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and reclaimed slopes.
<br /> 3) Vertical highwalls during mining operations have been as high as eighty feet per
<br /> bench, and ninety feet or taller in Coto, and have (based on observations since
<br /> 2002 and observations at nearby operations in the same setting) been stable.
<br /> However, to comply with MSHA and good engineering standards, highwalls shall
<br /> be limited in the future to 40 feet height, with minimum bench width of 20 feet (with
<br /> vertical highwalls), with recommendations that highwalls have a maximum slope of
<br /> 1 H:IOV.
<br /> 4) On the perimeter of the site, a minimum setback from the permit
<br /> boundary/property line is recommended to provide access to the top of the
<br /> headwall and post-reclamation final slope, provide for drainage erosion and
<br /> sediment controls (DESC), and increase the factor of safety, including reducing
<br /> potential for impact beyond the affected land boundary. This is 20 feet on the
<br /> north and east sides, and 5 feet on the south boundary. This area may be used,
<br /> within limits, for some activities, provided requirements for other purposes are met.
<br /> 5) As discussed above, the cemented nature of the deposit allows a much higher
<br /> angle of repose, but assumes continued ground moisture and other conditions.
<br /> Therefore, it is necessary to protect this exposed, undisturbed material. Proposed
<br /> backfilling with selected material, to be compacted to standards, and then in turn
<br /> protected by a biotic layer (soil and vegetation) that will further anchor and
<br /> stabilize the slope, will protect the material. On-site shale, as well as materials
<br /> imported, have the necessary physical characteristics for this use, even though
<br /> disturbed and recompacted. The use of selected large materials (concrete
<br /> footing and foundation rubble, slabs, and use of large boulders of suitable rock) will
<br /> further strengthen and enhance the stability. Nevertheless, it is not recommended
<br /> that final slopes exceed 1:1 without further analysis and design of suitable backfill
<br /> reinforcement (such as geomembrane/geogrid) or retaining walls (such as mass or
<br /> deadman-anchored systems). (The use of grouting or rock facing would provide
<br /> protection and stability but not meet the required stated goal of providing wildlife
<br /> habitat on the slopes.)
<br /> 6) Information for slope stability analyses may include, but would not be limited to,
<br /> slope angles and configurations, compaction and density, physical characteristics
<br /> of earthen materials, pore pressure information, slope height, post-placement use
<br /> of site, and information on structures or facilities that could be adversely affected
<br /> by slope failure.
<br /> 7) Assumptions and estimates:
<br /> Layer Classification Dry Friction angle Cohesion Permeability
<br /> density (degrees) (psf) (fpm)
<br /> (psf)
<br /> Ex.Soil ML, SM 100 30 1000 0.1
<br /> Glacial till SM, ML 140 45+ 2500, 0.001
<br /> (deposit) in (cemented), CL (cemented) 1800
<br /> situ
<br /> PI. Soil ML, SM, SP, SW 115 30 500 0.1
<br /> Glacial till SM, ML, CL 150 45+ 2500 0.001
<br /> (deposit) (compacted)
<br /> placed
<br /> 8) As the profile is above the water table (saturated ground water conditions are not
<br /> present), pore pressure information is not applicable.
<br /> Technical Revision to M1985-001 J&J Pit, LaPlata County, dated 24 AUG 2016 Page 26
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