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Mr. Mark Johnson <br /> January 24, 2020 <br /> Page 3 <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 structures around the Nix mine are,for the most part,considered critical structures. Discussions with the author of <br /> the memo, Mr.Tim Cazier, indicate the FOS will be adopted by the MLRB. The FOS are for both static and seismic <br /> (from an earthquake)stability analyses. For generalized strength assumptions and critical structures,a FOS of 1.5 is <br /> considered sufficient for static conditions and a FOS of 1.3 is considered suitable for seismic conditions. <br /> The stability of structures within 200 feet of the proposed mining limits was evaluated at nine(9)sections. The sections <br /> evaluated either at the tallest high wall or in the area of the closest structure in each mine cell under anticipated loading <br /> conditions around the perimeter of the site as discussed below. The computer program XSTABL was used for the <br /> analysis. The method for selecting the critical failure surface for each analyzed loading condition is the following. The <br /> Modified Bishop's Method of Analysis is used to find the critical failure surface by randomly searching with 20 termination <br /> points and 20 initiation points(400 failure circles)with 7 foot line segments over the slope surface and at the structure in <br /> question to determine the lowest factor of safety. Both static stability under anticipated mining conditions and seismic <br /> stability under peak ground acceleration loads were performed. Seismic loading was obtained from the U.S.G.S. Unified <br /> Hazard Tool. Review of the Hazard Tool indicated a maximum horizontal acceleration of 0.096g with a return period of <br /> 2,475 years for the site. <br /> The seven cross section locations were selected and analyzed as described below. All of the sections met adequate <br /> FOS as summarized below in Table 1. The section locations are shown on Figure 1. <br /> ► Cell 1: <br /> Section D This section is on the northwest side of Cell 1 and considers the tallest mine highwall of <br /> this cell where the mine highwall is closest to the Last Chance Ditch. The stability analysis for this <br /> section assumes a mine highwall sloped at 2 horizontal to 1 vertical(2H:1 V). The nearest structure is <br /> the Last Chance Ditch located approximately 31 feet to the northwest.The stability analysis on this <br /> section was run with potential failure circles initiating at the ditch and terminating in the area near the <br /> base of the mine highwall. <br /> Section F This section is on the west side of Cell 1 and considers the tallest highwall on the west <br /> side of the cell where the mine is closest to a fence. The stability analysis for this section assumes a <br /> mine highwall sloped at 2 horizontal to 1 vertical(2H:1 V). The nearest structure is the fence located <br /> approximately 25 feet to the west.The stability analysis on this section was run with potential failure <br /> circles initiating at the fence and terminating in the area near the base of the mine highwall. <br /> Section G This section is on the south side of Cell 1 and considers the case of the closest structure <br /> to the mine(the Rural Ditch).The stability analysis for this section assumes a mine highwall sloped at <br /> 2 horizontal to 1 vertical(2H:1 V). The nearest structure is the Rural Ditch located approximately 17 <br /> feet to the south.The stability analysis on this section was run with potential failure circles initiating at <br /> the Rural Ditch and terminating in the area near the base of the mine highwall. The analyses indicated <br /> a sufficient FOS. However,this FOS was the lowest of all the sections analyzed. The mine limit was <br /> moved another 20 feet to the north. The actual FOS will be greater than the FOS presented below <br /> (Table 1). <br /> ► Cell 2a: <br /> Section B This section is on the northeast side of Cell 2a and considers the case of the tallest <br /> highwall and the closest structure(a gravel road). The stability analysis for this section assumes the <br /> presence of a slurry wall and a mine highwall of 3H:1 V. The gravel road is located approximately 45 <br />