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TR-9 Geotechnical Review Memo April 25, 2025 <br /> <br />While under the assumption of Mine revocation, it is understood that Mine personnel will no longer be <br />present. However, the potential risk for human safety remains given the location of the site. The Mine is <br />located on public land that is managed by the Bureau of Land Management which is frequented by the <br />public for outdoor recreation. Additionally, the area has cultural and historical significance for Native <br />American tribes in the region. Given that the area has the potential for public interaction, the risk to <br />human safety still exists and is considered “critical”. As such, the corresponding minimum factors of <br />safety are 1.5 and 1.3 for static and seismic conditions. <br /> <br />Material Strength Property – Cohesion <br /> <br />Material strength properties used within the Report were assumed and back-calculated values used in <br />KUE’s engineering report which was approved under TR-6. With no site-specific material strength <br />testing, material strength properties were taken from published and verified typical values for the <br />encountered limestone and interbed material at the Mine. Using conservative site parameters, a back <br />analysis was conducted by setting the factor of safety to just below 1 which is the minimum criteria for a <br />failure. The purpose of this back analysis was to corroborate the published value for cohesion used in the <br />geotechnical model. For cohesion of the interbed material, empirical values were listed at 40 psf with the <br />back analysis calculated at a value of psf. As an additional measure of conservatism, KUE reduced <br />the back-calculated cohesion to psf which was used in KUE’s geotechnical analysis. For more <br />information summarizing the Division’s review of KUE’s assumptions and parameters, please refer to the <br />Division’s review memo for TR-6 dated September 29, 2023. <br /> <br />As discussed in the Board Policies – Section 30, Table 1 section of this memo, GGI uses the designation <br />of “non-critical”. As part of this designation, GGI notes that the additional degree of conservatism <br />regarding the cohesion value for the interbed material is no longer relevant and original back-calculated <br />value of psf. is used in the Report. However, as also discussed earlier in this memo, given that the <br />area has the potential for public interaction, the risk to human safety still exists and the site is still <br />considered “critical” in a revocation scenario. As the site is considered “critical”, the interbed cohesion <br />value of psf assigned with the “critical” designation should be consistent and maintained through the <br />engineering analysis for the revocation scenario. <br /> <br />Additional Comments <br /> <br />When reviewing the Report, calculated factors of safety are provided within the discussion however no <br />associated geotechnical slope stability analysis results were provide for the Division’s review. The <br />resultant model analyses are necessary for the review to ensure that the discussed site parameters and <br />assumptions are consistent and accurately applied to the slope stability models. Additionally, there were <br />no discussions regarding the seismic parameters applied to the pseudo-static models. However, it should <br />be noted that blasting was discussed but not analyzed in the Report. Under the revocation scenario, <br />blasting will no longer be present and is longer relevant. It was also determined in KUE’s report, <br />approved with TR-6, that the pseudo-static seismic acceleration parameters are more conservative and <br />control the analysis. <br /> <br />Adequacy Items <br /> <br />The following is a summary of the Division’s comments/questions discussed and observed during the <br />previous sections of this memo: <br /> <br />• As the site is considered “critical”, the interbed cohesion value of psf assigned with the <br />“critical” designation should be consistent and maintained through the engineering analysis for <br />the Mine which includes the revocation scenario. Please have GGI provide updated slope <br />stability analyses with the use of a cohesion value of psf for the interbed material. <br /> <br />• Please have GGI provide details regarding the seismic parameters applied to the pseudo- <br />static models.