1990-01-18_REVISION - M1988112 (7)

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Memo to S. G. Renner Battle Mountain Amendment page 2 Topic 25: Pseudostatic Stability of the Tailings Structure Your initial adequacy comment indicated that Battle Mountain shouldi provide further design specifications to achieve a minimum pseudostatic (earthquake loading) factor of slope stability of at least 1.25 for the tailings structure. Mr. Dorey of SRK has responded with a clarification of the their earlier submittal. Very briefly summarized, Mr. Dorey first expounded upon the choice of the equivalent acceleration factor used in performing their pseudostatic analysis. Battle Mountains seismological consultant, Dr. Charles Glass, indicated that the maximum credible earthquake (MCE) for the immediate portion of the Sangre de Cristo Fault which would impact the tailings structure is of Richter Magnitude 6.8 to 7.0. He projects a probability of occurrence for this event of from 0.6 to 12 percent in a 100 year period. An earthquake of this magnitude, if it were to occur, would impose a maximum instantaneous acceleration of approximately 0.6 g upon the tailings embankment. Dr. Pat Rogers of the CGS, who has personally investigated the Sangre de Cristo Fault, believes Dr. Glass' conclusions are appropriate. SRK does not believe it is appropriate to use the instantaneous maximum acceleration of the MCE to complete the pseudostatic analysis. They present two justifications for their assertion: (1) The extremely low observed seismic activity of the fault during the past 110 years of scientific observation; and, (2) the standard methodology for conducting pseudostatic analyses converts from MCE acceleration to an "equivalent acceleration" for conduct of the analysis. Rob Dorey reasons that the 10 year project life implies a probability of from .06 to 1.3 percent for the occurrence of the MCE during the project life. Further, he observes, virtually no historical activity has been observed in the past 110 years of record, and that Kirkham and Rogers, in CGS Bulletin 43, concluded that the last exhibited movement of the fault occurred between 1,940 and 4,715 years ago. Rob Dorey concludes that the extremely low probability of the specific MCE, makes it more appropriate to base the selection of the "equivalent acceleration" for the analysis upon accepted projections of the potential for regional seismic activity, such as Algermissen et al (1982). Based upon Algermissen et al, the standard accepted by the National Building Code, the suggested peak ground acceleration for this project area is 0.13 g, projected to have "an associated 90 percent probability of not being exceeded in 250 years". Neither Dr. Rogers or I disagree with SRK's rational. In responding to your preliminary adequacy comment, SRK has also reexamined their original pseudostatic analysis of the tailings structure. Rob Dorey has extended the analysis, including the performance of a parametric sensitivity analysis. The parametric sensitivity analysis evaluates how sensitive the result of the analysis is to the variation of the various input parameters. In short, how much does the calculated factor of safety change, if we vary one or more of the input parameters, such as material shear strength or the factor of acceleration? The results of the sensitivity analysis are depicted graphically in Attachment 9 (Figure D.6-17 ).