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Mr. Jeremy Dueto <br /> February 2, 2021 <br /> Page 2 <br /> STABILITY ANALYSES <br /> Two(2)sections were evaluated at the tallest high wall in each mine cell based on the sections provided by Al. The <br /> computer program XSTABL was used for the analysis. The method for selecting the critical failure surface for each <br /> analyzed loading condition is the following. The Modified Bishop's Method of Analysis is used to find the critical failure <br /> surface by randomly searching with 20 termination points and 20 initiation points(400 failure circles)with 7 foot line <br /> segments over the slope surface to determine the lowest factor of safety. Seismic loading was obtained from the <br /> U.S.G.S. Unified Hazard Tool. Review of the Hazard Tool indicated a maximum horizontal acceleration of 0.071g with a <br /> return period of 2,475 years for the site. <br /> The two maximum cross section locations were selected based on the sections provided by Al and analyzed as <br /> described below. <br /> ► 3 horizontal to 1 vertical reclamation slope: This section was modeled with a top elevation of 4,607 feet that <br /> slopes at a 3:1 mine slope(horizontal to vertical) until reaching bedrock at an elevation of 4,517 feet(90 feet in <br /> height). A slurry wall was modeled 25 feet from mine limit. The slurry wall was also modeled 25 feet from the <br /> silt pond located exterior to the slurry wall. The slope of the silt pond was also modeled at 3 horizontal to 1 <br /> vertical. The stability analysis on this section was analyzed for reservoir stability requirements on the slope <br /> interior to the slurry wall. <br /> ► 2 horizontal to 1 vertical temporary silt pond slope: This section was modeled with a top elevation of 4,608 <br /> feet that slopes at a 2:1 mine slope(horizontal to vertical) until reaching bedrock at an elevation of 4,530 feet <br /> (78 feet in height). The stability analysis on this section was analyzed for temporary highwall requirements <br /> assuming Al will dewater the mine during mining and that the cell will eventually be filled with silt from wash <br /> plant operations. <br /> MATERIAL PROPERTIES <br /> The material index and engineering strengths assumed in this slope stability report are discussed below. <br /> Overburden <br /> The strength properties for the insitu silty to clayey sand overburden were based on field testing data and on our <br /> engineering judgment;the following parameters have been used to model the overburden. <br /> Dry Unit Moist Unit Saturated Unit Cohesion C'psf Friction Angle 0' <br /> Weight(pco Weight(pco Weight c <br /> Native 103 114 126 50 28 <br /> Alluvial Sand and Gravel <br /> The sand and gravel is generally a medium to coarse-grained sand that is medium dense to dense and locally gravelly. <br /> The alluvial sand and gravel was modeled as follows: <br /> Dry Unit Moist Unit Saturated Unit Cohesion C'psf Friction Angle 0' <br /> Weight(pco Weight(pco Weight c <br /> 119 129 130 0 35 <br /> Bedrock <br /> Bedrock below the alluvium is sandstone and claystone and interlaminated to interbedded claystone and sandstone <br /> bedrock. Sandstone is typically stronger than claystone. Claystone is generally a weak bedrock. To be conservative, <br /> we modeled the bedrock as claystone. For the claystone bedrock,two potential strength conditions were considered. <br /> These strength conditions are referred to as: 1) peak strength,and 2)residual strength, <br />