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~~ DIVISION OF MINERALS AND GEOLOGY Dephrtment of Natural Resources 1313 Sherman 51., Room 215 Denver, CO 80203 Phone: (303) 866-3567 FAX: (J03) 832-8106 ti~ ~~ 'revs Roy Romer Gvemor Michael B. Long July, 21 1993 oivi:ian 0irec,or TO: Larry Oehler FROM: Allen Sorenson RE: Design Report for the First Upstream Raise at the San Luis Mine Tailing Impoundment, Battle Mountain Resources, Inc., File No. M-88-112 Battle Mountain Gold (BMG) and their consultan#s, Steffen, Robertson, and Kirsten.(SRK), have prepared and submitted a design report which addresses the suitability of the in-situ tailing as a foundation for the phase II upstream raises. Subm>_ tal of this report is a result of commitments made by BMG in the ~pproved mine plan to verify design assumptions through monitoring, testing, and analysis as the tailing impoundment was developed. As part of the monitoring program incorporated into the San Luis Project mine plan, BMG was to install and `monitor'piezometers within the tailing emban}anent. Readings of the five vibrating wire piezometers installed in the embanlanent indicate that a structure is unsaturated,. as designed, however, inctinsi tencies in temperature `readings provided ly.'the'piezometers:demo strate that the instruments are~not functioning properly, and the piezometric data cannot lie used with reliability. It is critical, in terms of embatLlonent stability, that the phreatic surface, or lack thereof, within the embankment be monitored. BMG /~2 should be required to install properly functioning piezometers within the embankment sufficient to fully cliara~terize the saturated or unsaturated state of embankment materials prior to construction of the phase II raises. Installation of the piezometers must be carefully supervised to preclude the possibility of damaging the synthetic liner beneath the embankment. LIQUEFACTION AND STABILITY ANALYSES In adequacy responses submitted during consideration of the San Luis Project permit amendment, the operator stated: STATE OF COLORADO The Phase 2 raises will be founded upon unsaturatied tailings sand and the embankment itself, therefore, the design has assumed liquefaction in this area cannot occur.