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A=COM <br /> July 16, 2020 <br /> Mr. Ron Hickman <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 Il Design, <br /> Henderson Mill Tailings Dam, near Parshall, Colorado <br /> Dear Ron: <br /> AECOM Technical Services (AECOM) was contracted by Climax Molybdenum Company- <br /> Henderson Mill (Henderson) to analyze and design stabilization measures (buttress) for Phase lI of <br /> the 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. 32 (TR-32), which presents the design and construction drawing for the <br /> second stage of the 3 Dam buttress, was submitted to the Division of Reclamation, Mining and <br /> Safety (Division/DRMS) for approval in June 2020. On July 15, 2020, DRMS provided an internal <br /> review letter dated July 14, 2020 addressed to Mr. Peter Hays from Mr. Zach Trujillo, both with <br /> DRMS, outlining his review of TR-32 and his request for additional information. Presented below <br /> are the additional information requests identified by the DRMS shown in italics followed by <br /> AECOM response. <br /> DRMS REVIEW REQUEST FOR ADDITIONAL INFORMATION AND AECOM RESPONSES <br /> DRMS Comment I <br /> Please have Henderson provide the additional stability analysis for the 1 Dam to ensure to the <br /> Division that long-term stability will remain if material is borrowed from the I Dam benches. <br /> AECOM Response 1 <br /> There was a similar question posed during the review of the Phase I Buttress. The response <br /> provided below is excerpted from the 2018 TR-29 adequacy review response since it still <br /> applies. The original stability design of 1 Dam was based on an overall design slope of <br /> AECOM <br /> 6200 S Quebec St <br /> Greenwood Village,CO 80111 <br /> www aecorn corn <br /> C\USERS\ZANA KARIMI\DOCUMENTS\HENDERSON STABILITY\DRMS PH 11 RESPONSE FINAL DOCX <br />