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I I I I I I I I I I I I I I I I I I I normal pool elevation to near the outlet works inlet elevation. Rapid draw down represents emergency evacuation of the reservoir, or normal drainage operation if the drawdown rate is on the order of 1 to 2 feet per day. Earthquake loading was represented by applying an outward horizontal and upward vertical acceleration of O.lOg to the potential slide masses. 2.4.2 Pore Pressures Pore water pressures for steady-state seepage loading conditions can be estimated as hydrostati c pressures bel O\~ the steady-state phreati c surface. The 1 ocati on of the steady-state phreati c surface was estimated us i ng data collected from piezometers installed in the boreholes drilled for the geotechnical investigations and Casagrande's theoretical procedure. The steady-state phreatic surface for Barnes Meadow Dam is shown on Figure V.3. The phreatic surface method can also be used for estimating pore water pressures under rapid drawdown conditions. The phreatic surface from the steady-state seepage condition was modified by conservatively assuming that the reservoi r water surface is lowered instantaneously, and that the phreati c surface coincides with the upstream face of the embankment beneath the riprap. The rapid drawdown phreatic surface is also shown on Figure V.3. 2.4.3 Material Properties The principal soil properties used in stability analyses are density and shear strength. Fi e1 d and 1 aboratory soi 1 s tests, as well as past experi ence wi th similar materials, were employed to arrive at the Barnes Meadow Dam embankment properties shown on Figure V.3. Exploratory borings in Barnes Meadow Dam revealed a relatively homogeneous embankment consisting of medium dense to dense, silty sand with gravel, cobbles, and occasional boulders. The borings also indicated the dam is 1231 H -57-