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Geotechnical Evaluation of Mine-Induced Selsmlcity on Monument Dam <br />Dorsey & Whitney, LLP <br />• discharge channel, and in a generally westerly direction along the stream channel. A <br />granular drain would be provided at the downstream toe of the landslide to collect seepage <br />through the landslide. Seepage collected by the toe drain would be discharged by a system of <br />pipes at a location downstream of the buttress fill. Materials used in construction of the toe <br />drain would be filter-compatible with adjacent soil and would be sized to convey the <br />expected flow. <br />As discussed above, an interceptor drain would be considered in addition to the stability <br />berm and buttress fill. The potential location of an interceptor drain is shown in plan view on <br />Figure 7.1 and in section view on Figure 7.3. The objective for this drain is to intercept <br />primarily surface water and also shallow water uphill of the dam/landslide and remove it <br />from the system. The potential benefits of removing water at this location are expected to be <br />positive, but have not been evaluated in this study. <br />7.2 Stability Analysis with Preventive Measures <br />7.2.1 Representative Cross Sections <br />Representative cross sections for analysis of earthfill dam and the landslide are as shown on <br />Figures 5.1 and 6.1, respectively. Material properties for the dam and landslide are as <br />described in Sections 5 and 6, respectively. Similarly, water pressures in the dam and <br />landslide were considered at the high and threshold levels as described in Sections 5 and 6, <br />respectively. <br />7.Z.2 Results of tl-e Analysis <br />Earthfill Dam <br />The calculated factor of safety for the static steady-seepage analysis of the downstream slope <br />with stability berm and buttress fill is about 1.6, which exceeds the recommended minimum <br />factor of safety of 1.5. This stability model was subsequently evaluated for yield acceleration <br />using pseudo-static analysis methods. The calculated yield acceleration was about 0.13 g. <br />The calculated yield acceleration of 0.13 g significantly exceeds the required yield <br />acceleration of 0.04 g associated with a median peak ground acceleration of 0.08 g at the dam <br />site that could occur as a result of E Seam mine-induced seismicity. <br />Landslide <br />The calculated factor of safety for the static steady-seepage analysis of the downstream slope <br />with stability berm and buttress fill is about 1.6, which exceeds the recommended minimum <br />factor of safety of 1.5. This stability model was subsequently evaluated for yield acceleration <br />using pseudo-static analysis methods. The calculated yield acceleration of 0.11 g <br />GEI~' 25 0603500fi-03-16MOnumm[DVnGwmdvticalEVal <br />March 2006 <br />