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4-5 Although a fixed single value for each material was selected for the stability assessment, sensitivity of the results was reviewed to determine if reduced conservatism in the selection of a parameter for a particular material would greatly affect the resultant factor of safety. Conclusions drawn concerning this subjective parametric analysis are discussed below. 4.1.2 Piezometric Conditions 1 The effects of ground water levels and seepage are critical factors in the stability evaluation of the tailings dam. The piezometric levels used for the stability analysis were based on the hydrologic model conditions as discussed in Section 3.3 and shown in Figure 8. The observed piezometric conditions were incorporated into the analyti— cal model for Case A (existing conditions) as shown in Figure 9. For the two future cases, maximum operating pool and raised embankment, it was necessary to extrapolate the existing data. Phreatic surfaces through the embankment associated with the higher pond levels were estimated using approximate theoretical methods. The assumption was made that flow through the core would be consistent with standard flow through a homogeneous dam core. The piezometric levels in the bedrock were assumed to increase in direct proportion to the increased differen— tial head between the pond and the ground surface downstream from the dam toe. The extrapolated piezometric surfaces for each case are shown on Figure 9. 4.1.3 Analysis Results The minimum factor of safety against slope failure was calculated for each case for two modes of failure (circular and black), and for static and earthquake conditions. The pseudostatic earthquake analysis was performed with a ground acceleration of 0.025 based on the seismic risk for the project location (Corps of Engineers, 1977). The results of the stability analysis are presented in Figure 9. 1" !!! ;0e ��