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N <br />The emerging water, although clear, is increasing. According to <br />Mr. Pete Evans, Colorado Mine Land Reclamation Board Geologist, in a <br />telephone conversation on July 21, 1982, captured surface seepage has <br />increased from 10-15 gpm in 1978 to 200-230 gpm in the spring of 1982. <br />Numerous facilities are located in the possible flood plain downstream <br />from the dam. Miners Gulch Road (gravel) crosses 2100 feet downstream <br />and State Highway 149 crosses 4000 feet downstream from the dam. <br />Several cabins, which might be affected by a dam breach, are located <br />along or near the left bank of the Rio Grande River about 7000 feet <br />downstream from the dam. <br />COMMENTS AND CONCLUSIONS <br />Based on existing facilities downstream from the dam, this structure <br />should be classified in the moderate to high hazard category in the <br />event of a dam failure. Considering future development possibilities <br />downstream, the high hazard potential rating would appear to be the most <br />appropriate. <br />Our figures indicate, based on available data, that the outlet facili- <br />ties, combined with storage above the invert elevation of the spillway, <br />can handle the 6 -hour probable maximum precipitation (PMP) with little <br />or no freeboard remaining. MSHA's recommended design storm for high <br />hazard potential dams is the PMP 6 -hour storm. <br />With respect to embankment stability, MSHA guidelines recommend that an <br />impounding structure have minimum static and seismic factors of safety <br />of 1.5 and 1.2 respectively, under maximum anticipated phreatic <br />conditions. According to the Dames 6 Moore report of July 1, 1975, <br />prepared for Homestake Mining Company, static and seismic factors of <br />safety were 1.42 and 1.08 respectively. However, for seismic analysis <br />Dames 6 Moore used a pseudostatic horizontal loading condition of <br />10 percent gravity which is somewhat larger than that required for <br />the seismic zone in which the dam is located. <br />Reportedly, the Colorado State Engineers Office has requested a new <br />stability analysis. We concur with this request and suggest that several <br />analyses be conducted based upon (1) existing conditions, (2) a full <br />reservoir with accelerated seepage, (3) increased embankment height <br />without remedial measures, and (4) maximum embankment -maximum pool with <br />remedial work completed. The analyses are to be performed by the owners' <br />consultant, D'Appolonia Consulting Engineers, Inc. <br />The consultant has previously presented two alternatives to the company <br />to eliminate problems developing as a result of excessive seepage flow <br />through the structure. One entails curtailing the seepage by use of <br />slurry walls and sealing of the reservoir to the extent feasible. The <br />other alternative consists of a gravity collection and return flow <br />system. The consultant may also examine the feasibility of foundation <br />grouting. To assist in determining the alternatives available, subsur- <br />face data was obtained by drilling six borings, conducting various tests <br />as related to the borings, and installing standpipe -type piezometers in <br />each of the borings drilled. <br />