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ECOSA Evaluation AdrianBrown <br />directed to detention basins, with the most downgradient structures designed to contain twice the 10- <br />year, 24 -hour precipitation event. <br />3.4 Groundwater Management <br />During construction and reclamation of the facility, and for a period of approximately 20 years <br />thereafter, infiltration of precipitation to the surface of the ECOSA will be retained within the <br />overburden until the field capacity of the placed overburden is reached. After that period, infiltrated <br />water will flow out of the facility into the underlying colluvium and bedrock at an expected rate of <br />approximately 69 gpm. Based on the expected rate and the existing neutralizing capabilities of the <br />overburden, water exiting downward from the base of the ECOSA into the underlying bedrock is <br />expected to be of similar water quality to that of the water in the regional ground water table flowing <br />from the Carlton Tunnel. Appendix I, Volume II provides a complete discussion of the expected <br />performance of the ECOSA construction design. <br />3.5 Stability <br />3.5.1 During Construction <br />ECOSA is constructed by end - dumping an essentially dry, cohesionless material from the top of the <br />ECOSA, and later from progressively lower benches as the surface of the ECOSA is conformed to its <br />final 2.5:1 slope. During construction, therefore, all OSA faces are meta - stable at their angle of repose, <br />with a factor of safety at or close to unity. If the face is locally steepened, it will ravel or fail back to its <br />angle of repose over time. <br />Under earthquake loading, this equilibrium can be disturbed. The initial ground motion caused by the <br />earthquake accelerates the entire OSA, with the velocity increasing rapidly, and the OSA materials <br />being compressed. When the ground decelerates the velocity reduces, until in general it returns to close <br />to zero. During this phase of the motion the overburden is decompressed by the inertia of the moving <br />material, which causes a reduction in normal stress within the OSA. The frictional forces within the <br />material reduce to below the angle of repose, allowing a short term period of failure in the overburden <br />mass. <br />This process has been modeled for ECOSA in Plate 8, by evaluating its displacement response to a <br />prototype major earthquake, scaled to a maximum acceleration of the design earthquake of 0.14 g (1.37 <br />m/s The analysis indicates that the outer surface of ECOSA would be displaced up to approximately <br />2.4 meters (8 feet) assuming that there was no frictional resistance to the movement. The actual <br />movement would likely be smaller. The movement would be generally parallel to the face of the OSA, <br />with the crest of the OSA moving down less than 1.9 meters (6 feet). The movement would <br />immediately cease after the major earthquake motion ceased. <br />This evaluation indicates that under a major localized earthquake there is potential for the crest of the <br />ECOSA to descend as much as 6 feet. If a haul truck were present on the crest, this might cause damage <br />and possibly injury to the driver. However, it does not seem likely that there would be damage to <br />1385E.20120224 6 <br />