Abstract Description: Motivation/Problem Statement: Ripshin Dam is an earthen dam located in Roan Mountain, Tennessee that impounds a 56-acre private lake. The dam was originally constructed in two phases from 1927-1928 and 1945-1946 and repairs were made to the dam in 1992 that included raising the dam to a 42 feet height and constructing a concrete chute spillway. In 2021, S&ME began consulting for a siphon drain replacement for the dam. Additional challenges (“curve balls”) were discovered during dam evaluation, design, and construction that significantly affected the dam repairs construction schedule and cost.
Methods/Procedures/Approach:
Due to the steep inclination of the downstream slope (steeper than 2.5H:1V), a geotechnical exploration and historical records review were performed that revealed the dam core was constructed using semi-hydraulic fill methods. Stability analyses indicated the dam did not meet industry stability factors of safety, and a chimney and blanket drain system and earthen stability berm were designed to stabilize the downstream slope. The dam repairs were permitted, and construction began in September 2023.
An original 30-inch CMP was present near the lower center of the dam, and records were inconclusive on how the CMP was abandoned. During lake drawdown, a depression was observed at the toe of the upstream slope, and it was recommended that the embankment be breached, and the 30-inch CMP be removed. Construction observations indicated the pipe was grouted and conditions were more favorable than thought. The Owner elected to leave a portion of the CMP in place. The upstream end of the CMP was excavated, evaluated, and backfilled with grout. Much of the dam core material and original embankment soil were removed.
In September 2024, embankment filling was 5 feet below the primary spillway inlet when Hurricane Helene hit the region, and the site experienced 14 inches of rainfall. Storm flows were passed using pumps and new siphons (which had never been tested). Due to rising water levels, a 4 feet deep temporary spillway was excavated near the right abutment to pass additional storm flows. Stormwater was rerouted without overtopping the crest. The dam repairs were substantially completed in December 2024.
Conclusions/Implications:
Changed conditions are often encountered during dam design and construction. Communication between the Engineer, Owner, and Contractor are important when changed conditions are encountered. Construction schedules and costs can be significantly impacted, particularly from additional dewatering costs. Planning for storm events during construction is necessary to help prevent overtopping the dam.
Learning Objectives:
Learn about dam geotechnical evaluations of dams with semi-hydraulic fill and mitigation methods.
Learn about changed dam conditions and communication between the Engineer, Owner, and Contractor for a private dam project.
Learn about temporary dewatering measures and handling extreme storm water events during construction.
