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4-6 <br />For Case A, the factors of safety for the circular and block failure <br />modes were the same with a factor of safety of 1.4. For Cases B and C, <br />the block failure factor of safety was 1.2, less than the circular <br />failure factor of safety of 1.3. The factors of safety for earthquake <br />conditions were between 1.1 to 1.3 for the various cases studied. The <br />block failure mode is therefore the critical failure mode for the <br />stability analysis. Further results discussed below are for the block <br />mode. In all cases, the criti'cal..failure surface passes through .the <br />middle of the dam:into the 'alluvium, through the fractured bedrock zone, <br />and exits just beyond...the embankment toe. . The critical surfaces are <br />shown In Figure:9. <br />The generally accepted factor of safety for static conditions is 1.5 and <br />for earthquake loading is 1.0. MSHA and the Colorado State Engineer <br />also consider these values acceptable. T96i?&jCoirs'rfof..;_safety for the <br />lower- tailings.., dam..'for earthquake" -loading -conditions ari--above „the k <br />: _ <br />minimum acceptable values The factors of safety for, static loading <br />conditions-are"slightly'below the acceptable value. <br />The stability analysis is relatively sensitive to the selected values of <br />strength parameters for the weathered rock foundation materials and to <br />the prescribed phreatic surface within the embankment, if the phreatic <br />surface reaches the critical failure surface. The critical surface <br />passes through the weathered rock zone for all cases analyzed. The <br />phreatic affects the upper part of the circle only for Case B. <br />As discussed above, the highest degree of conservatism in parameter <br />selection exists for the embankment fill and foundation soils. Signifi- <br />cant conservatism is not applied to the jointed and weathered bedrock <br />strength parameters. Increasing the parameters within reasonable upper <br />limits for strength of the fill on alluvial soils parameters to reduce <br />conservatism in the analysis would not significantly increase the <br />calculated factor of safety. <br />