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Upper Pit 6900 Access Road I Design Summary American Gypsum <br /> Circular failure modes were analyzed using user input ranges. Overall slopes were evaluated for <br /> static and pseudo-static conditions. <br /> Static conditions evaluate the road under normal loading conditions, while pseudo-static <br /> conditions evaluate the road under seismic (earthquake) loading by applying a horizontal <br /> acceleration to the stability model. Based on a United States Geologic Survey (USGS) Custom <br /> Hazard Map for the area (USGS, 2018), the Peak Ground Acceleration (PGA) corresponding with <br /> 10% probability of exceedance and 475-year return period is 0.072g. The horizontal acceleration <br /> used in the analyses is one-half of the PGA or 0.036g. <br /> 3.4 Material Properties <br /> Material properties for slope stability analyses were based on results from Tierra Group's early <br /> 2018 geotechnical investigation (including drilling and rock strength testing) near the Upper Pit <br /> and Tierra Group's 2015 bulk sample from an on-site intraburden stockpile. Foundation material <br /> properties were conservatively assumed to match rock categorized as "Poor Rock"from the 2018 <br /> geotechnical investigation. Rock properties were derived using laboratory data (uniaxial <br /> compressive strength test) and borehole log descriptions (Geological Strength Index (GSI) <br /> (Marino and Hoek, 2000) and disturbance factor), as well as RocLab software (intact rock <br /> parameter, Rocscience, 2011). These properties were input into SLOPE/VV software to create a <br /> Shear/Normal strength functions for the foundation material. Material properties for the road fill <br /> (Structural Fill) are from a direct shear test performed on the 2015 bulk sample. For the bulk <br /> sample, the cohesion was decreased by just over 50% to account for material variability. <br /> Table 3.2 summarizes the material properties used in the analyses and Attachment B contains <br /> laboratory test results. <br /> TABLE 3.2: MATERIAL STRENGTH PROPERTIES <br /> Unconfined Intact <br /> Unit Friction <br /> Compressive pi Rock Disturbance Cohesion <br /> Material Weight Strength GSI Parameter Factor (psf) Angle <br /> (pcf) 0(psf) (RocLab) <br /> Foundation 125 62,656 40 12 0.7 <br /> Structural Fill 122 - - - - 700 34.1 <br /> Note: [1]GSI is a system of rock mass characterization used to better describe rock mass properties in numerical <br /> analyses. <br /> 3.5 Model Results <br /> Table 3.3 presents the analyses results. Attachment C contains slope stability analyses output. <br /> TABLE 3.3: SLOPE STABILITY RESULTS <br /> Resulting FOS <br /> Analyzed Section <br /> Static Pseudo-Static <br /> Section E 2.77 2.58 <br /> Section F 1.55 1.44 <br /> AG Upper Pit 6900 Access Rd 20181015 October 2018 Page 4 of 9 <br />