of safety. The presence of fine-grained layers in the cobble/gravel material was conservatively modeled, and while these weaker layers produce small de- creases in the overall factor of safety,the decrease is not severe. We expect the overall factor of safety for the cobble/gravel slopes to be on the order of 1.5 or greater. Not surprisingly, the fine-grained overburden soil has a lower factor of safety when excavated at a 2h:Iv slope angle. In addition,this material will most likely be erodible. Stability concerns in this material may be addressed by excavating a slightly flatter slope angle in the overburden soil, on the order of 2.5h:ly. Alternately,the face of the excavation in the overburden soil could, in our opinion,be adequately stabilized