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Climax Molybdenum <br />A Freeport-McMoRan Company <br />February 24, 2017 <br />TE <br />'ed! <br />✓0 Violations: <br />Sent Via Email and Certified Mail#: 7016 1370 0000 2426 7680 <br />COLORADO OPERATIONS <br />Henderson Mine and Mill <br />P.O, Box 68 <br />Empire, CO 80438 <br />Phone (303) 569-3221 <br />Fax(303)569.2830 <br />RIECiElVED <br />NAR 012017 <br />DIVISION oi` REG <br />Mr. Peter Hays MINING L MMA770N <br />Division of Reclamation Mining and Safety & SAP9- Y <br />1313 Sherman St, Rm. 215 <br />Denver, CO 80203 <br />RE;: Climax Molybdenum Company, Henderson Mill, Permit No. M-1977-342, Proposed <br />Technical Revision No. 28, Original Number 3 Dam Regrade <br />Dear Mr_ Hays: <br />Climax Molybdenum Company (CMC) is submitting this request for a Technical Revision (TR -28) to the <br />Henderson Reclamation Permit for the re -sloping and regrading of the embankment on the original <br />number 3 Dam. <br />Henderson Mill requests approval to re -slope the embankment face in order to increase the slope stability <br />strength based on a recent geotechnical analysis of the facility. The Tailing Storage Facility (TSF) serves <br />as the impoundment for Mill tailings and process water. It encompasses around 1500 acres and is <br />bounded by both natural topography and two dams constructed of milled tailings. <br />Background <br />The Henderson Mill stores tailings and process water within the TSF, located north-west of the main Mill <br />complex. This facility is an unlined containment area bounded by natural topography and two tailings <br />dams (#1 dam and #3 dam). The dams are equipped with foundation and horizontal drains to maintain <br />the phreatic surface level within the TSF and collect seepage water. The original #3 dam was constructed <br />of processed tails starting in 1976 with active deposition continuing through the early 1980's. The #3 dam <br />area was in -active for a period of years until the #3 dam stepback was constructed in 2003 with active <br />deposition on the stepback starting in 2004. Annual deposition has continuing on the stepback through <br />present day. <br />A geotechnical investigation on the #3 dam area was initiated by Henderson in 2016 to better <br />characterize existing tailings and foundation material contained in, and below, the impoundment in order <br />to ensure the operational and long term stability of the TSF, The Henderson TSF Engineer of Record <br />(EOR) was contracted to perform the field investigation and engineering analysis, which was completed in <br />early 2017. Analysis indicates that improvement to the original 3 dam embankment slope stability is <br />required. This operational stability change is due to having a better definition of the upper foundation <br />material including strength properties and geometry. The area requiring improvement is confined to the <br />southern half of the original 3 dam. To increase the slope stability strength in an effective and timely <br />manner the decision has been made to re -slope the existing embankment from the current slope, ranging <br />from 2.911:1V to 3.85HAV, to a 4H:1V slope. By decreasing the slope angle the ratio of resisting force <br />over driving force will increase resulting in a slope stability improvement. Even though the northern half of <br />3 dam, currently at a 3.85HAV slope, shows acceptable slope stability strength, the decision is being <br />