2023-02-08_REVISION - M2003016 (2)

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Mr. Brennan Middleton September 30,2022 Page 4 Bedrock Bedrock below the alluvium is claystone. Claystone is generally a weak bedrock. For the claystone bedrock,two potential strength conditions were considered. These strength conditions are referred to as: 1)peak strength,and 2) residual strength. Peak strength is the maximum shear strength the claystone bedrock exhibits. The shear strength is made up of both cohesion (diagenetic bonding)and internal friction. Under short-term conditions for unsheared claystone,peak strength governs behavior. If a sheared surface or sheared zone is present within claystone because of faulting,slippage between beds due to folding,past shrink-swell behavior,stress relief,weathering,or from a landslide,the cohesion along the sheared surface is reduced to zero,and the angle of internal friction is decreased,due to alignment of clay minerals parallel to the shear plane. Under these conditions a claystone exhibits its lowest strength known as residual strength. Residual strength bedrock occurs in discrete zones, parallel with the sheared surface or zone,whereas fully softened strength occurs over a broader area(not used in this modeling). Based on data from site investigations,the residual strength claystone was modeled in a 1-foot thick,weathered layer overlying the peak strength bedrock as follows: Unit Weight Cohesion Cpsf Friction Angle 0'° (pc� Peak= 115 Peak= 100 Peak=24 Residual= 110 Residual=0 Residual= 17 Soil-Bentonite Slurry Wall The proposed slung wall will consist of a mix of the overburden, alluvial sand, and imported bentonite. The resulting mix will produce a non-Newtonian fluid with some shear strength characteristics based on a reduced friction angle of the overlying overburden. Based on engineering judgment,we modeled the slurry wall as follows: Unit Weight Friction Angle 0'° *0 Cohesion C psf g 105 0 0 STABILITY ANALYSES RESULTS The stability analyses assumed the mining will be per the previously mined slope. The plan includes dry mining in the cells as the water level in the cell is controlled by the slung wall. Mine slopes within the slurry wall lined cell will not exceed 3h:1 v. Setbacks listed in Table 1 (below)indicate the setback from the structure to the existing mine slopes. The factor of safety shown below is the minimum factor of safety of the three conditions listed above. TABLE 1 -SLOPE STABILITY RESULTS AND SETBACKS Location Critical Structure Static Factor Seismic Factor of DRMS Draft FOS (height(ft), Structure Setback From of Safety at Safety at Structure Requirement sloe) Mine Limit ft Structure (0.09g horizontal Static/Quake South(22', Underdrain 30 2.44 1.72 1.511.3 3h:1 v Southeast(28', Concrete 39 2.89 2.10 1.511.3 3h:1v) Ditch