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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />17 <br /> <br />Figure 3. Illustration of how downstream progressive <br />sloughing due to saturation could lead to dam failure <br />[Adapted from [5]] <br />Stoping <br />If the collapse is in the upward direction, it can lead to <br />a near vertical cavity in the embankment. Particle <br />movement is driven by gravity and the stoping process <br />progresses in a more vertical/upward direction (rather <br />than horizontal/upstream) and may eventually express <br />itself as a sinkhole near the dam crest. <br /> Figure 4. Stoping Leading to Formation of a Sinkhole in a <br />Narrow Sloping Core [2] <br />Concentrated Leak Erosion <br />Erosion of material along the sides of an opening or <br />crack is considered concentrated leak erosion. This can <br />also be known as a scour mechanism, as the material is <br />scoured from the sides of the void by the force of <br />moving water. Plastic soils and some unsaturated silts <br />and sands can hold an opening or crack that would be <br />susceptible to concentrated leak erosion. Causes of <br />cracks or openings include differential settlement, <br />hydraulic fracture along a low stress zone (at conduits <br />and low compaction zones), desiccation, collapse <br />settlement around poorly compacted material (at <br />conduits and vertical walls), collapse of foundations <br />soils and animal burrows or rotting tree roots causing <br />voids. Ponding and seepage with particle transport on <br />the downstream face, around conduits or at the <br />downstream toe, are all signs that concentrated leak <br />erosion could be occurring. <br /> <br />Figure 5. Common Crack Locations for Concentrated Leak <br />Erosion, [3], [4]