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Mr, Mark Johnson <br /> June 17, 2019 <br /> Page 2 <br /> STABILITY ANALYSES <br /> Recently, Division of Reclamation and Mining Safety(DRMS)staff drafted a policy regarding stability analyses of <br /> neighboring structures. The draft summarizes adequate factors of safety(FOS)for non-critical and critical structures. <br /> The power line is considered to be a critical structure. Discussions the author of the memo, Mr.Tim Cazier, indicate the <br /> FOS will be adopted by the MLRB. The FOS are for both static and seismic(from an earthquake)stability analyses. For <br /> generalized strength assumptions and critical structures, an FOS of 1.5 is considered sufficient for static conditions and <br /> an FOS of 1.3 is considered suitable for seismic conditions. <br /> The stability of the power line in Cell 2 was evaluated at one section considered the most critical due to its height and the <br /> presence of thicker, weaker,overburden, The section modeled was 10 feet of overburden sandy clay overlying 32 feet <br /> of sand and gravel, on top of weathered sandy claystone and unweathered sandy claystone bedrock. Because Cell 2 is <br /> already mined and reclaimed as a reservoir with 3:1 (h:v)slopes, we modeled both full and empty reservoir conditions. <br /> The 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 a broad range of the slope surface and at the power line. Both static stability under anticipated <br /> conditions and seismic stability under peak ground acceleration loads were performed. Seismic loading was obtained <br /> from the U.S.G.S. Unified Hazard Tool. Review of the Hazard Tool indicated a maximum horizontal acceleration of <br /> 0.085g with a return period of 2,475 years for the site. <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 sandy clay overburden were based on our engineering judgment; the following <br /> parameters have been used to model the overburden. <br /> Dry Unit Moist Unit Saturated Unit Cohesion C'psf Friction Angle V° <br /> Weight c Weight c Wei ht c <br /> 103 114 126 50 28 <br /> Alluvial Sand and Gravel <br /> The sand and gravel is generally a fine to coarse-grained sand locally grading to sandy gravel and is the thickest unit in <br /> the mine highwall. The alluvial sand and gravel was modeled as follows: <br /> Dry Unit Moist Unit I Saturated Unit Cohesion C'psf Friction Angle 0'0 <br /> Weight c Wei ht c Wei ht c <br /> 119 130 1 137 0 1 35 <br />