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COLORADO GEOLOGICAL SURVEY <br /> 1801 1911 Street <br /> Golden,Colorado 80401 • • - •, <br /> June 16,2017 Karen Berry <br /> State Geologist <br /> Eric Scott <br /> Colorado Division of Reclamation, Location: <br /> Mining and Safety SE '/ of NE '/ of Section 2, <br /> Department of Natural Resources T8S,R79W of the 6"'pM <br /> 1313 Sherman St., Room 215 39.38340, -106.17910 <br /> Denver, CO 80203 <br /> Subject: Climax Mine Overburden Storage Facilities Stability Analysis; <br /> CGS Unique No. SU-17-0005 <br /> Dear Mr. Scott: <br /> As requested,we have reviewed the analysis for long-term stability for the Climax Mine Overburden Storage <br /> Facility. For this review we received the North 40 and McNulty Overburden Storage Facility Design Report <br /> (Golder Associates, May 5,2017). Golder has completed an outstanding evaluation, analysis and design report <br /> for this project. Our review is not comprehensive but focused on the long-term stability analysis and <br /> specifically the pseudo-static analysis of cross-section C-C' where the factor of safety is shown as 1.004. We <br /> generally concur with the slope-stability conclusions presented by Golder,including their conclusions about <br /> pseudo-static conditions,but offer the following comments. <br /> Seismic Risk: Golder states on page 21, ..... because the Climax mine is not located in a highly seismic area, <br /> the use of residual strengths is not strictly necessary in most areas. Nonetheless, residual strengths were used <br /> for the OSF stability evahiations, which are believed to provide an inherent level of conservatism in the <br /> design. "There are differing opinions on how recent the activity is of the nearby Mosquito Fault. However,it is <br /> generally considered a late Cenozoic fault capable of generating large earthquakes. To calculate the probability <br /> of occurrence detailed paleoseismic studies are still needed. Due to an unquantified seismic risk,we would <br /> caution against considering the design inherently conservative for the pseudo-static condition. The potential <br /> seismic activity at this location warrants the 20%reduction in strength tests recommended by Hynes-Griffin <br /> and Franklin(1984) for pseudo-static conditions as indicated in Golder's report. The conclusion of this being <br /> an overly conservative analysis is potentially misleading and should not be allowed to influence changes <br /> to design or construction changes from those proposed. <br /> Minturn Formation: The Minturn Formation has the lower shear strengths used in the overall analysis. Two <br /> site conditions at cross-section C-C' increase the overall strength of the evaluated slope and lend credence to <br /> the shear strength used for the Minturn here. These two conditions are: a favorable geologic orientation of the <br /> Minturn Formation(dipping up to 40 degrees into the slope),and failure planes that must pass through <br /> multiple beds and/or formations with differing strengths, including material with higher strength than the <br /> Minturn Formation. <br /> In general failures in the Minturn Formation can occur at strengths significantly lower than the 31 degree <br /> strength measured and used in the slope stability analysis (with a 20%reduction for the pseudo-static <br /> condition). Even when discounting cohesion,this is a high strength for the Minturn Formation. Large scale <br /> dip-slope failures in the Minturn, specifically at Dowd's Junction, Colorado, fail along slopes around 18 to 20 <br />