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iiiiiiiiiiiiiiiim • STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Department o(Natural Resources 1313 Sherman Sr, Room 215 Denver, CO 80203 Phone: (30l) 866-3567 FAX: (3031 8328106 September 27, 1993 TO: Larry Oehler FROM: Allen Sorenson RE: Phase II, Raise I Embankment Lift, Battle Mountain Resources, Inc., San Luis Project, File No. M-88-112 OF' ~~~0 ,e , ~~~(r~y^~•) ~~ •~a rJ' n• .f~~ ~ rB 16 Rny Romcr Governor Michael B. Lnnp Division Director This memorandum is the result of review of the letter from Battle Mountain Gold (BMG) dated September 13, 1993 which included responses to Division concerns and an update on raise construction. Piezometers BMG has proposed installation of five standpipe piezometers across the embankment crest to replace the malfunctioning pneumatic piezometers and provide pertinent information on the phreatic surface within the dam. The standpipes will be completed at an elevation 10 feet above the liner interface beneath the @mbankment, and although this w}ld-configuration will not yield any information on a phreatic surface that might exist within the lowest portion of the embankment, BMG has completed slope stability analyses that demonstrate any saturated zones within the lowest ten feet of the embankment would not affect the stability of the structure. Since the piezometers proposed by BMG will yield the pertinent information needed to ensure embankment stability, and completion 10 feet above the liner interface will not threaten the integrity of the liner, the Division can approve the BMG proposal. Liquefaction and Stability Analysis BMG has provided the Division with the requested analysis of the affect of earthquake induced settling of the tailings on embankment stability. BMG has concluded that the amount of settlement that could occur is within the range of strains that could be safely tolerated by the structure. I would agree with their conclusions, however I would recommend that the Division require that the tailing embankment undergo a complete inspection for stability related concerns, and that a certified report, including any recommendations, of that inspection be submitted to the Division within 30 days in the event that a magnitude 6 or greater earthquake occurs within 200 kilometers of the site, and that such a report be required to be submitted within 7 days for any magnitude 7 or greater earthquake occurring with 250 kilometers of the site.