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1 <br />' SECTIONTWO seeua~ennal~s~s <br />Based on our understanding, Climax may store the precipitation from extreme storm events <br />' (PMP) within the impoundment. During the PMP event the decant pond elevation is expected to <br />rise to neaz the dam crest, saturating tailing within the engineered portion of the emb;uikment <br />shell. This will likely reduce the overall dam stability. The pool is expected to rise rapidly <br />' during the PMP event and dewatering of the impoundment to normal operating pond elevation <br />was assumed to take approximately 6 weeks. <br />' A seepage analyses was performed to evaluate changes in the embankment seepage conditions <br />and estimate the phreatic surfaces that result from storage of the PMP event. Analyses were <br />performed for both the current dam elevation and ultimate design dam elevation of 8900 feet. <br />' The estimated phreatic surfaces were then used later to estimate embankment stabiliri. <br />2.1 PRIOR STUDIES <br />' Seepage conditions of the Henderson Tailings Dam, for normal operating pond and post-closure <br />conditions, were previously evaluated by Woodwazd-Clyde in 1995. The results of these <br />' analyses are summarized in a report titled "Undrained Static and Seismic Stability Analyses and <br />Post-Closure Stability and Seepage Analysis, Henderson Tailing Dam, neaz Pazshall, Colorado", <br />dated Apri12000. The finite element seepage model of the dam used for the 1995 ev~duations <br />' was modified for this study. Modifications were limited to adjusting the model for the current <br />embankment height and increasing the decant pond elevation to reflect the expected elevation of <br />the decant pond following the PMP event. <br />' 2.2 APPROACH AND METHODOLOGY <br />' Our evaluation of changes in seepage conditions resulting from the PMP event for the: existing <br />and ultimate height dam elevations consisted of the following six steps: <br />I. The finite element seepage model of the dam, developed during the 1995 evaluations, was <br />' modified to represent the current embankment height conditions (crest El. 8810 feet) with the <br />normal operating pond level. This seepage model was calibrated for steady state :seepage <br />conditions by adjusting the material conductivity values such that the computed phreatic <br />' surface closely matched the actual observed piezometer readings in the dam. <br />2. A transient seepage model was run to evaluate the change in the phreatic surface resulting <br />' from the PMP event using the calibrated material properties as the "base case" values. For <br />the transient analysis the decant pond elevation was instantaneously increased from its <br />normal operating level of 8800 feet to the maximum elevated level of 8809 feet a,~d then held <br />' at this elevation for the model period. The simulation was run for a period of six weeks. <br />3. A pazametric analysis was performed to estimate the sensitivity of the results to v,uiation in <br />' the tailing sands (engineered shell) conductivity. The tailing sand conductivity w:rs increased <br />by 1 order of magnitude from its "base case" value and the transient seepage analysis <br />repeated using the new conductivity. The phreatic surface at the end of six weeks was again <br />' estimated. <br />4. The seepage model was then adjusted to reflect the ultimate height embankment conditions <br />(crest EI. 8900 feet) with the normal operating pool elevation of 8890 feet. The calibrated <br />Vjw N:IPROJECTSYBeM1B_HENOERSON_MILL BMGSUB 006.0 PRO,I_DELN41#11LLRJ.D0~:1]4JUL-01\\ 2-I <br />1 <br />