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