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Mr.Anthony Adza -4- March 24, 2016 <br /> Calculated <br /> Safety Recommended Minimum Safety <br /> Loading Condition Factor(') Factor <br /> Rapid Drawdown and Peak Bedrock 1.2 1.2 <br /> Strength <br /> Rapid Drawdown and Residual Bedrock 1 8(2) 1.0 <br /> Strength <br /> Notes: <br /> 1. Safety factors are presented for failure surfaces that extend beyond the property boundary. <br /> 2. The critical safety factor for this loading condition is 1.2,which corresponds to a failure surface that is <br /> located within the overburden clay. The presented safety factor of 1.8 corresponds to a failure surface that <br /> was forced to extend through the residual bedrock layer. <br /> Rapid drawdown is an unlikely loading condition for a gravel pit because the rate of drawdown is <br /> controlled by dewatering. If only steady state seepage loading is considered, acceptable safety factors <br /> could be provided beyond the property boundary if the mining highwall was located 110 feet from the <br /> property boundary. Model outputs are presented in Attachment C. <br /> RJH did not evaluate the local stability of the I H:1 V mining highwall. <br /> Recommendations <br /> Based on the analyses presented in this letter,we recommend that the mining highwall extend no <br /> closer than 175 feet from the property boundary at locations where there are neighboring structures <br /> within 200 feet of the mine. If mine dewatering is restricted to prevent rapid drawdown loading <br /> conditions,the highwall could extend as close as 110 feet from the property boundary. These <br /> recommendations are based on the highwall being no steeper than I H:I V and a seepage barrier being <br /> installed around the perimeter of the pit. <br /> If it is desirable to extend the mining highwall beyond the recommended limits, additional subsurface <br /> investigations and site-specific testing could be performed to support more rigorous stability analyses. <br /> Specifically, laboratory testing could be performed to more thoroughly evaluate the strength of the <br /> overburden clay. <br /> Please contact me if you have any questions about the information contained in this letter. <br /> Sincerely, <br /> RJH CONSULTANTS, INC <br /> Adam B. Prochaska, P.E.,Ph.D <br /> Project Engineer <br /> RJH/jmm <br /> 16109.16-03-10-Engineering Evaluation Letter doc <br />