1990-01-18_REVISION - M1988112 (7)

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Memo to S. G. Renner Battle Mountain Amendment page 3 In performing its original pseudostatic analysis of the tailings structure, SRK assumed a material shear strength for the non-tailings embankment with an angle of internal friction = 35 degrees and cohesion = 0. This original analysis projected a factor of safety (F OS) of 1.0 under a equivalent acceleration factor of 0.15 g. The sensitivity analysis determinedi that if the fill material's shear strength is increased by employing an angile of internal friction = 38 degrees and cohesion = 200 psf, the FOS (Factor of Safety) at 0.25 g is approximately 1.4. SRK believes that this less conservative material strength is more realistic for the sandy gravel source material located at the embankment site than the original material shear strength assumptions. In my opinion, Mr. Dorey's material strength opinion is not unrealistic for the relatively granular native materials of the site. However, ~ examination of the original permit application's geotechnical appendices did not locate any laboratory shear strength test results for those native materials. The Division should stipulate that the materials intended for construction of the embankment be tested to determine their shear strength, prior to embankment construction, in order to verify Mr. Dorey's opinion. Should the material strengths prove less than expected the embankment design could be amended to increase the factor of safety. In summary, SRK has clarified and justified their derivation of the 0.15 g acceleration factor for the pseudostatic stability analysis of the tailings structure. Further, they have addressed your concern regarding the projected pseudostatic factor of safety. Their approach is consistent and appears to be conservative. As in any analysis of this nature, the ultimate test of the accuracy of their assumptions and design decisions lies in verification of actual site conditions, material properties and structure performance. In my opinion they have demonstrated to my satisfaction that their design approach is prudent. It remains for them to verify their assumptions and projections. The original permit application and amendment application documents contain numerous commitments to monitoring and verification exercises. In order to avoid misunderstanding, I recommend that we require the applicant to submit a brief but comprehensive description of a "Monitoring, Verification and Reporting Program". This program description should clearly state the purpose of the program. It should include consideration of all forms of monitoring; geotechnical, hydrological, water quality, air quality, revegetation, etc. It should summarize the monitoring facilities to be installed and observations to be performed, including a schedule or frequency of monitoring. It should commit to the periodic reduction of the monitoring data and the comparison with projected design properties and performance standards. Appropriate reporting frequencies should be proposed, with no less frequent than annual report submittals. Exhibit L, "Reclamation Costs" should be amended to include bonding necessary to complete the monitoring and verification program in the event of default or premature termination of the project. Finally, the applicant's proposal should clearly state the applicant's commitment to have responsible professionals in charge of site operations and the conduct of the monitoring program.