Boone Dam Project
What Happened at the Dam?
In October 2014, a sinkhole was discovered near the base of the embankment at Boone Dam, and water and sediment was found seeping from the riverbank below. While sinkhole occurrence in East Tennessee is common, the locations of the sinkhole and the muddy discharge were indicators of potential issues with the safety of the dam.
TVA responded by assembling a team of TVA dam safety engineers and external experts to investigate the source of the observed seepage, and to determine whether dam safety may be compromised. The conditions of the embankment dam foundation were found to be favorable to a process termed “internal erosion,” in which voids develop within a dam and/or its foundation because of the action of flowing groundwater.
How Did TVA Respond?
Because internal erosion is one of the leading causes of dam failures worldwide, TVA staff and external experts implemented a number of interim risk reduction measures, including:
- Lowering the pool elevation to 1350’-1355’ (roughly 10 feet below winter pool levels)
- Assigning onsite inspectors to the dam for continuous surveillance
- Installing an automated network of sensors to monitor the dam for pressure and temperature changes every 15 minutes and for movement every 30 minutes
- Exercising an emergency action plan
- Constructing a tailrace filter to minimize further deterioration of the dam
How Did the Problem Develop?
The original designers and constructors of Boone Dam in the early 1950s encountered highly irregular bedrock known as “karst,” consisting of local pinnacles separated by 20- to 30-foot deep crevices, during excavation of the foundation. Near the surface of the bedrock and within these pinnacles, in a part of the ground termed “epikarst,” they encountered voids and soft muddy soils. To limit reservoir seepage underneath the dam, a deep excavation called a “cutoff trench” was created to remove the voids and soft soils within the rock pinnacles and epikarst. Additionally, grout was pumped into the foundation to fill any remaining voids beneath the embankment dam.
While the foundation treatment was state of the art in the 1950s, dam safety engineers now recognize the potential for deterioration with this type of construction.
In March of 2015, TVA discovered a well-developed, complex network of groundwater seepage paths coming from sources other than the reservoir. The land east of the dam is higher in elevation and contains numerous depressions and sinkholes. During large rain events, surface runoff flows underneath the dam and is a leading contributor to the observed seepage and sinkhole at the toe of the dam. This phenomenon occurs after large rain events, and was discovered by the intense instrumentation program initiated in November 2014. Further investigation has confirmed that deterioration of the cutoff trench has occurred as seepage flows continue to undermine the foundation of the embankment dam.
What Were the Risks?
As the water flows past soil particles, it exerts a drag force that could dislodge particles and carry them into nearby void spaces where they may be transported away from the footprint of the dam. As the velocity of the groundwater flow increases, the ability to erode and transport soil also increases. If left unaddressed, continued internal erosion may lead to enlargement of the network of voids at which time a large influx of water into the voids could cause rapid acceleration of internal erosion and eventual breaching of the dam.
A critical component of TVA’s process was a careful evaluation of the risks associated with a dam suffering internal erosion. In its current state, the dam cannot be relied upon to serve the functions for which it was constructed. In the unlikely event of a dam failure, risks to the public would include:
- Property damaged
- Economic losses
- Environmental impacts
- Loss of critical infrastructure
- Potential loss of life
Was the Public Safe?
Due to the complex and urgent nature of the situation at Boone Dam, TVA complemented its own dam safety engineers with nationally recognized experts in dam safety. Some of these were working directly with TVA’s staff; others served as independent checks. In addition, TVA engaged other owners of large dams, such as the United States Army Corps of Engineers and other large private utilities that have had similar seepage issues.
The experts’ review determined that the interim risk reduction measures used by TVA significantly reduced the risk of dam failure. The primary means to achieve this risk reduction was the restriction on pool elevation. The lower lake level slowed the process of deterioration and significantly decreased the amount of water that would flow downstream in the unlikely event of a breach. Therefore the lowered pool elevation had to continue until further remedial actions were completed.
In addition, TVA engaged local and state emergency management officials regarding conditions at the dam site, including developing specific plans to address potential emergencies and conducting mock exercises to execute those plans. TVA also initiated efforts to reinforce downstream facilities in a way that minimized potential risks from an unlikely failure of Boone Dam. TVA maintained open lines of communication regarding its activities to remediate the dam, especially where such communication might affect public safety or the community’s ability to respond to an emergency.
The safety of our workers, the community and businesses downstream of the dam and of the general public were and continue to be our overriding priority.
What Was the Fix?
TVA’s experienced team of dam engineers and safety experts evaluated a number of methods for repairing the dam. The wide range of options considered included:
- Removing the dam
- Constructing a new dam
- Building seepage filters
- Grouting the voids underneath the dam
- Constructing a composite seepage barrier
- Building berms to fortify the dam
TVA and the group of experts also considered such variables as:
- Durability of the repair
- Time to return to normal operation
- Impact to the public
- Risk for recurrence
- Environmental impacts
- Cost to ratepayers
As a result of the decision-making process and internal-challenge sessions, TVA identified a composite seepage barrier as the preferred option to remediate the problems at Boone Dam, pending additional environmental review. A composite seepage barrier created a positive cutoff from the reservoir and is made from non-erodible material. Therefore, this solution had a very low probability of a seepage connection within the reservoir recurring.
The composite seepage barrier consisted of two components constructed in a three-stage process:
- Stage 1: Grout curtain in the foundation soils and epikarst
- Stage 2: Grouting the underlying bedrock
- Stage 3: Concrete diaphragm wall through the dam and epikarst terminating in the underlying bedrock
The remediation approach was similar in concept to those used in other dams with karstic seepage issues in Kentucky and Tennessee, such as Wolf Creek Dam and Center Hill Dam.
How Long Did It Take?
The remediation required the following activities:
- Test grouting and sinkhole repair
- Exploration grouting from the crest
- Infrastructure construction
- Impact studies and NEPA review
- Epikarst grouting
- Rock grouting
- Diaphragm wall design and construction
- Site restoration
Through benchmarking of projects with similar complexity and scale, TVA estimated that the remediation project to return the reservoir to normal operations would require 5 to 7 years to complete.
How Did TVA Help the Community?
We committed to taking the time to repair this dam right. However, TVA also understood the serious impact that this extended drawdown had on local businesses and residents. We worked with the community on actions such as new ramp extensions, a new swim beach and other activities to help mitigate the extended drawdown and allow local resident to enjoy the water while work continued on Boone Dam. Community projects included:
- Boat ramp extension at Pickens Bridge Access area (the only area with current boat access)
- Boat ramp extension and new temporary public swim beach at Boone Dam tract 22R located along the right bank of the Holston River
- Boat ramp extension just north of Devault Bridge, also along the right riverbank
- These boat ramp extension included roads, parking areas and roadway improvements
In addition, TVA installed several buoy markers on Boone Lake to designate areas that present potential hazards to boaters because of rocks, tree stumps and debris uncovered by the shallow water. To protect the public during project construction, TVA installed buoy markers and barricade floats in the dam forebay area an average about 300 feet upstream of the dam, and also in the tailwater area about 1,200 feet downstream.