2017-12-01_REVISION - M1977342 - Laserfiche WebLink

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e SCOM AECOM 303 694 2770 tel /7� 6200 South Quebec Street 303 694-3946 fax Greenwood Village,CO 80111 www aecom corn October 5, 2017 Mr. Brandon Guite Climax Molybdenum Company Henderson Mill 19302 County Road 3 Parshall, Colorado 80468 Subject: Construction Completion Report: 3 Dam Reslope Grading Project, Henderson Mill Tailings Dam near Parshall, Colorado—AECOM Project No. 60535717—FINAL Rev 1 Dear Brandon, AECOM Technical Services, Inc. (AECOM) prepared this letter report,which presents a summary of the as-built conditions for regrading the downstream slope of the original 3 Dam at Henderson Mill. An as-built drawing showing the location of the work area is appended PROJECT OBJECTIVES AND DEPOSITION STATUS AECOM was contracted by Climax Molybdenum Company to provide design and field engineering services required to flatten the average downstream slope of the original 3 Dam, as part of ongoing efforts to increase the computed undrained slope stability factor of safety to the minimum established design criteria. Deposition activities on the 3 Dam Stepback were ceased until this reslope work could be completed. This report documents this work has been completed satisfactorily meeting the intent of the design and associated drawings. Henderson can resume spigot deposition on the 3 Dam Stepback in accordance with their deposition plan. We recommend Henderson review future deposition plans and when operationally possible, limit deposition on the Stepback Dam during spring runoff season. SUMMARY OF DESIGN The topographic information before construction indicates that the surface grade of the downstream slope of 3 Dam varied from 3.4H:1V to 3.5H:1V, with a crest El. of 8828 feet. At the beginning of the work, the downstream slope of 3 Dam was covered with about 8 inches of sandy and silty clay cover, with sparse to no vegetative cover. The design included sequential flattening of the downstream slope to a uniform 4H:1V grade, and placement of the excavated material in the area between the original 3 Dam and the Stepback 3 Dam. The crest elevation of the regraded slope was set at El. 8824.3 to 8825.2 feet. The surface casings of those piezometers in the original 3 Dam slope where significant excavation was completed were lowered and their respective operational alarms and threshold limits were reestablished for an undrained stability loading condition considering the upgraded geometry. Existing horizontal and foundation drains at the toe of the regraded slope were not altered. CONSTRUCTION OBSERVATIONS AECOM provided part-time observation of the construction work, which was completed by Henderson Mill construction personnel between June 7 and July 28, 2017. Grading of material excavated from the slope continued until August 4, 2017. Elliott Drumright of AECOM was requested to observe the work on June 8, 14, 27 and July 18. On each of those days, weather conditions were clear to partly cloudy, with temperatures of 50-75°F. Lisa Yenne also observed the work in-progress on June 21.