2014-03-07_REVISION - M1988112 (3)

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5.3.2 PFM -2 Internal Erosion or Piping of Embankment Materials Exiting on Downstream Face of the Dam near the Under -drain Pipe Outlets Failure Mode Description: This potential failure mode involves internal erosion of the low- plasticity dam embankment materials initiating and progressing in the vicinity of the under -drain outfall pipes. Seepage has been observed exiting the face of the dam several feet above the drain outfall pipes, located at the downstream toe near maximum section. Erosion of the dam fill materials would initiate under high seepage exit gradients on the downstream slope, due to elevated pore pressures in the downstream zone of the dam that is caused by two possible mechanisms: (a) clogging or partial clogging of the sub -drain system, and/or (b) reservoir filling and re- saturation of the tailing under flooding conditions. The dam section is essentially homogeneous and is not protected from internal erosion by a central filter zone. The upstream "filter blanket" shown on design and construction drawings may provide some filtering protection at the contact with the mine tailing, but available project records indicate this zone may have excessive fines (up to 35% passing No. 200 sieve) that prevent it from serving as an effective filter. The erosion process progresses as an erosion and seepage path "pipe" is formed within the dam embankment. The piping feature works its way backward from the unprotected exit on the downstream dam face, along the outside of the drain pipe and progresses by gradual enlargement of the seepage pathway which is held open in the area adjacent to the lower haunches of the pipe which forms a "roof' that allows the pathway to remain open and enlarge by scour erosion along the drain pipe periphery. The erosion must then progress vertically upward by a stoping mechanism through the tailing until it connects into the upstream ponded area which is the source of the head driving the internal erosion process. This results in a direct, open, high velocity seepage path through the tailing and under the dam along the drainpipe. Under increasingly higher flow velocities, more fines are washed out and the open area of the internal erosion "pipe" feature enlarges; a sinkhole develops on the upstream side within the ponded area as fines are continuously removed down - gradient. Intervention fails to seal the erosion feature. Rapid erosion of the highly erodible tailing and embankment soils occurs, and larger and larger materials are transported under the high flow velocities, leading to breach development, progressive downward scour erosion, and release of the pool at maximum section adjacent to the drainpipes. The following positive factors (making progression to failure less likely) and adverse factors (making progression to failure more likely) were identified and considered in the process of assigning node probabilities in the risk analysis. Positive Factors: 1. The dam is routinely monitored for internal pore pressures and outfall discharge rates. Deep piezometers in the dam indicate low pore pressures, and the drain system outflows San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 45 February 2014