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<br />STEEPEN ROBERTSON AND KIRSTEN <br />Consulting Engineers and Scientists <br />July 28, 1993 <br />SRI{ Project No. 14719 <br />Battle Mountain Gold Company, Inc. <br />P.O Box 310 <br />San Luis, CO 8 1 152-03 10 <br />Attn: Ms. Alana Scott <br />RE: SAN LUIS PROJECT PHASE II RALSE 1 DESIGN REPORT <br />Dear Ms. Scott: <br />We have reviewed the comments on the above referenced report by the Colprado Division of <br />Minerals and Geology Division as expressed in the memo from Mr. Allen Sorenson to Mr. <br />Larry Oehler dated July 21, 1993 and offer the following responses. <br />Piezometers <br />As no seeps have been observed along the downstream face of the main embankment and that <br />the pipes from the underdrain system beneath the embankment exhibit flonvs which would <br />indicate that the embankment is not widely saturated and a phreatic surface, if present, is quite <br />low. This is in accordance with the original design of the facility. As per the facility design, <br />piezometers were installed at the base of the embankment immediately above the VLDPE liner <br />which underlies the embankment in order to monitor the phreatic su#face within the <br />embankment. However, these piezometers have produced questionable varialtions in readings <br />which cast doubt on the validity of the data. Therefore, the State has requested replacing the <br />piezometers. <br />It is important to consider that the effect of a phreatic surface in the embankment is small unless <br />it develops to a significant height. Based upon accepted theories of saturated flow in porous <br />media, it is not possible, now that the tailings separate the embankment matedial from the free <br />water pond, to generate more than a minima] phreatic surface in the embankritent fill. <br />We do not advocate attempting to replace the current piezometers due to the potential damage <br />to the liner system which could result. Unless installed close to their original position directly <br />above the liner, the piezometers are unlikely to detect a phreatic surface sufficient to provide <br />meaningful data, therefore we do not recommend installing them higher in the embankment. <br />If they must be installed to receive the State's approval for the dam raise, we recommend that <br />a minimum clearance of 10 ft is maintained above the liner to preclude the possibility of liner <br />damage. <br />Slellen Robe~tsc^ ~~e ~rs;en tU 5.1. In.c. <br />3232 South Vance Street. L :~-:.rod. Colorado 80227, U.SA. <br />Tel (3031 985 t333 = .~ ~n~~e ~;G31 985 99x7 <br />Other oliices in U.S A C.:' - ~ ~.ir.yddm and F,fnCa <br />