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Knight Piesold Knight Piesold and Co. <br /> CONSULTING <br /> 1999 Broadway, Suite 600 <br /> Denver, Colorado 80202-5706 USA <br /> Telephone: (303) 629-8788 <br /> Facsimile: (303)629-8789 <br /> November 3, 2017 E-mail: denver@knightpiesold.com <br /> American Gypsum Company KP Project No.: DV101-00100/04 <br /> PO BOX 980, 740 Hwy 6 KP Doc. No.: DV-17-1390 <br /> Gypsum, CO 81637 <br /> Attn: Mr. Jared Rhea <br /> Subject: Eagle Mine <br /> West Pit Area Backfill and Reclamation Slopes Stability <br /> Dear Mr. Rhea: <br /> Knight Piesold and Company(Knight Piesold) is pleased to present this letter reviewing the slope stability <br /> of the West Pit area and its surrounding disturbance relative to the planned pit expansion, pit backfill and <br /> related disturbance slope mitigation. The letter aims to support the American Gypsum Company (AGC) <br /> response to Division of Reclamation, Mining and Safety(DRMS) regarding their design standard of care for <br /> slope stability/geotechnical analysis as documented in the DRMS Memorandum dated March 31, 2016. <br /> The DRMS memo outlines a basis and a general methodology to assess the stability of slopes that could <br /> present a hazard to the public, the environment or public infrastructure. Within their memo, minimum <br /> standards for slope stability factor of safety are provided and these vary for specific cases. In the absence <br /> of validated geotechnical information for the mined waste materials and the native alluvial or colluvial <br /> materials mantling the ground surface. In this circumstance, the DRMS memo allows an approach that <br /> utilizes generalized or assumed material parameters(i.e. shear strength inputs)and in turn requires slightly <br /> higher factors of safety(FoS). This approach is adopted for the review within this letter <br /> Figure 1 illustrates the West Pit area and its proposed post-backfilled/post-reclaimed configuration. Within <br /> this figure are shown two cross section positions that have been analyzed for slope stability for purposes <br /> of addressing the DRMS mandate. <br /> A limit equilibrium-based slope stability analysis was conducted on these two cross sections utilizing the <br /> SLOPE/W Version 8.0 software package. Material input parameters were selected consulting the technical <br /> literature and applying professional judgment. This is considered to satisfy the methodology of employing <br /> assumed material strength parameters as indicated in the DRMS memo.Accordingly,the targeted minimum <br /> FoS for the results will be compared with the DRMS values of 1.5 for static and 1.3 for pseudostatic analysis. <br /> The output from the slope stability analyses is attached. <br /> Analysis Summary <br /> Two indicative slope segments were analyzed using SLOPE/W for static and pseudostatic conditions. The <br /> areas are assumed to comprise either intact or mined and stockpiled anhydrite soil/rock. Cross section I is <br /> within the perimeter of the mine pit and entails a wedge of the mined waste material placed against the <br /> western pit highwall and the toe of the wedge is situated over the backfilled pit floor. Cross section J is <br /> located west of the pit on a ridge where mined waste material was placed in the past. Within this footprint <br /> intersected by this cross section, the mined waste is presumed to overlie a shallow layer of alluvial or <br /> colluvial soils overlying bedrock. Within the general proximity of the mine pit, the conditions are assumed <br /> to be relatively consistent.The materials identified are therefore either similar to intact anhydrite of the mine <br /> Martagemeni system carteed by <br /> ,SO 900,.!S 585661 <br /> ISO 14001-EMS 583662 <br /> OHSAS 18001 ONS 585663 <br />