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WCOLORADO <br />Division of Reclamation, <br />Mining and Safety <br />Department of Natural Resources <br />Date: July 14, 2020 <br />To: Peter Hays <br />From: Zach Trujillo <br />RE: Henderson Mine, DRMS File No. M-1977-342 <br />Slope Stability Analysis in Support of TR -32 <br />Peter, <br />As requested I have reviewed Climax Molybdenum Company's 3 Dam, Phase II Buttress Design and <br />associated geotechnical stability analysis (Report) for the Henderson Mine (Henderson). Henderson is <br />proposing to initiate Phase II of the 3 Dam buttress design which will include placing approximately <br />75,250 c.y. of buttress material, 1,000 c.y. of rockfill, and 900 cubic yards of filter material to the <br />currently constructed Phase I of the 3 Dam. This material will either be imported or borrowed from the 1 <br />Dam benches. It is stated in the Report that an, "[e]ngineering analysis has demonstrated the excess <br />material on the I Dam does not offer any structural support to the 1 Dam making it optimal borrow <br />source." However, this analysis has not been provided within the Report. <br />Additionally, as part of my review, information within the Report was used to perform a slope stability <br />analysis using Galena — Slope Stability Analysis System (Galena) as an estimated check by <br />approximately reconstructing the slope profiles and using material properties for the 3 Dam. Both models, <br />the Division's and Henderson's, utilized the Spencer Method with multiple surfaces which is suitable for <br />circular and non -circular failure surfaces for a two-dimensional (2D) analysis. In addition to the 2D <br />analysis, Henderson also provided a three dimensional (3D) analysis to more accurately simulate the <br />seismic performance of 3 Dam more accurately. Galena doesn't allow for a 3D analysis and so I was <br />unable to perform a stability check for that scenario. <br />When designing the addition to the 3 Dam buttress, Henderson used a 2D analysis as a way to design the <br />additional buttressing by using criteria of a stable slope and adjusting the additional buttress material to <br />come to a final buttress configuration. The final buttress configuration was then ran with the 3D modeling <br />code, FLAC31), using a site-specific 10,000 -year earthquake seismic loading which follows industry and <br />professional standards used for dams. The results of this pseudo -static analysis for the 3 Dam resulted in a <br />factor of safety of 1.3 and meets the requirements under Section 30, "Factors Safety for Slope <br />Stability/Geotechnical Analyses" (Section 30) in the Policies of The Mined Land Reclamation Board for <br />pseudo -static conditions. The results of my 2D stability check for the 3 Dam resulted in a 1.34 factor of <br />safety under the same pseudo -static conditions. It should be noted that while there was plenty of <br />discussion regarding pseudo -static conditions in the Report, there was no discussions on any stability <br />results under static conditions. <br />Henderson Mine Geotechnical Review Memo — TR -32 July 14, 2020 <br />OF C�lo <br />0 <br />1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http://mining.state.co.us I * * <br />Jared S. Polis, Governor I Dan Gibbs, Executive Director I Virginia Brannon, Director *+►* <br />