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RULE 6.5: GEOTECHNICAL STABILITY EXHIBIT <br /> There are no known geologic hazards on the proposed site. Based on a slope stability analysis, <br /> buildings or other structures within 200' of the Hayden Gravel Pit mining area will not be <br /> affected by mining excavation. Sufficient buffers will be maintained to neighboring property <br /> lines. Maps C-2 and F-1 shows these buffers. Map C-3 shows the mining and reclamation <br /> slopes of the mine. A standardized mining and reclaimed slope was analyzed for stability based <br /> on the anticipated typical thickness of overburden (15) and sand and gravel (23') in the mining <br /> pods. <br /> The material properties are derived from Table 2.5 in the SME Mining Reference Handbook 3, as <br /> there is no site-specific sample 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. From the SME table, sand and gravel with a mixed grain size has an internal angle of <br /> friction of 45 degrees and no cohesion. A layer of overburden/topsoil lies atop of the sand and <br /> gravel. It is described loose sand with a mixed grain size which has an internal angle of friction <br /> of 34 degrees and no cohesion. <br /> The final mining (21­1:1V), final reclamation (3H:1V), and active highwall (near vertical) slopes <br /> were analyzed. Mining will be conducted at the active highwall angle until the highwall has <br /> reached the half-way point of the final mining slope. Then the vertical active highwall slope will <br /> be knocked down to the final mining slope of 2H:1 V. Then it will be backfilled with to the final <br /> reclamation slope of 3H:1V. <br /> 1 . Mining and Reclamation Slopes <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 Hayden Gravel Pit slope stability analysis, the Bishop's Method of <br /> Slices was the iterative calculation used, and the software GALENA was used to model slopes <br /> and calculate the factor of safety. Since all three pods onsite will be mining and reclaiming the <br /> same deposit a single, standardized slope can be used to evaluate the slope stability at the <br /> Hayden Gravel Pit. <br /> Figure GS-1 shows the standardized slopes evaluated in GALENA: the active vertical highwall, <br /> the full mining slope, and the permanent reclaimed slope. <br /> The lowest factor of safety for permanent slopes was 2.03 at Final Reclamation. While the <br /> Active Highwall has a Factor of Safety below the CDRMS permanent slope minimum (1.5), the <br /> 3 Original source: Houk and Bray 1977 <br /> Hayden Gravel Pit GS-1 Lewicki& Associates <br /> December 2022 <br />