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File No. AA-2001-046 Owens Brothers <br />519/02 <br />-3- <br />West SSA results indicate a factor of safety (FOS) of 3.261. The South SSA results <br />indicate a FOS of 2.468. These values are well above the 1.25 FOS required to show <br />no impact on structures. <br />The second scenario, identfied as West-2 and South-2 is the reclamation slope <br />configuration of 3:1(H:V) to ten feet below the expected water level and 2:1(H:V) <br />thereafter, a set back of 25 feet, and the expected water conditions of the pit water <br />surface elevation equal to the piezometric water surface elevation. The West SSA <br />results indicate a factor of safety (F05) of 3.091. The South SSA results indicate a FOS <br />of 2.485. These values are well above the 1.25 FOS required to show no impact on <br />structures. <br />A third scenario, identified as West-3 and South-3, was analyzed with slope <br />configuration of 3:1(H:V) to ten feet below the expected water level and 2:1(H:V) <br />thereafter, a set back of 25 feet, and a water conditions where drawdown occurs in the <br />pit resulting in a higher piezometric surface in the overburden. For a head difference of <br />5 feet between the water surface in the pit and the piezometric surface, the resulting <br />FOS for the West and South sections are 2.587 and 1.807, respectively. These FOS <br />values are well above the 1.25 FOS required to show no impact on structures. This <br />scenario is a conservative situation because the material in the overburden is coarse <br />grained, therefore quick draining, and creating a head difference of 5 feet in this material <br />would be difficult. <br />A fourth scenario, ident~ed as West-4 and South, was analyzed to model a situation <br />where the working slope face is inadvertently excavated to 1:1 (H:V). The set back for <br />this scenario is 25 feet, and the water surface is at the same elevation as the <br />piezometric surface. The resulting FOS for the West and South sections are 2.197 and <br />2.171, respectively, well above the 1.25 FOS required. <br />Conclusion <br />The recommended offset is 25 feet from the top of the excavation slope to the structure. <br />The analysis shows that the resulting factors of safety for the two sections are well <br />above the requirements to show no impact on structures. H the slopes were <br />inadvertently mined to steeper than 2:1 (H:V), or if a sudden pit water drawdown <br />occurred, the analysis shows that the slopes would remain stable. <br />Thank you for your time and consideration in this matter and if you have any questions <br />or comments, please feel free to call. <br />Sincerely, <br /> <br />M. Danelle Volpe, P.E. <br />Civil Engineer <br />Banks and Gesso, LLC <br />enclosures: <br />cc: 20023-Studies-Current-DMG-SSAnalysis <br />