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Mr. Jeremy Dueto February 2, 2021 Page 3 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 as a result 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 other recent projects and engineering judgment,the residual strength claystone was modeled in a one-foot thick layer overlying the peak strength bedrock as follows: Dry Unit Moist Unit Saturated Unit Friction Angle 0' Weight(pco Weight(pco Weight(pco Cohesion C'psf 116 Peak= 126 Peak= 135 Peak= 100 Peak=26 Residual= 110 Residual= 133 Residual=50 Residual= 18 Soil-Bentonite Slurry Wall The proposed slurry wall will consist of a mix of the overburden clayey to silty sand, 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: Dry Unit Moist Unit Saturated Unit Friction Angle 0' Weight(pco Weight(pco Weight(pco Cohesion C'psf NA 112 115 0 0 STABILITY ANALYSES RESULTS The stability analyses assumed the mining will be per the mine plan. Dry mining will occur in the main cell as the water level in this cell will be controlled by slurry walls. The perimeter mine slopes of the main cell will be no steeper than 3H:lV. Dry mining will also occur in the silt pond cell due to dewatering activities. The silt pond cells will be no steeper the 2H:1 V. The factors of safety shown below is the minimum factor of safety of the conditions listed above. Summary of Stability Analysis Results Section Scenario Calculated Required Factor of Safety Factor of Safety Empty Reservoir 1.8 1.5 3H:1V;Main Reservoir Full Reservoir 2.1 1.5 Rapid Drawdown 1.3 1.2 Seismic 1.5 1.0 2H:1V;Silt Pond Temporary Mine Slope 1.4 1.3 1. Factor of safety using the Modified Bishop limit equilibrium method. 2. Seismic Analyses were modeled with a horizontal acceleration of 0.071 g using the Uniform Hazard Analysis.