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9(1) GEI Consultants, Inc. <br />May 22, 2003 <br />Project 99292 <br />Mr. Byron G. Walker <br />Colorado Division of Minerals and Geology <br />1313 Sherman Street, Room 215 <br />Denver, CO 80203 <br />6950 South Potomac Street <br />Suite 200 <br />Englewood, CO 80112 <br />303 •662 0100 <br />303 •662 8757 Fax <br />Re: Additional Analysis: Geotechnical Evaluation of Mine-Induced Seismicity on Bruce <br />Park Dam - <br />Dear Mr. Walker: <br />In response to your Telephone Contact Report dated December 11, 2002, included in Attachment <br />• A, we have performed additional analysis to evaluate the likely impacts of mine-induced <br />seismicity on Bruce Park Dam. Results of this analysis and our conclusions are presented in this <br />letter. <br />Background <br />Figure 3.4 in our report (included in Attachment B), Geotechnical Evaluations of Mine-Induced <br />Seismicity on Bruce Park Dam (GEI 2002), presented a comparison between the average <br />maximum accelerations generated by a 4.2 magnitude mine-induced seismic event and yield <br />accelerations associated with a factor of safety (FOS) of 1.0 from pseudo-static slope stability <br />analyses for the main dam. Since yield accelerations were greater than the average maximum <br />accelerations from the seismic event throughout the height of the dam and foundation soils, we <br />concluded that there would be no permanent seismic deformations from a magnitude 4.2 event. <br />Since magnitude 4.2 was the strongest event expected at the site, other events being smaller than <br />magnitude 4.2 would also not generate any permanent deformations in the main dam. <br />DMG recommended that a conservative approach be taken based on using yield accelerations <br />associated with a pseudo-static FOS of 1.2. <br />Analysis <br />We computed yield accelerations for various depths below the crest, for the downstream slope by <br />• considering potential failure surfaces that initiated upstream of the crest for FOS of 1.2. A <br />modified Figure 3.4 is included in Attachment C that also shows yield accelerations along the <br />depth below the crest for FOS of 1.2. These yield accelerations are greater than the average <br />maximum accelerations for magnitude 4.2 seismic event and permanent deformations of the dam <br />for a mine-induced seismic event of magnitude 4.2 should not occur. Since zero permanent <br />PR-16 <br />9929203-05.19 DMG later Offices Nationwide