2014-03-07_REVISION - M1988112 (3)

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situation at San Luis Tailing facility is believed to warrant a preliminary evaluation of this PFM, because it is understood that once floodwater enters the impoundment area the only way to remove it is by evaporation. Depending on the size and volume of flood entering the impoundment, a significant amount of time may be required to reduce the pool volume via recirculation and evaporation. This failure mode would be triggered by an earthquake of sufficient magnitude to cause deformation and formation of a transverse crack or multiple cracks through the dam embankment. The design earthquake for the facility was a 7+ magnitude event having a PHA = 0.6g. The statistical incidence and probability of transverse cracking under this level of shaking was obtained from the Bureau of Reclamation Best Practices Manual (USBR, 2012, Ch. 27, Fig. 27 -9b). For a peak ground acceleration of 0.6 g and earthquake magnitude of 7+ (per the design documents), the damage class for transverse cracking is Major (Class 3). The probability of transverse cracking is virtually certain (1.0). For this damage class, the maximum likely transverse crack width at the dam crest is 0.5 to 1.5 mm, which corresponds with a maximum likely crack depth of about 8 to 10 feet. The maximum relative crest settlement is 0.5% to 1.5% X the dam height = 9 inches to 2.5 feet of settlement. So if the dam settles 2 feet (lowering the crest elevation to about 8620), and the cracks extend 10 feet below that elevation, the flood pool elevation after the earthquake would have to be higher than approximately 8612 to 8613 to develop sufficient gradient through the cracks to initiate erosion (2 to 3 feet of head acting on the cracks). For this failure mode to develop, however, the earthquake must occur during the potentially long, but still not permanent window of time when floodwater is stored in the impoundment area, and the floodwater impounded must be high enough to intersect the cracks that develop. The transverse cracks must be sufficiently wide and deep that they intersect with the flood pool. The depth and velocity of water flowing through open cracks must be sufficient to initiate erosion and scour of the low- plasticity dam embankment and tailing materials along the sides and bottoms of the cracks. This is a concentrated leak erosion process, as distinct from the progressive internal or piping erosion process described under PFM -2. This potential failure mode was ruled out primarily due to the extremely low probability of the combined initiating events on the event tree. The flood probability was taken as 0.002 (500 -year event) for a major flood event that is assumed would be needed to impound water to within a few feet from the exposed upstream face of dam. The loading probability range was then modified by the earthquake probability for a major earthquake that would be needed to form a crack of sufficient size to initiate this failure mode, occurring in an assumed 2 -year window. The design earthquake for the project was estimated to have a recurrence interval of approximately 16,000 years, and would have a probability of exceedence in a 2 -year window of approximately 1.25 x 10 -4 (Pe= probability of event = 1 — [ 1 -1 /T]n, where T = return period and n = number of years). The combined initiating probability is therefore 2.5 x 10"x, which would place this potential failure mode well below the established risk guidelines for all levels of consequences. Another mitigating factor is that the site is accessible and inspections and monitoring occur regularly; this San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 49 February 2014