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<br />I <br />I <br />1 <br />1 <br />I <br />1 <br />1 <br />I <br />I <br />I <br />I <br />1 <br />I <br />1 <br />I <br />1 <br />I <br />I <br />I <br /> <br />A. Steady-State Seepage Conditions - The stability of the <br />downstream slope was analyzed with the reservoir at <br />normal pool elevation 9295 (G.H. 201. This is the <br />reservoir level that will control the development of the <br />steady-state phreatic surface in the embankment. <br /> <br />B. Operational Conditions - The stabil ity of the upstream <br />slope was analyzed for rapid reservoir drawdown <br />conditions from the normal pool elevation to the outlet <br />works inlet elevation. Rapid drawdown represents <br />emergency evacuation of the reservoir, or normal drainage <br />operation if the drawdown rate is on the order of 1 to 2 <br />feet per day. <br /> <br />2.3.2 Pore Pressures <br /> <br />Pore water pressures for steady-state seepage loading conditions can be <br />estimated as hydrostati c pressures below the steady-state phreati c surface. <br />The location of this phreatic surface can be estimated for homogeneous dams on <br />relatively impervious foundations by following graphical procedures developed <br />by Casagrande ("Seepage Through Dams," Boston Society of Civil Engineers, <br />19401. The steady-state phreatic surface for Twin Lakes Dam is shown on <br />Fi gure V. 2. Because no pi ezometers exi st in the embankment, the 1 ocati on of <br />the estimated phreatic surface cannot be verified with actual data. However, <br />the exi t poi nt on the downstream face appears to agree wi th the 1 ocati on of <br />visible seepage when the reservoir water surface was near normal pool. <br /> <br />The phreatic surface method can also be used for estimating pore water <br />pressures under rapi d drawdown condi ti ons. The phreati c surface from the <br />steady-state seepage condition is modified by conservatively assuming that the <br />reservoi r water surface is lowered instantaneously, and that the phreatic <br />surface coincides with the upstream face of the embankment. For embankments <br />with semipervious to pervious materials, partial dissipation of pore pressures <br />may be assumed. Based on visual and laboratory classification, and laboratory <br />permeability tests, the materials comprising the Twin Lakes Dam embankment can <br /> <br />-51- <br />