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Upper Pit 6900 Access Road I Design Summary American Gypsum <br /> For the road segment constructed in cut (from approximate Stations 0+10 to 2+50), the road will <br /> be excavated using dozers or excavators, assuming any rock encountered will be rippable and <br /> will not require drilling and blasting. Maximum anticipated cut depth is approximately 25 feet, <br /> located adjacent to the existing road. During construction, efforts will be made to minimize cut to <br /> the greatest extent possible while maintaining the original design intent. Where possible, <br /> excavated material will be used as road fill (cut to fill). The existing road must be maintained to <br /> continue accessing the Upper Pit during new road construction; the area between Stations 0+10 <br /> and 2+50 will be modified during construction to allow continued access to the Upper Pit via the <br /> existing road. <br /> The road will grade to drain at approximately 1% on either side of the road (from the road <br /> centerline) to prevent water accumulation on the road. Safety berms will be installed at the road <br /> edges (fill slopes only). Surface water runoff from the road will terminate in the natural drainage <br /> that reports to the existing sediment pond M501 as shown on Drawing 03 (Attachment A). <br /> 3.0 Road Slope Stability Analyses <br /> 3.1 Design Criteria <br /> The CO DRMS states that minimum slope stability factor of safety (FOS) requirements vary <br /> depending on level of investigation and engineering effort completed (CO DRMS, 2016). <br /> Table 3.1 summarizes minimum FOS requirements. <br /> TABLE 3.1: PIT SLOPE DESIGN CRITERIA(CO DRMS, 2016) <br /> Minimum FOS <br /> Type of Structure/ Consequence Generalized, Assumed, or Strength Measurements <br /> Failure Single Test Strength Resulting from Multiple <br /> Measurements Tests <br /> Non-Critical Structures-Static 1.3 1.25 <br /> Non-Critical Structures—Pseudo-Static 1.15 1.1 <br /> Critical Structures—Static 1.5 1.3 <br /> Critical Structures— Pseudo-Static 1.3 1.15 <br /> 3.2 Analyzed Sections <br /> Two sections were analyzed for slope stability: Section E, located on the west side of the haul <br /> road (highest cut section) and Section F, located on the east side of the road (highest fill section). <br /> Analyses assume groundwater is not present in the natural ground based on a geotechnical <br /> investigation Tierra Group performed in 2018. <br /> 3.3 Methodology <br /> Slope stability models were developed using SLOPE/W, part of the GeoStudio software <br /> (GEOSLOPE, 2018). SLOPE/W is a two-dimensional program that uses equilibrium principles to <br /> determine the lowest critical FOS for given material properties and geometry. Slope stability <br /> analyses used the Morgenstern-Price method, which satisfies both force and moment equilibrium. <br /> AG Upper Pit 6900 Access Rd 20181015 October 2018 Page 3 of 9 <br />