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Mr. Mark Johnson <br /> January 24, 2020 <br /> Page 5 <br /> Overburden <br /> The strength properties for the insitu silty to clayey sand overburden were based on field testing data and on our <br /> engineering judgment;the following parameters have been used to model the overburden. <br /> Dry Unit Moist Unit Saturated Unit Cohesion C'psf Friction Angle 0' <br /> Weight(pco Weight(pcO Weight WO <br /> Native 103 114 126 50 29 <br /> Alluvial Sand and Gravel <br /> The sand and gravel is generally a medium to coarse-grained sand that is medium dense to dense and locally gravelly. <br /> The alluvial sand and gravel was modeled as follows: <br /> Dry Unit Moist Unit Saturated Unit Cohesion C'psf Friction Angle Of <br /> Weight(pco Weight(pco Weight c <br /> 119 129 130 0 35 <br /> Bedrock <br /> Bedrock below the alluvium is predominately sandy claystone with local sandstone. Sandstone is typically stronger than <br /> claystone. Claystone is generally a weak bedrock. To be conservative,we modeled the bedrock as claystone. For the <br /> claystone bedrock,two potential strength conditions were considered. These strength conditions are referred to as: 1) <br /> 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 as a result 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 other recent projects and <br /> engineering judgment,the residual strength claystone was modeled in a one-foot thick layer overlying the peak strength <br /> bedrock as follows: <br /> Dry Unit Moist Unit Saturated Unit Friction Angle 0' <br /> Weight(pco Weight(pco Weight(pco Cohesion C'psf <br /> 116 Peak=126 Peak= 135 Peak=100 Peak=27 <br /> Residual=110 Residual= 133 Residual=0 Residual= 15 <br /> Soil-Bentonite Slurry Wall <br /> The proposed slurry wall will consist of a mix of the overburden clayey to silty sand,alluvial sand,and imported <br /> bentonite. The resulting mix will produce a non-Newtonian fluid with some shear strength characteristics based on a <br /> reduced friction angle of the overlying overburden. Based on engineering judgment,we modeled the slurry wall as <br /> follows: <br /> Dry Unit Moist Unit Saturated Unit Friction Angle 0' <br /> Weight(pco Weight(pco Weight(pcO Cohesion C psf <br /> NA 112 115 0 1 0 <br />