2014-02-13_REVISION - M1977493 (2)

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Review Assistance Memorandum — Mayflower Tailings Storage Facility (aka 5 Dam) February 13, 2014 Page 3 of 6 Section 7 — Stability Analyses 10. Section 7.1 — DRMS does not have specific requirements for factors of safety against slope failure for tailings dams. URS recommended using the conventional factors of safety for water impoundment dams, which is reasonable. The proposed seismic design criteria are based on a magnitude 7.0 OBE event with a return period of 475 years. These criteria were reviewed by URS and accepted by DRMS in 2008. It is unclear from the report how these criteria were selected. There does not appear to be a generally accepted "design" earthquake for use in the design of tailings dams. Several authorities have recognized that tailings dams must be managed in perpetuity following closure, and some have suggested that tailings dams should therefore be designed to survive the Maximum Credible Earthquake (MCE) to avoid the need for expensive retrofit measures at closure. The calculated seismic acceleration from the selected OBE event is 0.06g. From Table 2 -1, the calculated peak horizontal acceleration during a 5,000 -year event (required for a large high hazard water impoundment dam) would be 0.20g. Table 2 -1 shows the peak acceleration for a 10,000 -year event (generally equated with the VICE) to be 0.27g. 11. Section 7.4 — Table 7 -3 lists the factors of safety against failure of the design slope under steady -state and post- earthquake loading. The post- earthquake analyses appear to have modeled the dam under static loading conditions using the undrained soil strengths listed in Table 7 -2. While the design OBE ground acceleration is listed, there is no discussion in the report of an analysis performed under transient seismic loading. It is recommended that the stability of the existing and proposed dams be evaluated under a seismic load using undrained soil strengths. It is also recommended that the seismic load be varied to see what level of earthquake shaking the dams will be able to withstand. This analysis should be performed prior to raising the dam above its current level. Section 8 — Conclusions and Recommendations Section 8.1 — Conclusions 12. It is agreed that widespread liquefaction of the tails is not likely to occur for the OBE event. Failure due to liquefaction caused by other seismic events was not considered. 13. It is agreed that the proposed dam is likely stable under the loading conditions described (steady -state and undrained /post -OBE earthquake). Failure during a seismic event was not evaluated. 14. Loss of the dam by other failure modes (overtopping, "static" liquefaction, piping) was not addressed. Section 8.2 — Recommendations 15. It is agreed that the "observational approach" outlined in the report should be closely followed, as expressed by the statement on page 9 -1, "Only through periodic, updated inspections and ongoing monitoring can unsafe conditions be detected so that corrective action can be taken." It is recommended that all the bulleted items on page 8 -2 be incorporated into a monitoring and reporting program for the duration of the project. DRMS should establish formal requirements with the dam owner that include data acquisition, data evaluation, and data reporting.