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7 Photo 8: Compaction of clay layer at Lake Ann, CO Each layer was placed in 3-inch lifts and compacted using a vibrating plate compactor. The clay layers were also compacted with a jumping jack. The repair was completed on November 28, 2014. No issues have been reported since completion. Case Studies – Lessons Learned Anchor Dam, WY – Even with foundation treatment, some geologic conditions are always susceptible to sinkholes, and foundation treatment can do little to achieve preferred conditions. Anchor Dam is a concrete arch dam built on dolomite bedrock with carbonate karst in the abutments and gypsum karst in the foundation. The rock dissolves with groundwater over time, creating networks of cavities. During construction the cavities that were found were filled with concrete but the reservoir never held the volume of water intended. Water continuously leaks through the creek bed and under the dam. More than 50 sinkholes appeared and were plugged in the first twenty years after construction, but the reservoir continues to leak. Soon after construction, one sinkhole was reported to be approximately 300 feet in diameter and 50 feet deep. The dam still stands today but the reservoir is frequently empty and only holds water temporarily after rain and snow melt. Scholl Dam, CO – Poor underlying soil conditions can create sinkhole issues over broad areas, making localized fixes ineffective. At Scholl dam sinkholes in the upstream abutment have been repaired continuously, but the sinkholes continue to reappear at different locations. Repairs have included geomembrane, clay blankets, and multiple grout mixes and applications. Investigations have suggested that the sinkhole-prone area is founded on landslide material that has formed a matrix with voids that covers an area large enough for the sinkholes to find other seepage paths. The right abutment and groin have leaked since the first filling of the reservoir. So far the sinkholes have been unable to be mitigated with repair measures and there may be no viable permanent fix. Conclusion No matter how small a dam, there are unique properties and conditions that can potentially lead to a dangerous situation. Regular and thorough surveillance of a dam can help catch issues early, and with the help of experienced professional engineers most sinkholes can be mitigated or repaired in a timely and cost- effective manner. Without such help, small issues can lead to larger problems that may cost more time and money to repair. Under some circumstances, small issues can signal much larger issues below the surface that may lead to dam failure if dealt with improperly or left unchecked. Useful References [1] United States Department of the Interior: Bureau of Reclamation. (1987). Design of Small Dams [2] United States Department of the Interior: Bureau of Reclamation. (2012). Best Practices and Risk Methodology [3] United States Department of Agriculture: US Forest Service (2012), Pocket Safety Guide for Dams and Impoundments [4] D. Magill and R. Berry, Comparison of Chemical Grout Properties: Which Grout can be used Where and Why?, Avanti International, 2006, http://pilemedic.com/pdfs/comparison-of-chemical-grout- properties.pdf. [5] B. Babcock. (2013). “Sorting out the Grout.” World Tunnelling. June 2013.