2012-08-16_REVISION - M1977493 (2)

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Response: As part of the "worst case" evaluations (see Scenario 3, section 6.3 of the Supplemental Stability Analysis (SSA) in Attachment 1 to this letter), Golder evaluated the potential impact of elevated groundwater levels on OSF stability. The "worst case" Scenario 3 evaluation considered piezometric levels up to 50 ft above the existing ground surface within the OSF, in conjunction with placement of a high percentage of lower permeability sedimentary mine waste near the toe of McNulty. Figure 8 in the SSA provides a plot of the computed stability factor of safety versus the height of a phreatic surface within the OSF above the existing topography. The evaluation demonstrates a stable condition under such adverse conditions. In addition to the worst case stability evaluations considered in the SSA, Golder also completed a sensitivity assessment of the impact of elevated groundwater levels in the May, 2012 Design Report (TR -22). The results of this sensitivity assessment are documented in the Geotechnical Stability Analysis Appendix C. For the elevated phreatic surface sensitivity analyses in the Design Report, the "worst- case" phreatic condition was assumed to exist at the top of the existing native ground surface. The results of this sensitivity assessment showed that the factor of safety was relatively insensitive to an increased groundwater level, with the static factors of safety decreasing by only 0.02 to 0.08 from the base case. As a result, installation and monitoring of piezometers solely for the purpose of stability monitoring criteria was not a required component of the O &M Plan. Section 4 — Geometry and design criteria - Cross section E -E' was selected for analysis, however section C -C' has a lower FOS. It would appear that section C -C' would be a better choice to evaluate for worst case scenario stability. Response: Golder considers section E -E' representative of the McNulty OSF, which will contain more overburden material than the North 40 OSF. In addition, the differences in the FOS between C -C' and E -E' are generally small, averaging approximately 0.1. However, Golder has also performed the same "worst- case" evaluations on section C -C' and has updated the SSA to include these results, which are similar to those previously presented for Section E -E'. - The new cross section F -F' is stated to run perpendicular to the existing slope through the mapped landslide area — this does not appear to be the case from a brief inspection of the attached figure. Please reorient this section to be perpendicular to the existing slope through the landslide area and re- evaluate. It may be useful for Golder to use the most current published data for that area (USGS Copper Mountain Geologic Quad published in 2003 -2004) to make sure section F -F' is properly oriented. Response: Golder developed cross - section F -F' from a 2010 mine area survey, which is considered to be more accurate than the older USGS aerial data. The section is perpendicular to the existing slope through the potential landslide area, although it does deviate from perpendicular towards the downslope end of the section, outside of the slide area limits. The geometry of the cross - section downhill of the mapped slide area does not affect the back - calculated strength for the potential slide mass. The reason that the section was allowed to deviate from perpendicular with respect to the existing ground in this area is so that the section would be close to perpendicular to the proposed OSF slopes. Creating a section perpendicular to the OSF slopes is important for realistically assessing the impacts of OSF construction. Golder could rotate the section counterclockwise such that section F -F' passes through the toe of the 6