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~IIII~II~~I~II~~I~~ BATTLE MOUNTAIN RESOURCES. INC. September 13, 1993 5Ep 20 1993 Mr. Larry Oehler Division of Minerals and Geology 1313 Sherman Street, Room 215 Denver, Colorado 80203 ~/EMO~+~+ D COMPANY Re: Battle Mountain Resources, Inc. (BMRI) - San Luis DMG Permit No. M-88-112 Phase II, Raise I Embankment Lift Dear Mr. Oehler: This letter is written in response to the Division's August 13, 1993 review letter for the Phase II, Raise embankment lift. Piezometers The possible use of geophysical methods, specifically electrical and seismic surveying as suggested by the Division, to identify the potential phreatic surface under the main embankment has been reviewed. Shallow seismic methods require extensive data reduction and interpretation. The data manipulation would include site specific and condition specific correction factors with a tolerance resolution greater than what would likely be acceptable for locating the potential phreatic surface. Electrical resistivity methods can provide higher resolution results than shallow seismic methods. Electrical surveys identify changes in resistivity; certain conditions, such as the salinity of the tailings, may interfere with identifying resistivity boundaries which will increase the potential for interpretation errors. Also, electrical methods require significant data reduction and interpretation. Due to the potential for data reduction and interpretation errors associated with shallow seismic and electrical methods, the existing vibrating wire piezometer will be replaced by simple stand-pipe piezometer. Stand-pipe piezometers offer the advantage of requiring no data reduction, no data interpretation, and no instrumentation failures. The issue of potential liner damage during installation of the standpipes has been addressed through additional modelling efforts. Stability analyses of the critical downstream face, sliding wedge failure mode, such as depicted on figure 4.2 in the design report, indicates that including a phreatic surface which is 10 feet above the liner has little effect on stability of the structure. Under pseudostatic loading conditions with a seismic coefficient of O.15g the factor of safety decreases from 1.47 to 1.38 when a 10 ft high phreatic surface is included in the analysis. A SUBSIDIARY OF RA7T! F.NOUATAIN GOl,O COMPANY P.O. BOX 310 SAN LUIS, COLORADO 81 ISLO110 (719) 672-776? FAX (719) 672-3367