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Geotechnieal Evaluation of Mine•Induced Seismicity on Monument Dam Dorsey 8 Whitney, LLP • 5.4 Representative Cross Section We developed a representative cross section at the maximum section of the earthfill dam for static slope stability analysis based on material property information presented in Armstrong, 1983, which describes four soil layers within the earthfill dam consisting of sandy clay and lean clay. The representative earthfill dam section used in the static analysis is presented on Figure 5.1. 5.5 Water Pressures Water pressures through the dam and foundation were estimated based on the following: Case 1: Full reservoir at emergency spillway level and steady-seepage, which is about 10 feet below the crest of the dam. • The embankment is homogeneous and isotropic because it has no internal drainage systems. • The phreatic surface exits on the downstream slope above the toe of the dam as reported in Armstrong, 1983, and can be estimated using the Casagrande's solution (Casagrande, 1940). It is our opinion that these assumptions are somewhat conservative because seepage has been observed about 15 feet below the crest on the downstream slope (Hydro-Triad, 1979). The present operation of the reservoir, which maybe full for 3 or 4 months a year, likely does not fully develop steady-seepage conditions within the embankment dam. A second analysis was performed to model this condition, as follows: Case 2: The phreatic surface is estimated to be at lower levels within the dam because the reservoir is not full most of the year and steady-seepage through the dam has not developed. These water levels were also used for the pseudo-static analysis. 5.6 Material Properties Material properties, such as moist and saturated unit weights, and strength parameters were obtained from Armstrong, 1983, as discussed previously. Material properties of the GEI is 0603500F03-I6 Monwnmt Dam GmtNUVCil Evil March 2006