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1. Introduction <br /> Slope stability analysis is required for buildings or other structures within 200-feet of the <br /> proposed mine slopes as part of the 112 (C) permit application (refer to Figure 1). <br /> Approximately 10 to 15-feet of material will be removed for the project with 4:1 slope along <br /> the eastern side of the project and 3:1 slopes elsewhere. <br /> The purpose of this report is to provide a summary of the geologic and geotechnical models <br /> developed and discuss the slope stability factor of safety evaluation during construction and <br /> after construction. The scope of this project included: <br /> • Development of a geological and geotechnical model <br /> • Complete a slope stability model using Slope/W <br /> • Evaluate the safety of factor for the proposed mine. <br /> 2. Engineering Geology Cross Section <br /> The local geology in the area consists of highly to moderately weathered, Pierre Shale <br /> overlain with 12 to 15 feet of low plasticity clay. The highly weathered portions of the Pierre <br /> Shale are composed of very stiff, low plasticity, medium hard clay. Highly weathered, low <br /> plasticity Pierre shale (claystone) was observed about 12 to 15-feet below clay within our <br /> borings drilled near the top of the slope at the northeast corner of the site. The claystone <br /> has a blocky structure and is intensely to highly fractured. <br /> Pierre shale material varies from claystone to cemented dark gray shale. Some large clear to <br /> cloudy crystals are found withing the shale, likely to be selenite. <br /> Strike and dip angles of the shale bedding planes were not observed. Generally, bedding <br /> planes in shale typically found in the area are short and bedding plane slides are unlikely due <br /> to the current physical configuration of the site. <br /> Upper-level clays were stiff to very stiff, low plasticity, dry to moist, and brown. Refer to <br /> Table 1 below for material properties used in the analysis. <br /> Table 1. Local Geological Materials and Properties <br /> Material Name Material Description Material Properties <br /> Pierre Shale Moderately weathered, Unit Weight (pcf) = 122 <br /> blocky, moist, brown Cohesion (psf) = 600 <br /> claystone Phi (°) = 26 <br /> Native Clay Stiff to very stiff(medium Unit Weight (pcf) = 105 <br /> dense), dry to moist, brown Cohesion (psf) = 0 <br /> sandy clay and clayey sand Phi (°) = 22 <br /> Six (6) sections (Sections A through F) were analyzed for this study. Groundwater was not <br /> encountered in our seven (7) borings to depths ranging up to 25 feet below the existing <br /> 20-8315-Native / Pierre Mine 12/10/2020 4:28 AM <br />