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~~ <br /> <br />INTRODUCTION <br />Lafarge is currently seeking a revision to the approved Mining Limit of the Mann Lakes <br />operation, in Adams County. The property is bounded by 120a' Avenue on the north, the <br />South Platte River on the south and east and the Brantner Ditch on the west. The <br />proposed revision includes decreasing the setback from the centerline of the Brantner <br />Ditch from 200 ft to 140 ft. The rules and regulations of the Division of Minerals and <br />Geology require that any mining within a 200 foot setback of a man-made structure show <br />through engineering analysis that the proposed mining will not cause damage. The <br />accepted method of demonstrating this is through a slope stability analysis. This report <br />contains an overview of the geotechnical results and methodology used in the analysis of <br />the mining and reclaimed slopes and their affects on all man-made structures. <br />GEOTECHNICAL INVESTIGATION <br />In order to evaluate the slope stability of the proposed mining embankment, geotechnical <br />investigations were performed to obtain the soil types and classifications for the onsite <br />materials. No lab testing was done for on-site materials. Conservative soil strength <br />properties were estimated based using typical values for similar materials found in gravel <br />pits along the South Platte River in this region. Table 1 represents a summary of the soil <br />parameters that were used in the stability analysis. The thickness of the overburden and <br />weathered bedrock was assumed to average 3 ft. <br />Table 1. -Soil Properties <br />Description Max dry density <br />(pct) Saturated Density <br />(pct) Cohesion <br />(ps0 Direct Shear <br />(Friction angle) <br />Stable Bedrock 130 145 500 30 <br />Weathered <br />Bedrock 130 145 0 11 <br />Sand and Gtavel 125 138 0 32 <br />Overburden 110 120 0 25 <br />DESIGN ANALYSIS AND CRITERIA <br />The proposed mining slopes were analyzed using the XSTABL v5.lO5a computer <br />program. XSTABL was used to analyze the stability of the embankment during <br />construction. It was assumed that mining will result in a vertical high wall of undisturbed <br />alluvium azound the perimeter of the site. The proposed revision places this highwall <br />approximately 140 ft from the centerline of the Brantner Ditch. The phreatic surface was <br />assumed to follow a line extending from the toe of the highwall to the approximate water <br />surface elevation in the ditch. Gravel deposits near the Brantner Ditch are approximately <br />25 feet deep <br />