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-~. <br />~- <br />~~- <br />i <br />~~ <br />~~ <br />~~ <br />_~~ <br />~~ <br />~i~ <br />ii <br />1i~ <br />1~ <br />I =i <br />l:g <br />I~ <br />1 <br /> <br />3 <br />7 <br />I <br /> <br />Woodward-Clyde <br />Consultants <br />consistent with criteria used to date for this tailing dam. As shown on Table 1, the <br />factors of safety range from 1.78 at Study Section 8-8' to 1.96 at Study Section 11-11'. <br />The second series of slope stability analyses were performed to establish thre:~hold levels <br />for the embanlonent raised to crest Elevation 8900. Nere again the depth to the <br />phreatic surface within the embank®ent slope was varied to help establish the threshold <br />levels for the piezometers for the raised embank®ent. With the phreatic surface at a <br />depth of 10 feet below the embank®ent slope surface along the full length of the slope, <br />results of slope stability calculations at each of the study sections agaitt indicated <br />adequate factors of safety (greater than 1S). As shown on Table 1, the factors of safety <br />range from 1.52 at Study Section 10-10' to 1.61 at Study Section 7-7'. The critical shear <br />surfaces and corresponding factors of safety aze shown on Figures 7 through 11 for each <br />of the five study sections. The sheaz surface in all cases pass fairly deeply through the <br />full height of the tailing slope and exit just above the starter dam or near tha; toe of the <br />embankment slope in the case of Study Section 11-11'. <br />The stability at Study Section 10-10' was analyzed for the embankment raised to crest <br />elevation 8900 using a best estimate of the location of the phreatic surface. The phreatic <br />surface for the existing embankment was extended up for the raised embankment. The <br />estimated factor of safety for this case was 2.09, compazed to 1.52 for the cace: where the <br />phreatic surface is at the threshold levels. if the phreatic surface is higher than the best <br />estimate, the factor of safety would be closer to 1.5. <br />Based on the results of these stability analyses, we believe it is reasonable to establish <br />threshold levels for the existing piezometers and for piezometers to be installed in the <br />future as Dam No. 1 is raised at an elevation 10 feet below the ground surface at each <br />piezometer location. Thus, threshold level elevations can be established and these levels <br />used throughout the remainder of construction of Dam No. 1 providing there are no <br />significant changes in the key assumptions made for these analyses. ~ These key <br />assumptions aze summarized in the following section of this report. Generalntfotmation <br />for each of the existing piezometers including the threshold level elevations aze <br />summarized on Table 2. Comparing the February '92 water surface elevations to the <br />threshold level elevations for each piezometer, it is found that the measured water levels <br />are all well below the threshold levels. In general, the water levels for lower <br />~ua~.3 as-m-sv.rr 3-5 <br />