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L RAD PERATI N <br />Climax Molybdenum Ilenderson Mine and Mill <br />A Freeport-McMoRan CompanyP.O. Box 68 <br />PSH WHE Empire, CO 80438 <br />M-1977-342 TR 29 Phone (303) 569-3221 <br />January 26, 2018 4 Annual Fee/Report: CW Fax (303) 569-2830 <br />,! No Violations <br />Sent Via Email and UPS #: 1ZP70OUT0220009617 <br />RECEIVED <br />Mr. Peter Hays <br />Division of Reclamation Mining and Safety ,AN 3 0 2018 <br />1313 Sherman St., Rm. 215 <br />Denver, CO 80203 DIVISION OF RECLAMATION <br />MININGAND SAFETY <br />RE: Climax Molybdenum Company, Henderson Mill, Permit No. M-1977-342, Proposed <br />Technical Revision No. 29, Number 3 Dam Buttress Stage I <br />Dear Mr. Hays: <br />Climax Molybdenum Company (CMC) is submitting this request for a Technical Revision (TR -29) to the <br />Henderson Reclamation Permit for the Stage 1 construction of a buttress at the toe of the original and <br />stepback sections of 3 Dam in order to further improve the slope stability based on a recent geotechnical <br />analysis of the facility. <br />Background <br />The Henderson Mill stores tailings and process water within a single tailings storage facility (TSF), located <br />north-west of the main Mill complex. The TSF is an unlined containment area, encompassing around <br />1,500 acres, and is bounded by natural topography and two tailings dams (1 Dam and 3 Dam). The dams <br />are equipped with foundation and horizontal drains to maintain the phreatic surface level within the TSF <br />and collect seepage water. The original 3 Dam was initially constructed in 1976 and the stepback was <br />constructed in 2003. <br />A geotechnical investigation of the 3 Dam area was initiated by Henderson in 2016 to better characterize <br />existing tailings and foundation material contained in, and below, the impoundment in order to ensure the <br />operational and long-term stability of the TSF. The Henderson TSF Engineer of Record (EOR) was <br />contracted to perform the field investigation and engineering analysis, which was completed in 2017. The <br />analysis indicated that improvement to the original 3 Dam embankment was needed to increase the <br />computed undrained slope stability factor of safety. In order to achieve this, 3 Dam was regraded from the <br />previous range of a 2.91-1:1V to 3.851-11V slope to a constant 4H:1 V slope across the original dam <br />embankment. This was completed in the summer of 2017 under Technical Revision 27. As a result, both <br />1 Dam and 3 Dam currently meet static slope stability requirements when considering both drained and <br />undrained conditions and will continue to meet these requirements as the dams are raised. <br />Additionally, the post -earthquake stability analysis showed that stabilization measures are required as the <br />dam increases in height in order to meet the seismic design criteria selected by the Henderson Mill <br />(10,000 year earthquake event). The decision has been made to build a buttress on the original dam <br />embankment and stepback in a four -staged approach to increase the overall stability of the impoundment <br />while maintaining tolerable post -earthquake deformations that will prevent a loss of containment of <br />tailings or process water. Constructing the buttress in four stages will allow an observational approach to <br />be incorporated into the overall design strategy, adjusting prior to each construction phase based on <br />actual site conditions at the time. Based on modeling results and current production rates, construction is <br />anticipated to take place in four stages prior to reaching the corresponding crest elevations as follows: <br />0 Stage 1: crest elevation at 8883 ft (estimated year 2019) <br />