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Grand Island Kesources,LLC Page 15 <br /> Idaho Tunnel Portal—Slope Stability Analysis <br /> 4.0 ANALYSIS RESULTS <br /> Analysis of the slope under drained conditions indicates a minimum Factor of Safety(FoS)failure surface <br /> of 1.36 for a non-circular failure of the excavation slope. The minimum FoS for a failure surface which <br /> intersects the County Road was only slightly higher at 1.37. Figure 4 presents a summary of these stability <br /> analysis results and includes the critical failure surface as well as a summary plot of all trial failure surfaces <br /> color-coded by FoS. These analyses demonstrate that the lower FoS failure surfaces pass entirely through <br /> the colluvium and regolith soil materials due to the slope geometry. The position of the weak hard rock <br /> and decomposed rock beneath the slope has little to no effect on the overall stability. <br /> During spring thaw, several areas of seepage were observed coming through the shotcrete facing <br /> (Photograph 2). This seepage indicates that portions of the slope may become saturated during seasonally <br /> high water levels. The seepage is not present everywhere and does not discharge uniformly from the <br /> slope which suggests isolated areas of seepage flow rather than complete saturation. Table 1 presents a <br /> summary of perched water depth with the Alluvial Rock unit versus FoS. As indicated in this table, a <br /> perched water depth of 15 ft,or about half the thickness of the material, results in a FoS of 1.0. Figure 5 <br /> depicts the stability section with the assumed perched water table and critical failure surface from this <br /> analysis. <br /> Including the open collapse void in the stability section reduces the minimum FoS to 0.74 for a non-circular <br /> and 0.90 for a circular failure surface respectively,indicating a condition of instability. Since the slope was <br /> observed to be stable despite the presence of the tunnel and open void, these results serve to <br /> demonstrate that the shear strength adopted for the regolith and colluvial soils is conservatively low. <br /> These results are presented in the summary included in Table 1. <br /> Including the 200 psi cellular concrete void fill within the stability section increases the FoS considerably. <br /> The minimum FoS for all trial failure surfaces passing through the cellular void fill is above 4.0. <br /> Table 1—Stability Analysis Summary <br /> Water Depth Minimum FoS <br /> Model Casetft) Janbu, Bishop, <br /> non-circular circular <br /> Fully Drained 0 1.36 1.46 <br /> Perched Water 5 1.32 1.46 <br /> Perched Water 10 1.12 1.40 <br /> Perched Water 15 1.00 1.24 <br /> Open Void 0 0.74 0.90 <br /> Cellular Concrete 0 >4.0 I >4.0 <br /> Applied GeoLogic LLC 5/7/2020 <br />