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ACOM <br /> March 29, 2018 <br /> Mr. Brandon Guite <br /> Climax Molybdenum Company <br /> Henderson Mill <br /> 19302 County Road 3 <br /> Parshall, CO 80468 <br /> Subject: Response to DRMS comments for 3 Dam Buttress Stage 1 Design,Henderson <br /> Mill Tailings Dam,near Parshall, Colorado <br /> Dear Brandon: <br /> AECOM Technical Services(AECOM)was contracted by Climax Molybdenum Company- <br /> Henderson Mill (Henderson)to analyze and design stabilization measures (buttress) for the <br /> Henderson Mill No. 3 Tailings Impoundment(3 Dam)to meet Freeport's internal seismic design <br /> criteria. The buttress design is based on current state of the practice and was reviewed by an <br /> independent technical review board(TRB)of recognized experts within the tailings dam industry. <br /> A technical revision No. 29(TR-29),which presents the design and construction drawing for the <br /> first stage of the 3 Dam buttress,was submitted to the Division of Reclamation, Mining and Safety <br /> (Division/DRMS)for approval on January 26,2018. On February 28, 2018, DRMS responded to <br /> TR-29 with a letter addressed to Mrs. Amber Parmet with Climax Molybdenum Company outlining <br /> adequacy review concerns. Presented below are each concern identified by the DRMS shown in <br /> italics followed by AECOM response. <br /> DRMS REVIEW CONCERNS AND AECOM RESPONSES <br /> DRMS Comment 1 <br /> In the Stage 1 Buttress Design section of the technical revision letter, the Operator states an <br /> engineering analysis demonstrated excess material on 1 Dam does not offer any structural support <br /> to 1 Dam, making it an optimal borrow source. Please provide the Division with a copy of the <br /> engineering analysis report for review. <br /> AECOM Response 1 <br /> The original stability design of 1 Dam was based on an overall design slope of 4:1(horizontal <br /> to vertical). Steeper benches are constructed as part of normal operations as the impoundment <br /> steps upstream. These benches serve as access routes during operation until the following <br /> AECOM <br /> 6200 S Quebec St <br /> Greenwood Village,CO 80111 <br /> www.aecom.com <br /> M•\DCSIPROJECTSWVTRW0551494_HEN3_BUTTRW"ELNERABLESIDRMS REPORT\DRMS COMMENTS\FINAUDRMS RESPONSE_AECOM_FINAL DOCX <br />