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Mr. Anthony Adza -3- March 24,2016 Table 1 Material Properties Selected Peak Residual Moist Drained Drained Unit Saturated Friction Drained Friction Undrained Material Weight Unit Weight Angle cohesion Angle Strength cf) (Pcf) (de sf de sf Overburden Clay120 125 28 0 N/A 500 Sand and Gravel 122 132 30 0 N/A N/A Bedrock N/A 140 22 500 14 5000 Barrier Wall N/A 125 28 0 N/A 500 LOADING CONDITIONS Stability analyses were performed for steady state seepage and rapid drawdown loading conditions. Steady state seepage loading considered that groundwater exterior of the barrier wall was at its natural level and groundwater interior of the barrier wall had been dewatered to the top of bedrock. Rapid drawdown loading considered that water interior of the barrier wall had been quickly drawn down from the natural groundwater level to the top of bedrock and pore pressures interior of the barrier wall did not have time to dissipate. We conservatively considered that negligible pore pressure dissipation occurred interior of the barrier wall during rapid drawdown. Peak drained strengths were used for evaluation of steady state seepage. Stability during rapid drawdown was evaluated using bilinear strength envelopes that used the lesser of the drained strength and undrained strength. This technique was used to conservatively prevent the use of undrained strengths that exceeded the drained strength at low stress levels. Additionally,we evaluated both loading conditions considering that the upper 5 feet of bedrock was at residual drained strength, which is very conservative. RESULTS Analyses were performed to identify the recommended setback from the property boundary to the pit highwall such that the slope stability safety factors beyond the property boundary exceeded industry- accepted minimum values,which are provided in Table 2. We identified that the highwall would need to be at least 175 feet from the property boundary;this setback is controlled by rapid drawdown loading and the undrained strength of the overburden clay. A summary of analysis results is presented in Table 2. Model outputs are presented in Attachment C. Table 2 Slope Stability Safety Factors for a Highwall 176 Feet from the Property Boundary Calculated Safety Recommended Minimum Safety Loading Condition Factor(l) Factor Steady State Seepage and Peak Bedrock 2.1 1.5 Strength Steady State Seepage and Residual Bedrock Strength I 2.0 1.0 16109-16-03-1 0-Engineering Evaluation Letter