Rapidan Dam Emergency Repairs, Blue Earth County, Minnesota

Have you ever inspected a dam and wanted to run away? This was the case on February 14, 2002, when an inspection of the Rapidan Dam in SW Minnesota revealed a serious problem. Only a cooperative effort by Blue Earth County (BEC), St. Paul District Corps of Engineers (COE), Minnesota Department of Natural Resources (Mn/DNR), Federal Energy Regulatory Commission (FERC), SMC the general contractor, and Barr Engineering Company could successfully stabilize a dam that appeared to be in imminent danger of failing. Ground Penetrating Radar testing of the foundation for the 60-foot-high hollow Ambursen dam revealed the possible presence of voids between the concrete and the sandstone foundation. The February 14 inspection revealed a situation that was significantly worse than could have been imagined. This hollow concrete dam had a scour hole under the buttresses that was more than 18 feet deep and extended more than 30 feet upstream of the dam's toe. The buttresses were cantilevered over open water, instead of bearing on sandstone bedrock. Based on the field observations, it was evident that the dam was precariously close to complete failure. Due to access limitations and safety, the inspection was limited to two of the 11 bays. Stability analyses prepared based on the field observations, confirmed that the dam had safety factors that were at or slightly above 1. The dam should have failed. With spring rains possible at any time, it was necessary to act immediately. BEC, the dam owner, authorized Barr to evaluate the likely causes of the current conditions and identify options for stabilizing the dam. Evaluation and designs were completed and bidding documents prepared within two weeks. The dam owner did not have the resources to complete the work and approached the Mn/DNR Dam Safety staff for additional funding. Mn/DNR provided matching funds for the project, a local contractor was selected and construction began at once. Once site access conditions improved, additional soundings were obtained showing the 200-foot-long spillway was undermined along its entire length with scour depths between 17 and 29 feet. Again, matters were more serious than previously thought since the initial design had been for two isolated scour holes. This new information increased the estimated cost to complete the project. The COE, following a request for emergency assistance from Governor Jesse Ventura, continued the project once BEC and Mn/DNR funding was exhausted. The COE managed the placement of over 9,000 ton of rock fill (4000 ton of rock fill was placed under the earlier funding) and 2,300 CY of concrete in less than two weeks. By April 20, the threat of imminent failure of Rapidan Dam was reduced - the result of fast action and hard work by a team of professionals who recognized the urgent need to stabilize the dam before the arrival of spring rains, which would surely have caused a collapse. 13 pp., 4 figures.