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SECTIONFOUR Conclusions and Recomrnendatlons 1 ' 4.1 CONCLUSIONS The results of the undrained static stability analysis show that the calculated factor of safety for the existing embankment is slightly reduced by the elevated phreatic surface modeled, but is ' within accepted criteria. The calculated factor of safety for the ultimate height dam is generally unaffected by the elevated phreatic surface and meets acceptable criteria. The results of the stability analysis also shows that the calculated factors of safety fo•r post- earthquake conditions aze reduced by the elevated phreatic surface for both the existing and ultimate height dam conditions. Comparing results shown on Figure 12 to those shown on ' Figure 10, it also appeazs that there is a small increase in the factor of safety within 10 years after drawing down the PMP pond. The transient seepage analysis shows that while the phreatic surface near the dam crest gradually decreases in elevation with time, the phreatic surface near ' the toe of the slope rises slightly in elevations in response to drainage. This trend in the shape of the phreatic surface with time is highly sensitive to the conductivity of the tailing sands and to the seepage condi±ions existing immediately after drawing down the PMP pond. Th.; calculated theoretical factor of safety for post-earthquake conditions is above 1.0 for both dam :heights. These calculated post-earthquake factors of safety aze judged to be acceptable because of the conservative assumptions used in the analysis, as discussed in the previous section of this report. 4.2 RECOMMENDATIONS 1 1 J The results of our analysis show that the dam meets acceptable stability criteria. Thc~ undrained static stability of the dam is generally unaffected or only slightly affected by the raised phreatic surface and the post-earthquake stability is reduced by the elevated phreatic surface. The lower factors of safety calculated for post-earthquake conditions (about 1.05) aze judged to be acceptable because of the conservative assumptions used in the analysis, as discusse(i above. Even with the series of conservative assumptions, the post-earthquake factors of safety are still greater than 1.0 and therefore are judged acceptable. Our results also show that storage of the PMP storm event or the storage of water ne;u• the dam crest can elevate the phreatic surface for long periods. The actual behavior of the dam should be verified by monitoring the dam performance closely following an extreme storm event. Following any event that produces a significantl}• elevated pond, we recommend that Climax monitor the dam including: • Perform an immediate dam inspection • Conduct weekly piezometer monitoring until levels stabilize or fall. • Review prior dam stability analysis results. Dam stability may need to be reevaluated and our model recalibrated to account for the observed changes in the phreatic surface and response to stored water. Climax should continue with the current tailing deposition procedures. It is important that Climax continue to control deposition in order to produce an engineered shell offree-draining sands. Climax should periodically verify the dam construction using intrusive field techniques such as drilling or piezocone soundings. ~~` NIPROJECT516B4A61B_MENDERSON_MILL_BARGtiSU80VBO PROD_DELMFI#tILL R3.D(IC\]4JULd1\\ 4-I