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<br /> <br /> <br /> <br />2 <br />SINKHOLES: The Hole Story <br />…Issues are Deeper than you <br />Think! <br />Introduction <br />A sinkhole is a depression or void caused by collapse of <br />surface materials due to movement of water removing <br />underlying material. When this removal of material <br />forms an enlarging tunnel, it is referred to as backward <br />erosion piping. Sinkholes are often an indication that it <br />is occurring. <br /> <br />Figure 1: Sinkhole formation (from Reclamation Best Practices Fig <br />26-3a) <br />The most common condition causing sinkholes to <br />occur in or near dams is concentrated seepage through <br />voids or cracks causing material to move. This can <br />occur due to: <br />• Karstic foundation that has voids due to <br />solutioning <br />• Loose or poorly graded materials that include <br />cobbles or boulders and poor compaction <br />• Poor treatment of foundation during <br />construction, where there is a prevalence of joints <br />• Leak in pipe that penetrates the dam, creating <br />increased seepage <br />• Animal burrows that create shortened seepage <br />paths when they become submerged <br />• Differential settlement creating cracks or voids <br />• Construction defects creating cracks or voids <br />There are several repair options depending on the <br />cause and size of the sinkhole. Sinkholes can be <br />benign and self-healing, but are generally the first sign <br />of a developing problem that could progress to a <br />major dam safety issue if not addressed in a timely <br />manner. <br />Prevalence in Small Dams <br />Construction of small dams increased in the U.S. in the <br />early 1900s due to an increased demand for water in <br />agricultural and mining actives. At the time there was a <br />lack of appreciation of the complexities associated <br />with design and construction of small dams. This led <br />to limited engineering with not enough emphasis on <br />subsurface exploration, design of filters and cutoffs, <br />treatment of foundation and quality control during <br />construction (gradation and compaction). Small dams <br />typically have less foundation investigation and <br />preparation due to the smaller head. However, with <br />smaller cross sections gradients can still be high. In <br />addition, more frequent cycles of high and low pool <br />levels can intensify issues such as cracking or animal <br />burrows. <br />In addition, failure of small dams is also assumed to be <br />less catastrophic and frequent inspection and dam <br />safety protocols may be de-emphasized. Signs of issues <br />may go unnoticed for long periods of time. If issues are <br />noticed, a small repair may be applied without proper <br />engineering. If such repairs do not treat the <br />fundamental issue, reoccurrence is inevitable. Signs of <br />a sinkhole may appear to be only a small surficial <br />depression easily repaired with placement of fill. <br />However, a seepage pipe and potential failure path <br />may exist just below the surface, readily available to <br />promote material transport when the reservoir level <br />rises. <br /> <br />Photo 1: Example of small depression that may go unnoticed <br />(Young Creek Dam)