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Mr. Raymond Lazuk June 28, 2017 Climax Molybdenum Company, Climax Mine 5 1661214 006 RSP RevO One recent trend that Golder has observed for large mine waste structures is the standards of care being adopted for critical structures are similar to the requirements developed for impoundments. Specifically,the earthquake reoccurrence intervals are being selected based on the design life and risk level, with an OBE developed to evaluate the relatively short-term operational life of the facility, and a MDE developed to evaluate performance of the facility under long-term conditions (e.g., post closure). It should be recognized that for the long-term closure scenario, the corresponding MDE represents a conservative long-term reoccurrence interval, which varies depending on the risk level posed by the facility, but is commonly defined as the 1-in-2,475-year event, or the event with 2%probability of exceedance over a 50-year period. The corresponding FoS calculated using the larger MDE will inherently result in lower FoS relative to the more short-term OBE event. However, evaluating the MDE allows the consequence of a larger, low probability event to be more realistically estimated.As a result,the actual risk to the facility is arguably less, provided an appropriate event is selected for the MDE and the consequences of the MDE (e.g., FoS or calculated displacement) are acceptable. Although criteria for dams are generally considered to represent a more stringent condition than for a non- impounding OSF, the International Commission on Large Dams (ICOLD) guidelines are useful for comparison with the project design criteria. For reference, consider the (COLD criteria from Bulletin 148, "Selecting Seismic Parameters for Large Dams" (ICOLD 2016),where ICOLD defines two different seismic events. The Safety Evaluation Earthquake(SEE), which is analogous to the MDE as defined in the Climax OSF evaluations, and the OBE. Under ICOLD's terminology, the SEE and OBE are both evaluated, but with different criteria for acceptable performance. Specifically: ■ SEE is defined as "the maximum level of ground motion for which the dam should be designed or analyzed... It will be required at least that there is no uncontrolled release of water when the dam is subjected to the seismic load imposed by the SEE". ICOLD notes that a higher level of performance should be expected under the OBE (e.g., a higher FoS). ■ OBE is defined as"the level of ground motion at the dam site for which only minor damage is acceptable...In many cases, it will be appropriate to choose a minimum return period of 145 years (i.e., a 50% probability of not being exceeded in 100 years)" The analysis presented in Golder(2017)was developed to be generally consistent with this philosophy. It is recognized that there are many documents that can be sited associated with regulatory oversight of the mining industry and which generally reflect the current the standard of care for the industry. One commonly referenced document is the Best Available Demonstrated Control Technologies (BADCT) document by the Arizona Department of Environmental Quality (ADEQ 2016). As a comparison, the prescriptive stability criteria for a large dump leach, which is considered analogous to the Climax OSFs, consists of: ■ Static Factor of Safety= 1.3 ■ Pseudo-static factor of safety = 1.0, or alternatively the predicted magnitude of displacements will not jeopardize the facility containment. • The earthquake to be considered in the pseudo-static evaluation is defined in different ways within the document, alternately as "the maximum earthquake likely to occur during a 100-year interval (80% probability of not being exceeded in 100 years)" and "largest earthquake with a 100-year return interval."The 1 in 100-yr event specified is considered analogous to the OBE developed for the Climax OSFs (Golder selected a 1-in-475-year reoccurrence interval which is typically acceptable for water retention structures). • Where human life is potentially threatened, ADEQ includes a requirement for consideration of the maximum credible earthquake. While Golder does not consider the Climax OSFs to be critical life threatening facilities to the general public similar to Golder es to DRM Comments 28JUN17 docxAssociates 1661214 006 RSP RevO Responses S