2014-03-07_REVISION - M1988112 (3)

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Table 7 PFM -2: Internal Erosion or Piping of Embankment Materials 5.4 POTENTIAL FAILURE MODES RULED OUT (CATEGORY IV) The following candidate PFMs were identified but ruled out from further consideration and not carried forward for detailed evaluation based on review of available information: Earthquake- Induced Cracking of Embankment with Flood Pool in Impoundment Leading to Scour Erosion and Breach of Dam. This potential failure mode involves the coincident occurrence of a major earthquake during a period of sustained storage of floodwater within the impoundment area. Normally these extreme loading conditions (flood and earthquake) are not combined for purposes of identifying potential dam failure modes. However, the operational San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 48 February 2014 Probability Range Node Description Comments Probability Descriptor Flood Normal Loading Event Pond must exist in the impoundment area to initiate this PFM; flood pools resulting from 0.002 1 major events will be larger and closer to the dam than the normal free -water pool Initiation — Erosion Use low end of published (USBR, Starts 2012, Ch. 26, Table 26 -19) range for initiation under normal pool 0.0015 0.0004 conditions and high end for flood pool conditions Continuation — Upstream "filter blanket" may unfiltered provide some filtering protection Neutral to Likely 0.7 0.7 Progression — roof Drain pipe haunches could form forms roof over a portion of the flow path; Very Unlikely 0.01 0.01 away from pipe would collapse; materials are non -plastic Progression — Long seepage pathways through Unlikely (Flood pool level) upstream zone fails to embankment and tailing deposits to Very Unlikely (Normal 0.1 0.01 fill in pipe pool) Intervention fails Site is accessible and inspections and monitoring occur regularly; Unlikely (Flood Pool) to failure would progress slowly and Very Unlikely (Normal Pool) 0.1 0.01 could get materials to site and intervene in a timely manner Dam Breaches For major flood events, larger pool of impounded water may be Likely (Flood Pool) to released by breach; small pool Unlikely (Normal Pool) 0.9 0.1 during normal operations may not be sufficient to fully breach dam Estimated Probability of Failure 2 x 10.10 3 x 10-11 Consequences Category 4 1 5.4 POTENTIAL FAILURE MODES RULED OUT (CATEGORY IV) The following candidate PFMs were identified but ruled out from further consideration and not carried forward for detailed evaluation based on review of available information: Earthquake- Induced Cracking of Embankment with Flood Pool in Impoundment Leading to Scour Erosion and Breach of Dam. This potential failure mode involves the coincident occurrence of a major earthquake during a period of sustained storage of floodwater within the impoundment area. Normally these extreme loading conditions (flood and earthquake) are not combined for purposes of identifying potential dam failure modes. However, the operational San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 48 February 2014