2017-07-07_REVISION - M1977493

• Permit No: M1977493
• IBM Index Class Name: REVISION
• Doc Date: 7/7/2017
• Doc Name: Adequacy Review
• From: Climax Molybdenum
• To: DRMS
• Type & Sequence: TR25
• Email Name: ECS
• Media Type: D
• Archive: No

Golder
(2_-i)A_ ssoC1ates
June 28, 2017 1661214 006 RSP RevO
Mr. Raymond Lazuk
Environmental Manager
Climax Molybdenum Company, Climax Mine
Highway 91 —Freemont Pass
Climax, Colorado 80429
RE: RESPONSES TO DRMS TR-25 REVIEW COMMENTS REGARDING SECTION C-C,
Dear Ray:
This letter has been prepared by Golder Associates Inc. (Golder)to provide additional technical information
in response to the adequacy review comment received from the Colorado Division of Reclamation, Mining
and Safety(DRMS) (DRMS 2O17b) during the review of the TR25 submittal (Golder 2017) for the Climax
North 40 and McNulty overburden storage facility (OSF) expansion. Specifically, the DRMS review noted
that the modeled pseudo-static factor of safety (FOS) for design section C-C' is not consistent with the
guidance provided in a recent DRMS guidance memorandum (DRMS 2O17a)on industry standards of care
relative to FOS. DRMS stated that "the long term stability of the McNulty overburden storage facility is a
primary concern" and has requested "additional discussion or information regarding the submitted post-
closure seismic (pseudo-static)factor of safety for Section C to demonstrate that the provided post closure
FOS of 1.0 is appropriate and acceptable".
1.0 OVERVIEW
Golder(2017) provides the Golder final engineering evaluations and analyses for the North 40 and McNulty
overburden storage facility (OSF) expansion designs. Both OSFs are located at the Climax Molybdenum
Company — Climax Mine (Climax). The report includes results of the field investigation, laboratory
investigation, operations and monitoring (O&M) recommendations, and design calculations performed in
support of the ultimate OSF designs.The report presents the final operational water management designs,
demonstrates the geotechnical suitability, and presents the closure and reclamation strategies for the North
40 and McNulty OSF expansions. Climax submitted the Golder(2017) report to the DRMS to facilitate an
adequacy review as a formal Technical Revision 25 (TR-25) to the Climax Reclamation Permit
No. M-1977-493, as most recently amended by AM-06 (Climax 2010).
DRMS (2O17a) developed a recent guidance memorandum to document the DRMS understanding of the
current standard of care for slope stability FOS. The memorandum presents the basis of the guidance is in
part due to the recognition of inherent geologic, hydraulic, and geotechnical uncertainties. The guidance
memorandum also notes that the recommended slope stability factors of safety, based on uncertainty and
public safety categories, are intended to be consistent with the Colorado Department of Transportation
(CDOT) and the Colorado Geological Survey (CGS) criteria. For structures such as a mine OSF, the
recommended static and pseudo-static factors of safety are 1.25 and 1.1, respectively, where strength
measurements are available from multiple sources or tests. Golder interprets the Climax OSF's to be
consistent with this category, as we have completed multiple geotechnical characterization programs at the
Climax OSF sites. For a critical structure, e.g., a water retention dam, public highway, or structure that has
a high likelihood of imposing imminent public danger(e.g.,where the potential for significant displacements
exist), the recommended static and pseudo-static factors of safety are 1.3 and 1.15, respectively, where
strength measurements are available from multiple sources or tests.
1661214 006 RSP RevO Responses to DRMS Comments 28JUN17 docx
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