2012-08-16_REVISION - M1977493 (2)

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McNulty OSF and the mapped slide area. However, doing so would result in the section being further from perpendicular to the existing slopes through the mapped slide area. The rotated section would also be further from perpendicular to the proposed OSF grades. Golder measured the slope of the existing ground at several locations within the mapped potential landslide area. The slope was measured by dividing the drop in elevation by the distance between the uphill and downhill limits of the mapped potential slide area. The average slope of the existing ground within the mapped potential landslide area is approximately 17.3 %. The slope through the mapped potential slide area, as measured along section F -F' is 19.4 %, indicating F -F' is located in a relatively steep portion of the mapped slide area, and that F -F' is oriented close to perpendicular to the slope. - Although a seismic event is used to justify the lower acceptable FOS value of 1.0 for worst -case scenario, it does not appear that any seismic events were considered during the worst -case evaluations. Although DRMS agrees that a significant seismic event is unlikely, it may still be worthwhile to evaluate how it could affect the proposed design in conjunction with other factors. Response: It is prudent and common practice to develop designs which are stable under unfavorable conditions, such as elevated groundwater or seismic loading. However, in general, it is not common to design for all unanticipated worst case conditions occurring simultaneously (i.e., elevated groundwater, lower than expected foundation strength, weaker than expected overburden, and a simultaneous seismic event). This is because the probability of such a simultaneous occurrence is typically considered too low to justify the overly conservative designs that would be necessary to mitigate against such conditions. However, in response to the comment, Golder has rerun Scenario 3 in the attached, revised SSA (low shear strengths and elevated phreatic surface) to also include the operational base earthquake (OBE), and the results provided a factor of safety marginally greater than unity (FS= 1.01). Attachment 2 to this letter are the jpg Slide output files that illustrate the critical failure surfaces for cross - sections E -E' and C -C' referenced on Figure 13 of the revised SSA. The conditions for these cross - sections are as follows: 1. Lower bound overburden strength. 2. "Mean" water table. This water table was varied as shown in Figure 13 in the revised SSA. 3. OBE seismic conditions and operational slope configuration. These sections are from a limit equilibrium analyses using the program SLIDE, with factors of safety > 1.0, so by definition, they represent a no displacement condition. - There was no discussion in the worst case modeling about what parameters (if any) the OSF may be especially sensitive to during operation/post closure that may require special attention or mitigation. There was also no discussion regarding risks to existing infrastructure. The supplemental stability "worst- case" evaluations included sensitivity analyses to evaluate the impact of weak foundation strength, elevated groundwater, and low overburden material 7