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August 17, 2011 <br />The Union Milling Company <br />PO Box 36099 <br />Denver, Colorado 80236 <br />Attention: Mike Elder <br />John Danio <br />Subject: Slope Stability Evaluation <br />Tailings Pond Embankment and Future Tailings Slope <br />Union Mill <br />Leadville, Colorado <br />Project No. DN45,584 -130 <br />1 <br />"Planning, Design, and Analysis of Tailings Dams" by Steven G. Vick, 2 Printing, 1990 <br />CTL I THOMPSON <br />CTLIThompson, Inc. analyzed the stability of the proposed Union Milling <br />Company tailings pond embankment and future tailings slope and presented <br />results in a letter dated July 8, 2011. As part of our evaluation, we performed <br />direct shear tests on samples of tailings materials you provided. A gradation <br />analysis was performed on the sample and it contained a minus 200 fraction of <br />about 50 percent. Based upon information in Vick', single -stage cycloning <br />reduces the minus 200 fraction of the tailings by about 30 -40 % We attempted to <br />duplicate this reduction on the sample used in the previous direct shear tests. The <br />sample tested represented a reduction of only 12 %. <br />After our July 8 letter, we prepared an additional sample and achieved a <br />reduction of 30 %. The gradations for the tailings sample and the "modified <br />sample" are shown on Figure 1. A direct shear test was performed on the modified <br />sample and results are shown on Fig. 2. During this test, the shear rate was <br />increased because we observed soil particles had migrated out of the sample <br />during the previous tests which may have influenced the results. An internal angle <br />of friction of 35 degrees was measured during the latest test. This result <br />compares well with published literature. <br />Vick indicates that cycloned materials typically flow as a thick, ropy <br />discharge and assume an angle of repose of 3:1 (18 degrees) to 4:1 (14 degrees). <br />If we use the internal angle of friction of 35 degrees and a slope angle of 14 to 18 <br />degrees, the infinite slope factor of safety for 3:1 and 4:1 slope is 2.1 and 2.8; <br />respectively. These values represent the minimum factors of safety assuming no <br />water surface develops in the tailings materials above the embankment. <br />We also used the results of the latest direct shear testing to evaluate slope <br />stability including a water surface develops in the tailings materials. Example <br />results are shown in Appendix A. We assumed the cycloned sand would form <br />"wedges" with 3:1 outer and internal slopes. We analyzed two potential water <br />1971 West 12th Avenue 1 Denver, Colorado 80204 I Telephone: 303 - 825 -0777 Fax: 303 - 825 -4252 <br />