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p ~. ~ ~ <br />Wooaward~clyde <br />consultants <br />at a shallow depth through the full height of the embanlmzent and not down into the <br />foundation. The factor of safety corresponding to this surface is 2.97. <br />A summary of the factors of safety for the existing height embanlmlent at each of the <br />study sections is presented on Table 1. The lowest factors of safety were found for sheaz <br />surfaces that pass through the lower portion of the embankment which correspond to <br />study sections on the right side of Dam No. 1. The factors of safety for each study <br />section indicate a mazgin of safety considerably above commonly accepted minimum <br />valuGS. <br />3.4 THRESHOLD LEVEL ANALYSIS <br />Additional slope stability calculations were performed to establish threshold levels for <br />the piezometers within the existing Dam No. 1 embankment and for the raised <br />embarilanent with a crest at Elevation 8900. We understand that these threshold levels <br />will be used to assist in evaluating the stability of Dam No.l based on morutoring of <br />piezometer water levels. It should be emphasized that regular careful visual observation <br />and documentation of conditions on Dam No. 1 should continue to be made. 'The visual <br />observation of the embanlonent is a vital component in the overall assessment of the <br />stability of Dam No. 1 and provides eazly indications of problems that may not be <br />reflected by piezometer water levels. For example, local variation in tailing sand <br />properties could cause the phreatic surface to exceed the level which would be predicted <br />based on the neighboring instruments. A locally elevated phreatic surface may cause <br />seeps and moist azeas which could be noted during visual observation. Remedial actions <br />could then be implemented based on the operating and emergency response plans. <br />The first series of slope stability analyses were performed to establish threshold levels <br />for piezometers for the existing height embank®ent. The depth to the phreadc surface <br />within the embank®ent slope was varied to help establish threshold levels for <br />piezometers. With the phreatic surface at a depth of 10 feet below the embanlonent <br />slope surface, results of slope stability calculations made at each of the study sections <br />indicated adequate factors of safety. For this study, an adequate factor of safety is <br />defined as greater than or equal to 1S. This value has been selected to conform to U.S. <br />f~ Army Corps of Engineers criteria for long term stability of embankment dams and is <br />~_ <br />nssuei3 os~-svvr 3-4 <br />