2014-03-07_REVISION - M1988112 (3)

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type of failure would progress slowly and the operator would have opportunity to intervene in a timely manner, further reducing the likelihood of this failure mode developing. Seismic - induced liquefaction and excessive deformation cause loss of freeboard and overtopping of dam leading to failure. Although the earthquake loading used for design of the San Luis Tailing Facility is significant (PGA = 0.6 g, Magnitude = 7 +), the dam, its foundation, and the tailing materials immediately up- gradient from the dam are currently unsaturated. Liquefaction therefore is not an issue and strength loss of the dam or upstream tailing under seismic shaking does not lead to a credible failure mode for the dam. Deformations (settlement) on the order up to 2 feet are predicted for the design earthquake, but there is sufficient freeboard on the existing ponded area such that overtopping would not occur. The previously ruled out failure mode described in the preceding paragraphs considered a situation in which the design earthquake occurred at a time when there was much more water in the impoundment area following a major flood event. However, as the risk analysis for that potential failure mode concluded, the likelihood of that combination of loading is so remote that the overall risk is well below tolerable risk guidelines. A similar conclusion would likely be the case for this PFM, even if the extended period of inundation were to re- saturate the tailing and/or a portion of the dam embankment. Debris blocking of South Diversion Ditch drop structure causes overtopping of the structure during flood events smaller than a 100 -year flood. This PFM is a subsidiary contributing factor related to development of PFM -1 and does not by itself lead to a unique credible failure mode. Floods more frequent (less volume and flow) than a 100 -year event are unlikely to result in failure of the dam or release of tailing for the reasons described in the discussion of PFM -l. Clogging of under -drain system results in elevated pore pressures within the dam, seepage face emerging high on downstream dam face and slope instability resulting slumping, crest elevation lowering and loss of freeboard. This PFM is a subsidiary contributing factor related to development of PFM -2 and does not by itself lead to a unique credible failure mode. Slope instability and lowering of the dam crest on its own would not lead to overtopping and failure of the dam under normal operating conditions because of the large amount of freeboard and large distance from the dam to the existing small free -water pool. If the under -drain system were to become clogged and at the same time a major flood event inundated the facility there would be a plausible failure mode to consider. However, the likelihood of those combined conditions occurring simultaneously (under -drain malfunction and major flood) is considered remote. All of the piezometers monitoring the dam embankment consistently indicate very low pore pressures, and the drain outlets are flowing, indicating that the under -drain system is functioning properly. The seepage face that has been observed emerging on the downstream slope a few feet above level of the drain outfalls should be investigated and its cause better understood, as discussed in Section 6. That situation is believed to be a localized condition that does not present San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 50 February 2014