1995-08-08_REVISION - M1988112 - Laserfiche WebLink

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• iii iiiiiiiiiiiuiii , STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Deparlmem of Nawral Resources 1313 Sherman St., Room 215 y Denver, Culorado 80203 Il ~ Phone 0031 A66~3567 I FAX: 0031 832-810h DEPARTMEtdT OF NATURAL August 8, 1995 RESOURCES Roy Romer Ms. Sally Hayes co~rmor Battle Mountain Resources InC IamesS Lochhrad , . P.O. Box 310 E~ecul rve Director M~ehari a ~nnti San Luis, CO 81152-0310 D~~~:~o~ Di~rcwr RE: Phase II, Raise 2, Embankment Design Report, Sau Luis Project, Permit No. M-88-112 Dear Ms. Hayes: I have received and reviewed SRK's Embankment Design Report which addresses the geotechnical suitability of the tailings material as a foundation for phase II raise 2 of the tailings embankment. Pursuant to the approved permit, BMRI is required to verify embankment design assumptions prior to construction of raise 2 through monitoring (piezometers), testing (cone penetrometer probes, etc.) and stability modeling (static and dynamic conditions). Section D.6.5.4. I of the approved permit states "raises will be of an upstream construction type and will be placed on dewatered consolidated tailings". SRK recognized, from the available test data, there is an interval of "saturated" tailings that will underlie a portion of the upstream section of raise 2. As a result, SRK completed a very thorough pseudostatic model of the embankment, considering liquefaction conditions in the tailings, using the required .15g seismic coefficient. The results of the modeling effort indicate that the embankment, in general, would remain stable even though there is potential for some deformatioc during the MCE (maximum credible earthquake). SRK predicts "the displaceme)t alo2:g thn critical potential failure surface for the downstream face" (predicted to occur a[ the interface of the VLDPE liner and the bottom of the main embankment) "is estimated ro be on tha order ojl-3 feet jot a PGA (peak ground acceleration) of 0.6 g arising from a magnitude 7+ earthquake. For the upstream face of the embankment raise, and a PCA of 0.6g, the displacement is on the order of /-/.S jeer". SRK predicts there will be some defonnation (settling) of the tailings and [he upstream face of raise 2 during the post-liquefaction phase of a seismic even[. Since the upstream crest of raise 2 will be founded on approximately 20 feet of saturat~zd tailings, and the downstream crest founded on 16 feet of unsaturated tailings, there could be .6 and .5 feet of settlement beneath the upstream and downstream crests of the raise, respectively. However, even though the pseudostatic models imply that the conceptual raise design provides a stable structure, the fact remains: saturated tailings exist within the foundation material proposed for raise 2.