2014-03-07_REVISION - M1988112 (3)

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fall into a typically steady range, indicating that the under -drain system is functioning well. 2. The outfalls of the drainpipes were buried by runoff sediment at the time of the first inspection (March 2013). The seepage that was observed at that time was emerging from the downstream face of the dam about 5 feet vertical above the buried pipe outfalls, and may have been a result of physical blockage of the pipe outlets, not internal clogging of the pipes by some other mechanism. The pipe outfalls were later excavated and exposed and subsequent inspections revealed that all three pipes were flowing. 3. Under normal operating conditions there is a very long seepage path from the current, shallow free -water pool (normal operation pool) and the downstream toe of the dam; the long seepage paths make it highly likely that the erosion process, if it initiates at the downstream toe of the dam, would be interrupted by collapse of the developing piping feature somewhere along the long pathway and the low hydraulic gradients would be insufficient to keep the process going. 4. Low plasticity tailing and embankment materials cannot support and sustain a stable open erosion "pipe" feature; if piping initiates in the dam away from or above the drain pipes the erosion "pipe" feature will likely collapse on itself and the process will cease before the "pipe" can progress back to the ponded area. 5. There has been no direct evidence of clogging of the drainpipes or evidence of this type of erosion process initiating during the lifetime of the facility (over 20 years) even under conditions with more saturated tailing and larger operating pools. Adverse Factors: 1. After the pipe outfalls were exposed and all three pipes were found to be flowing, the wet seeping area on the dam face above the pipe persisted. Removing the obvious physical blockage of the pipe outfalls may not have completely solved the problem of elevated pore pressures at the toe of the dam. 2. The embankment and tailing soils appear to be highly erodible; erosion gullies are prevalent on the dam face and in the groin areas. 3. There are no internal filters in the dam to mitigate the risk of piping and internal erosion, although the upstream "filter blanket" may provide some filtering protection. 4. During and following major storm events (much larger than a 100 -year event), there may be a large flood pool stored within the impoundment area for some extended time. Under current operating conditions, it is assumed that floodwaters would be circulated via infiltration, collected in the underdrain system on the liner, and then pumped -back up to the impoundment area from the collection pond. Depending on the magnitude of the storm and the size of the impounded flood pool there may be a long duration of re- saturation of the tailing and higher potential for this PFM to initiate if the underdrain system is overwhelmed and pore pressures in the downstream zone of the dam become elevated. The proximity of the flood pool to the dam also is important as the closer it is San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 46 February 2014