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Mr. Lee Sampson <br />March 28, 2022 <br />Page 3 <br /> <br />Agapito Associates, Inc. <br />is comprised of 32 to 37 ft of colluvium overlying the bedrock. The soil conditions consist of a <br />medium-stiff to hard, sandy to gravelly clay, with occasional clayey sand or gravelly layers to <br />depths of 7 to 22 ft. At deeper depths, the clay was medium-stiff to stiff down 27 to 37 ft. <br /> <br />The bedrock consists of very low- to moderate-strength interbedded sandstone and claystone. <br />These units are typical of the Williams Fork Formation encountered at the mine site, where the <br />formation consists of interbedded sandstone, siltstone, claystone, shale, and coal. <br /> <br />Groundwater was measured at depths of approximately 16 to 22 ft below the existing ground <br />surface during drilling in the boreholes. Shannon and Wilson2 noted that groundwater conditions <br />during drilling were not considered a static measurement, but rather reflect short-term conditions. <br />Therefore, groundwater fluctuations may occur depending on the time of year and local conditions <br />particular to the site. <br /> <br />The spoil itself will comprise excavated bedrock consisting of sandstone, siltstone, and mudstone. <br />The unsorted spoil at the mine has previously been estimated to have the following gradational <br />characteristics4: <br /> <br /> Cobbles and boulders 50% <br /> Gravel 28% <br /> Sand 2% <br /> Fines (No. 200 sieve) 20% <br /> <br />Significant sorting of the spoil is however anticipated from end-dumping over the edge of the pile. <br />Previous work at other areas of the mine indicated that the spoil material has a steep overall angle <br />of repose of around 38°.2 <br /> <br />Table 1 summarizes the mean geotechnical properties for the spoil and soil obtained from <br />laboratory testing.2 As the geotechnical conditions for the site have not changed since the 2009 <br />study, Agapito considers these properties appropriate for the updated assessment. <br /> <br />Table 1. Mean Spoil and Spoil Properties from Laboratory Testing <br />Material Unit Weight (pcf) Friction Angle (°) Cohesion (psf) <br />Spoil 110 35 750 <br />Clay 125 31 0 <br />pcf = pounds per cubic foot; psf = pounds per square foot <br />SLOPE STABILITY ANALYSIS <br />The stability analysis was carried out using the limit-equilibrium software SLOPE/W.1 As this <br />program utilizes a two-dimensional (2D) analysis, two sections lines were selected to assess the <br /> <br />4 CTL/Thompson, Inc. (1992), “Underdrain Filter Requirements, West Pit and Section 16 Pit Excess Spoil Fills,” letter <br />report to Colowyo Coal Company, job no. 18,683, April 27th.