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' SECTIONTHREE Sta611ityAnalyses <br />' Assumptions <br />' The post-earthquake stability analysis is based on assumptions that we believe are conservative. <br />Therefore, the conclusions presented in the next section of this report consider these ~~ssumptions <br />including: <br />' - Tailing liquefaction has occurred -Prior analysis shows that the dam has a low potential <br />for liquefaction. Therefore, the associated loss in sheaz strength during an earthquake has a <br />low risk of occurring. We have, however, conservatively assumed in our analysis: that <br />liquefaction will occur. <br />- Remolded undrained shear strength is assumed - An undrained strength ratio of <br />' Su/p =0.53 was measured in undisturbed tailing samples and an undrained strength ratio of <br />Su/p"=0.28 was measured in remolded tailing samples as described in our report slated April <br />2000. In our opinion, and in this case the remodeled strength is a reasonable and <br />' conservative representation of the liquefied post-earthquake strength of the tailing. The <br />remodled undrained strength was conservatively used to represent the post-earthquake <br />undrained shear strength of the tailing, as discussed above. Based on the liquefaction <br />' potential analysis, this assumption is very conservative and it represents the lowe~:t undrained <br />strength possible for the tailing. If liquefaction does not occur, as the liquefaction potential <br />analysis indicates, a strength loss of 20 to 30 percent relative to peak strength is more likely <br />' to occur during strong shaking, based on published literature. A small increase in the <br />undrained strength above the remodeled strength would result in a much higher post- <br />' eazthquake factor of safety than indicated in the analyses in this report. <br />- Stormwater is ponded for six weeks -Our seepage analysis and estimated phreatic surface <br />assume that the maximum pool elevation is sustained for 6 weeks and then removed . This <br />' assumption increases the estimated seepage quantity from the temporazy pool into the tailing <br />and results in a higher phreatic surface. <br />- High hydraulic conductivity used for analysis -The higher hydraulic conductivity was <br />' assumed for the model and used to estimate the phreatic surface after six weeks o1'the <br />sustained pond. This higher conductivity would allow more water to infiltrate the tailings <br />' and raise the phreatic surface more quickly. <br />- "Base Case" conductivity used for dewatering -The lower "base case" conductivity was <br />used to model drawdown of the tailing following the PMP event. This assumption would <br />' result in a slower drawdown. <br />The assumptions above are each conservative and then combined make for a very conservative <br />' analysis. Thus, we judge that a calculated factor of safety above 1.0 for the post-earthlquake <br />stability is suitable and appropriate. As such, the dam meets acceptable dam safety criteria. <br /> <br /> <br /> <br />ViW NWROJECTSK84A61B HENDERSON MILL_BARG\SUD DP6.0 PROD DELMH~MILLR3.D0(~\YNUL-01\\ 3-3 <br /> <br />