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RULE 6.5: GEOTECHNICAL STABILITY EXHIBIT <br /> Colorado Geologic Survey mapping for the North River Pit shows the middle unit of the Pierre <br /> Shale underlays the gravel and eolian deposits onsite'. There are no known geologic hazards <br /> on the proposed site. Based on a slope stability analysis, buildings or other structures within <br /> 200-ft of the North River Pit affected area will not be affected by mining excavation. Sufficient <br /> buffers will be maintained between the slope crest and the permit boundary. A standard slope <br /> was analyzed for stability to determine if there was offsite impact within 200-ft of the mining <br /> area. <br /> The material properties are derived from Table 2.5 in the SME Mining Reference Handbook2, as <br /> there is no site-specific strength data of the material available. Therefore, all materials are <br /> matched to a classification from this table that best matches the materials in terms of <br /> description. The native alluvial material is best classified as sand and gravel with a mixed grain <br /> size. A layer of soil and sand lies atop the sand and gravel deposit. This material is best <br /> described as loose sand, mixed grain size. The Pierre Shale underlays the other materials and <br /> is defined as a soft sedimentary rock. A summary of the material properties can be seen in <br /> Table GS-1. <br /> Table GS-1. Material Properties <br /> Material Unit Weight(lbs/ft) Cohesion Friction Angle <br /> Sand and gravel,mixed grain size 110 0 45 <br /> Loose sand,mixed grain size 99 0 34 <br /> Soft sedimentary rock(shale) 110 20,000 25 <br /> The final reclamation (3H:1V) slope and active mining slope (OH:1V) were analyzed. <br /> 1 . Methodology <br /> Factor of Safety is expressed in terms of strength divided by stress as a ratio. It is arrived at by <br /> an iterative computer process where a slope failure is assumed, the strength and stress of that <br /> slope failure are calculated, and those values are compared to determine a lowest factor of <br /> safety. In the case of the North River Pit slope stability analysis, the Bishop's Method of Slices <br /> was the iterative calculation used, and the software GALENA was used to model slopes and <br /> calculate the factor of safety. One slope (see Figure GS-1) was analyzed to look at the factor of <br /> safety. <br /> ' Geologic Map of the Florence Quadrangle, Fremont County, Colorado. Colorado Geological Survey. <br /> 2018 <br /> 2 Original source. Hoek and Bray 1977 <br /> North River Pit GS-1 Il Lewicki& Associates <br /> April 2025 <br />