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<br /> <br /> <br /> <br />3 <br />Signs of Sinkhole Activity <br />Sinkholes can often be identified by whirlpools forming <br />upstream of the dam. Whirlpools may be significant <br />and easily observable (Photo 2), or they may be slight <br />enough that they simply result in leaves or grasses <br />floating in a circular pattern, which is the more <br />common scenario. <br /> <br />Photo 2: Whirlpool upstream of dam (U.S Forest Service) <br />Excessive seepage on abutments, embankment, toes, <br />or further downstream from the dam, can signal an <br />issue. Muddy seepage downstream can be an <br />indication that transportation and sediment is <br />occurring and material is being piped out of the dam. <br />Holes and depressions on or near the crest can signal <br />uneven settling and material transport below grade. <br />During low pool, small depressions and holes upstream <br />of the dam, on the embankment, abutments, or even <br />reservoir banks can be indications of sinkhole <br />formation. <br /> <br />Photo 3: Sinkhole on upstream embankment (Paxton Dam) <br />Potential Breach of Dam <br />Sinkhole activity can lead to a dam breach if not <br />treated. The overburden or embankment migrates into <br />voids due to seepage from the reservoir. Backward <br />erosion piping occurs causing sediment transport out <br />of the foundation or dam. The material transport is not <br />stopped by a filter or cutoff and a sinkhole forms <br />upstream of the dam as the erosion increases. In some <br />situations, the failure path is high enough that the <br />erosion only results in a loss of water storage above <br />the sinkhole. However, more commonly, the sinkhole <br />and associated pipe is low enough to mobilize <br />sufficient material to undermine the embankment, <br />causing a full breach with possible downstream <br />consequences. <br />Fixes <br />Upon identifying a sinkhole, the first and most <br />effective action to be taken is to lower the reservoir <br />pool. Any reduction in gradient will help slow erosion <br />but lowering the pool to below the sinkhole is ideal if <br />possible. Depending on the size and cause of the <br />sinkhole, several repair methods are possible. <br />Clay Blanket - If the sinkhole is due to a localized issue, <br />such as loose or poorly graded material, excavation of <br />the sinkhole and surrounding area and placement of a <br />clay layer can be a simple repair. The thickness of the <br />clay layer should be based on gradients and material <br />properties and determined by a soils engineer. The <br />clay layer should be covered by native soil or rocks to <br />protect the clay from erosion. Compaction of the <br />excavated area before placement as well as <br />compaction of the clay layer is important. To use this <br />method effectively, there should be no signs of a pipe <br />leading from the sinkhole, as the clay may erode into <br />the pipe upon raising the reservoir. <br />Reverse Filter - For a larger sinkhole, another repair <br />option may be the placement of a more robust reverse <br />filter. The sinkhole area should be excavated to <br />remove voids and loose material as well as expose the <br />flow path if possible. The number of layers of the filter <br />varies based on depth needed and materials available, <br />but the filter should have a minimum of three layers <br />below the native ground material. Rock should be <br />placed at the bottom of the excavation followed by <br />gravels then sand, before being covered with native