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Mr. Raymond Lazuk June 28, 2017 Climax Molybdenum Company, Climax Mine 2 1661214 006 RSP RevO Golder understands that the DRMS is in the process of reviewing the Golder(2017)TR-25 submittal. Based upon the initial DRMS review, one issue has been raised as a possible concern, warranting a technical response. The potential issue is that "the modeled pseudo-static factor of safety (FoS) for design section C-C' during operations and closure is given as 1.09 and 1.00 respectively." The intent of this letter is to provide additional technical background information and justification for the pseudo-static stability results as presented in Golder 2017. 2.0 OSF DESIGN BASIS As documented in Golder(2017), the Climax McNulty and N40 OSF were designed to be consistent with the project design criteria. It is Golder's opinion that the geotechnical design criteria, geotechnical characterization completed, and geotechnical modeling performed meet or exceed the current domestic and global industry standards of care for the geotechnical design of mine waste impoundments and OSFs. Golder evaluated the stability of the OSFs with seven cross-sections, the locations of which were selected to represent the most critical or worst-case stability conditions(i.e., steepest foundations, highest stockpile locations, etc.). These design sections were generally oriented perpendicular to the foundation and OSF slopes, in areas with steep grades and the greatest fill height. Design section C-C' is located near the southern margin of the McNulty OSF, near the northern limits of the N40 OSF, where the overburden is relatively shallow but toes out on moderately steeply native topography. There is approximately 500 feet of relatively flat terrain between the toe of the OSF and state Highway 91. For the assessment of seismic criteria, Golder developed an operational basis earthquake(OBE)to assess the stability response of the OSFs from a seismic event during active operations. Consistent with current standard of care for mine waste stockpiles and low to moderate risk impoundments, a 1-in-475-year return interval event was selected, which represents a 0.2% annual probability of exceedance interval during operations, or a 10% probability of exceedance over a 50-year period. For assessment of post closure conditions, Golder conservatively used the maximum design earthquake (MDE) which corresponds to the 1-in-2,475-year return interval. It is Golder's opinion that the return intervals for the design earthquakes selected meet or exceed the current standards of engineering practice for non-high hazard structures,e.g., the Climax OSFs. Note that the use of low, moderate and high hazard as typically used for dam safety evaluations is not considered to be consistent with the definition of non-critical versus critical as used in the DMRS (2017) memo. It is also noted that these standards of care may differ from those used for public transportation projects such as those that CDOT and CGS routinely review. The stability design criteria used by Golder for development of the Climax OSF designs are summarized below, with the corresponding applicable DMRS criteria from DMRS (2017) provided in parenthesis (): ■ Active Operations Criteria: • Minimum allowable static factor of safety is>_1.4 (>_1.25 for non-critical structures, >1.3 for critical structures) • Minimum allowable seismic (pseudo-static) factor of safety is _>1.0, or an acceptable level of displacement(>_1.1 for non-critical structures>_1.15 for critical structures) • Operational basis earthquake (OBE) PGA is 0.06g — representing the 1-in-475-year event(the DMRS (2017) guidance does not define a seismic reoccurrence interval) ■ Closure and Post-Closure Criteria: • Minimum allowable static factor of safety is>_1.5 (>_1.25 for non-critical structures, >_1.3 for critical structures) • Minimum allowable seismic factor of safety is >_1.0, or an acceptable level of displacement(?1.1 for non-critical structures>_1.15 for critical structures) • Maximum design earthquake (MDE) PGA is 0.14g— representing the 1-in-2,475-year event(the DMRS (2017)guidance does not define a seismic reoccurrence interval) - - - Golder 1661214 006 RSP RevO Responses to DRMS Comments 28JUN17 docx ,=dates