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Pte- ~k.>1 V of I L <br /> <br />EDNA MINE <br />MOEFAT AREA <br />PHREATIC SURFACE <br />In accordance with the Settlement Agreement dated Novemt,er 19, <br />1987, .a description of the pre~iicte,i steady state eater cor~~iitior~s of <br />!,he embank.mer~t below the proposed se~iimer~t por~~i are descrit~ed ire this <br />report. <br />With the addition oi' the sediment pond iri the Moffat area, the <br />steady state seepage conditions may be altered in the emb.ar~k.mer~t from <br />the er.istir~g conditions. The location of the phreatic surface is <br />determined by the eater elevation of the pond and the location of the <br />pond from the edge of the slope. The eater elevation of G999.G feet <br />was used to determine the steady state seepage within the embankment. <br />This water elevation corresponds to the elevation of the primary <br />spillway of the proposed pori,i. Although higher eater levels could <br />occur ire the pond, the duration would be short and steady state <br />conditions at this higher eater elevation woul,i not be reached. <br />The methods developed by Yang H. Huang were used to predict the <br />+,heoretical phreatic surface of the embankment below the proposed <br />sediment pond. An e::plan.ation of Huar~g's method is included in Chapter <br />4 `phreatic Surfaces' in the book.. St,ahlity_..Ar~31yg_is..,of_._E3rth_..Slo_pes.. <br />Huang has developed charts to determine paints on the predicte~J <br />phreatic surface following procedures developed by Cas.agr.an~ie in 1937. <br />These charts were used to determine the exit point of the phreatic <br />surface in the creek. bank.. A copy of these charts ar~d .a typical cross <br />section of a pond embankment, showing critical parameters, is shown ir•, <br />Figure 1. The location of the pond is shown in detail or. E;:hit~it 4.G-3 <br />of the Moffat Area permit revision application. <br />A typical cross section of the topography of the slope in the area <br />of the proposed sediment pond is shown in Figure 2, This configuration <br />is typical of the southern section of the designed pond, in that the <br />dam area is 100X incised with no fill at the top. This area was first <br />~.ised to determine the surface seepage elevation in the einb.ank.ment <br />slope. Two other areas, one .at the primary discharge point ar~~i another <br />at the maximum point of fill along the Siam, were also check.e,i. Hu.ang's <br />methods are used to predict 3 points along a prob.aGle phreatic surface <br />perpendicular to the ~i.am face. The phreatic surface shown in Fig~~re 2 <br />exited at the highest location oY the 3 test sites at an elevation oi:' <br />approximately G971.°, feet. This elevation would place the phreatic <br />surface exit within the. Trout_C_reek drainage channel mak..ing it <br />diYficult to differentiate ground seepage from the normal water flow of <br />Trout Creek,. At the other two test sites Huar~g's methods could riot tie <br />used to accurately determine the elev.atior~ of .an exit point of the <br />phreatic surfaces t~ecause the data, measured from cross sections, fell <br />below the realm of the charts. However, ore can predict from the <br />charts that the phreatic surface e::it locations should be cell below <br />the outslope bank. of the d.am and therefore loner than the elevation <br />determined ire Figure 2. The parameters used in each of the 3 test <br />cases are listed or. the following page. <br />.~ - <br />