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Geotechnical Evaluation of Mine-Induced Seismicity on Monument Dam <br />Dorsey 8 Whitney, LLP <br />6. Available subsurface borehole information and instrumentation for the earthfill dam <br />and the landslide is inadequate to allow full characterization and understanding of the <br />geotechnical issues contributing to the existing dam safety concerns, <br />The relatively shallow depth of the E Seam mine area results in a reduced potential <br />for mine-induced seismicity compared to mines at greater depths. Approximately <br />80 percent of the E Seam area has less than 1,000 feet of overburden and is <br />considered to be capable of a microseismic event of M 2.0 or less. Approximately <br />20 percent of the E Seam area has between 1,000 feet and 1,400 feet of overburden <br />and is considered to be capable of a seismic event in the range of M 2.0 to 2.3. The <br />gently rolling terrain overlying the E Seam combined with the more ductile geologic <br />strata in the roof of the mine reduces the potential for rapid changes in confining <br />stress at mine depth and associated potential for sudden releases of energy in the mine <br />roof and floor. Further, active engineered measures for stable support of the E Seam <br />mine are expected to contribute to infrequent, yielding, plastic-type deformation <br />behavior for mine openings as opposed to sudden releases of energy. <br />8. Pga at the dam site for the expected M 2.0 event associated with 80 percent of the E <br />Seam area that has less than 1,000 feet of overburden, and calculated for an average <br />horizontal distance of about 3,500 feet from the dam, is 0.077 g. Pga at the dam site <br />for the expected M 2.3 event associated with 20 percent of the E Seam area that has <br />between 1,000 feet and 1,400 feet of overburden, and calculated for an average <br />horizontal distance of about 9,000 feet from the dam, is 0.06 g. These accelerations <br />are about equal to, or slightly less than, historic mine-induced seismicity recorded in <br />the region since 1973, and aze slightly higher than historic natural seismicity recorded <br />in the region since 1973. <br />9. Earthfill Dam: The results of a stability analysis of the earthfill dam using material <br />properties identi$ed by Armstrong and Associates in 1983 generally verify the <br />findings of the 1983 Armstrong stability analysis that the downstream slope of the <br />dam is unstable (calculated factor of safety is less than 1.0) for steady-seepage <br />conditions with the reservoir at the emergency spillway elevation. The recommended <br />minimum factor of safety for this type of analysis is 1.5. The calculated factor of <br />safety for the static analysis of the downstream slope of the earthfilf dam using a <br />lowered phreatic surface that was selected to represent limited duration of reservoir <br />filling (conditional full storage restriction) to emergency spillway level is 1.09, which <br />indicates the downstream slope is marginally stable for these conditions. The <br />recommended minimum factor of safety for this type of analysis is 1.5. The capacity <br />of the earthfill dam to withstand mine-induced seismicity for conditions of limited <br />duration of reservoir filling was estimated as a yield acceleration of 0.03 g. <br />UCI~ 28 0603500SOI.16 Momm~mt Dam GmterMiW Evil <br />March 2006 <br />