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Mr.Chris Oestreich <br /> May12,2023 <br /> Page 3 <br /> ► Plumb Ditch Section: This section is on the southwest comer of the mine where the Plumb Ditch is at its closest <br /> (-40 feet)to the mine limit. This section was conservatively analyzed with five(5)feet of sandy clay <br /> overburden overlying forty-one(41)feet of sand and gravel on top of claystone bedrock. A low permeability <br /> slurry wall is modelled 20 feet from the mine limit. The top three(3)feet of the claystone was modelled at <br /> residual strength reflecting the weathered nature at the top of the claystone. Unweathered claystone was <br /> modelled below the top 3 feet. This section was modeled for static conditions,static with a surcharge load, <br /> seismic conditions,and seismic with a surcharge load. As discussed below, all FOS required by the DRMS <br /> were met. The surcharge load modelled was a 10-foot high stockpile with a contact pressure of 1,200 pounds <br /> per square foot(psf). <br /> MATERIAL PROPERTIES <br /> The material index and engineering strengths assumed in this slope stability report are discussed below. <br /> Overburden <br /> The strength properties for the in-situ overburden were based on our experience at similar sites and engineering <br /> judgment;the following parameters have been used to model the overburden. <br /> Unit Weight Cohesion C'psf Friction Angle 010 <br /> c <br /> 114 50 26 <br /> Alluvial Sand and Gravel <br /> The sand and gravel is generally a fine to medium-grained sand overlying a fine to coarse grained sand that is typically <br /> medium dense and locally gravelly. The alluvial sand and gravel unit was modeled as follows: <br /> Unit Weight Cohesion C'psf Friction Angle 0'° <br /> 130 0 35 <br /> Bedrock <br /> Bedrock below the alluvium is claystone and sandstone. We conservatively modeled Claystone because it is weaker <br /> than sandstone. For the claystone bedrock,two potential strength conditions were considered. These strength <br /> conditions are referred to as: 1)peak strength,and 2)residual strength. <br /> Peak strength is the maximum shear strength the claystone bedrock exhibits. The shear strength is made up of both <br /> cohesion(diagenetic bonding)and internal friction. Under short-term conditions for unsheared claystone,peak strength <br /> governs behavior. If a sheared surface or sheared zone is present within claystone because of faulting,slippage <br /> between beds due to folding,past shrink-swell behavior,stress relief,weathering,or from a landslide,the cohesion along <br /> the sheared surface is reduced to zero,and the angle of internal friction is decreased,due to alignment of clay minerals <br /> parallel to the shear plane. Under these conditions a claystone exhibits its lowest strength known as residual strength. <br /> Residual strength bedrock occurs in discrete zones, parallel with the sheared surface or zone,whereas fully softened <br /> strength occurs over a broader area(not used in this modeling). Based on data from site investigations,the residual <br /> strength claystone was modeled in a 3-foot thick,weathered layer overlying the peak strength bedrock as follows: <br /> Unit Weight 'ps Friction Angle 0'° <br /> Cohesion C f g <br /> Peak=124 Peak=100 Peak=26 <br /> Residual=110 Residual=0 Residual=14 <br />