2012-08-16_REVISION - M1977493 (2)

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OSF "Worst Case" Failure Scenario As a standard preparedness exercise, using assumptions consistent with the current OSF design and FOS modeling, what are the failure threshold scenario(s) for the proposed design? (Seismic event, precipitation event, material saturation levels, over - loading, other ?) What are the "plausible threshold limits " for combinations of risk factors that will need to be mitigated, monitored for or avoided? Have any analyses have been completed (other than rolling rock) to determine the failure risk scenarios for impacts to existing infrastructure (such as that listed in section 2.1.3 or Highway 91). Response: The geotechnical stability analysis presented in TR -22 were performed in accordance with generally accepted standards of practice, using methods and input parameters considered to be representative of actual conditions or, in many cases, conservative. Examples of conservative assumptions applied in the TR -22 analysis include: • Residual shear strength (i.e., post -shear strengths) were applied for all relevant materials to conservatively account for the potential of pre - sheared conditions or potential strain softening materials. • Cohesion was generally neglected, even if laboratory testing showed cohesive strength properties. • The stability analysis included a sensitivity analysis to evaluate the impact of higher than expected piezometric levels. • The stability analysis included a series of sensitivity analyses to evaluate the potential impact of lower than expected overburden material strength on the factor of safety. The conditions modeled in the TR -22 analysis are considered to cover the plausible range of conditions expected within and beneath the OSFs. However, in order to further evaluate potential "worst- case" conditions, Golder completed a supplemental stability analysis (see Attachment 1). The supplemental stability included consideration of three additional "worst - case" scenarios: • Scenario 1— low foundation strength • Scenario 2 — ice lenses form in overburden • Scenario 3 — concentrated sedimentary overburden combined with elevated groundwater levels The supplemental stability evaluation provided in Attachment 1 shows that the proposed OSFs will remain stable even under these highly adverse "worst- case" conditions. Climax Responses to DRMS August 6, 2012 Email with Comments to July 26, 2012 Golder Associates Supplemental Geotechnical Stability Analysis Section 2 — Methods and assumptions - Groundwater levels used in the analyses were collected during Oct -Nov (low flow times), and may not be representative of baseline conditions especially during periods of snowmelt /runoff — would shallower GW surface in the native materials have an adverse impact on stability of existing surfaces as a foundation? 5