2014-02-13_REVISION - M1977493 - Laserfiche WebLink

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Page 2 of 4 prior to construction. This design will be required to include inspection and evaluation of the existing drain to determine its suitability for extension. Subsurface Material Characterization Section 5.1.3 of the SSA does not expressly state the slimes are not susceptible to earthquake- induced liquefaction, and does not consider the potential for loss of strength due to elevated pore pressures, even though it appears that a substantial deposit of this material will underlie the dam at a significant depth. Please address. Section 5.1.4 of the SSA: The description of the sludge layer location is somewhat confusing. The layer is estimated to be up to 3' thick at the beach at the upstream end of the current pond (1200' from the dam), increasing to about 30' thick at the "back" of the pond. Figure C -1 (Appendix C) shows the "back" of the pond to be farther upstream than the beach upstream of the dam, so the sludge layer does not appear to underlie the projected final dam. Please clarify. Seepage Analysis Section 6.3 of the SSA: The description of the calibrated model as using a phreatic surface 1 foot below the latest reading from Piezometer TH2006 -5 -1 (upstream of the blanket drain) but 8 feet above the highest reading for Piezometer P -4 (downstream of TH2006 -5 -1, and which appears to penetrate the blanket drain) is somewhat confusing. Is the highest reading in P -4 the latest reading? Using a higher phreatic surface would be conservative, but the description appears inconsistent. Piezometer locations are not shown on the model results. Please clarify. Stability Analysis In a seismic stability design criteria recommendation submitted by Climax to DRMS dated January 22, 2008, the proposed design criteria stated that existing and future TSF design would be evaluated for stability under two levels of design earthquake events: the OBE (Operating Basis Earthquake) for the operational period of the facility, and the MDE (Maximum Design Earthquake) for the post closure period; with the MDE being set equal to the MCE (Maximum Credible Earthquake). These criteria were accepted by DRMS in an approval letter to Climax dated February 14, 2008. The proposed seismic design criteria submitted for the Mayflower TSF are based on a magnitude 7.0 OBE event with a return period of 475 years. 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 and the peak acceleration for a 10,000 -year event (generally equated with the MCE) would be 0.27g. No data have been provided to show how the TSF design would withstand a post - closure MCE event. Please provide a suitable evaluation as was done with the OBE event. Section 7.4 of the SSA, Table 7 -3, lists the factors of safety against failure of the design slope under steady -state and post- OBE 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 and MCE ground accelerations are listed, there is no discussion in the report of any analysis performed under transient seismic loading. Please evaluate the stability of the existing and proposed Mayflower TSF under the OBE and MCE seismic load, as approved in 2008, using undrained soil strengths. Please also evaluate the TSF under a varying seismic load to determine what level of earthquake shaking the TSF will be able to withstand (if the TSF cannot be shown to withstand the MCE). SSA Recommendations and Monitoring /Reporting DRMS will require that all bullet items listed in section 8.2 of the SSA be implemented and incorporated into the monitoring and reporting program for Mayflower TSF. DSB has recommended that pore pressures in the slimes under the construction of new berms and beaches also be closely monitored to minimize the risk of buildup of excess pore pressure. Please describe how Climax intends to address this issue.