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Geotechnical Evaluation of Mlne•Induced Seismicity on Monument Dam <br />Dorsey & Whitney, LLP <br />10. Landslide: The landslide is active and has failed (moved) on numerous occasions <br />since 1979. The effective mobilized friction angle in the failure zone of the landslide <br />was back-calculated to be 15 degrees, which is considered to be near its lowest <br />residual strength value. A stability analysis model to represent conditions during the <br />drier months of the year resulted in a factor of safety of 1.03, which indicates only <br />marginal stability. The capacity of the landslide to withstand mine-induced <br />seismicity for conditions representing the drier months of the year was estimated as a <br />yield acceleration of 0.007 g, or approximately 0.01 g. <br />11. The results of the stability analysis, coupled with the observed field conditions and <br />instrument measurements, indicate the landslide is likely the component of the <br />dam landslide system that presents the most critical stability issue during reservoir <br />operations consistent with the SEO's conditional full storage restriction. However, <br />for full reservoir, steady-seepage conditions the downstream slope of the earthfill dam <br />is unstable, and is only marginally stable for conditions representing limited duration <br />of reservoir filling (conditional full storage restriction). The record of repeated, <br />episodic landslide movement and the results of the stability analysis that indicate the <br />landslide failure zone is approaching its lowest residual strength value suggest a <br />worsening stability condition associated with degradation of strength with time. <br />• 12. Preventive Measures: The calculated factor of safety for the static steady-seepage <br />analysis of both the dam with stability berm and buttress fill and landslide with <br />buttress fill is about 1.6, which exceeds the recommended minimum factor of safety <br />of 1.5. The calculated yield acceleration of 0.11g, or higher, significantly exceeds the <br />required yield acceleration of 0.04 g associated with a median peak ground <br />acceleration of 0.08 g at the dam site that could occur as a result of E Seam mine- <br />induced seismicity. <br />13. An acceleration of 0.08g at the dam site that could occur as a result of E Seam mine- <br />induced seismicity will not cause material damage to the dam and landslide system if <br />preventive measures as outlined herein are implemented. <br />GEI <br />29 <br />060350 0603-16 Mwwumt Dvn Gmtcclutiul Eval <br />March 2006 <br />