TVA Successfully Completes Dam Safety Modification Project for Ages

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When a sinkhole was discovered near the base of the Boone Dam embankment, Tennessee Valley Authority Dam Safety Engineers got to work determining the best solution. The complexity of the work and the quality of the repair make it a worthy POWERFUL Best Plant Award Winner.

With a name like Tennessee Valley Authority (TVA), most people can easily assume that the public corporation founded in 1933 serves customers in “The Volunteer State,” but TVA actually has a much broader reach than that. The Tennessee Valley region spans approximately 80,000 square miles across seven states in the southeastern United States. In this region, TVA supplies electricity to 153 local power companies serving 10 million people, as well as 57 large industrial customers and federal facilities. Beyond that, TVA also provides flood control, navigation, and land management for the Tennessee River system, and assists local power companies and regional governments in their economic development efforts. This is obviously no small mission.

Dams are the tools TVA uses to manage water and its impact on the Tennessee Valley. It operates river dams at 49 sites, 29 of which produce hydroelectricity, and dam safety is a top priority for the agency. That’s why, when a sinkhole was discovered in October 2014 near the base of the Boone Dam embankment, and water and sediment were found seeping from the bank below, TVA took the matter very seriously.

Urgent action needed

A team of TVA dam safety engineers and outside experts investigated the source of the observed seepage to determine if dam safety had been compromised. The investigation revealed that the foundation of the embankment dam had been affected by internal erosion, which is when voids develop due to groundwater flow. Internal erosion is one of the main causes of dam failure in the world.

Following this discovery, a number of interim risk reduction measures were quickly implemented, including lowering the elevation of the Boone Reservoir pool to between 1,350 feet and 1,355 feet (approximately 10 feet below the level of the winter pool); assign on-site inspectors to the dam for continuous monitoring; install an automated network of sensors to monitor the dam for pressure and temperature changes every 15 minutes and for movement every 30 minutes; exercise an emergency action plan; and construction of a tailrace filter to minimize further deterioration of the dam. Next, TVA’s team of dam engineers and safety experts assessed dam repair methods.

An extensive decision-making process was used whereby TVA identified a composite seepage barrier as the preferred option to remedy the problem. A composite seepage barrier would create a positive reservoir cut-off and could be fabricated from a non-erodible material. Therefore, once deployed, the solution would have a very low probability of an infiltration connection reoccurring in the reservoir.

“This project was the largest dam safety modification project undertaken in the agency’s 89-year history, and TVA had never executed a project with similar technical requirements for the creation of a water barrier through an active dam,” Chris Saucier, technical director of the TVA project, told POWERFUL. “However, TVA has executed significant dam safety modification projects over the past decade, and the experiences gained from these projects related to building highly efficient and safe work crews have proven invaluable. . Ultimately, the project reinforced my personal belief that our people will always be able to meet the technical needs of a project, and that the biggest challenges are those that involve how we protect ourselves and work together to meet the expectations of the citizens that TVA serves.

A seven-year effort

To plan the work, TVA used other similar projects to develop a benchmark schedule. After the emergency repairs, the first thing on the to-do list was an extensive injection testing program. “We started the first injection test sections at the toe of the dam in July 2015,” said Kevin Holbrook, Senior Director of Construction Projects for TVA. POWERFUL. The BARNARD and Nicholson construction groups participated in this work. As part of the work, two Limited Mobility Grout (LMG) lines were used for void filling and permeability reductions in soft soils and weathered rocks in the epikarst zone.

During this time, several challenges have arisen on the project, including managing the highly weathered rock on which the dam was originally built in the 1950s and addressing public concerns about the safety of the dam. Yet the biggest hurdle revolved around space.

“The most specific challenge with completing the project was the limited dam crest work area, which originally consisted of a work surface approximately 20 feet wide by 800 feet long,” said Holbrook. “To overcome this challenge, TVA lowered the crest of the dam by 10 feet and also constructed buttresses upstream and downstream, which had multiple benefits for the project. One of them being the final constructed work platform which was approximately 120 feet by 800 feet long.

Holbrook continued, “A large level of project planning and sequencing still had to be orchestrated during construction so that the project could utilize four independent operations that had specific spacing requirements to maintain dam safety at all times. The team used production rates from past projects and the test section to develop the detailed sequencing plan that allowed the narrow work area to be the most productive while meeting our requirements for job security. dam and personnel security. »

Successful projects require teamwork

A joint venture consisting of Treviicos South Inc. and Nicholson Construction Co. executed the construction of the seepage wall. It included 307 secant pile installations 50 feet in diameter from 80 feet to 180 feet deep. “To carry out this task, the contractor had to build on-site infrastructure to include a concrete batching plant, a grit plant and a water treatment plant which supported the construction,” explained Holbrook.

Meanwhile, Phillips & Jordan Inc. worked on rockfill berms on the upstream and downstream faces of the dam; improvement of downstream drainage; widened the ridge to provide a better working footprint for the cut-off wall; additional redundancy of internal erosion protection; and increased dam stability during construction and operation. Baker’s Construction Services Inc., a Tennessee-based site preparation contractor, helped manage vegetation and debris and helped upgrade publicly accessible site facilities.

At the height of the project (Figure 1), the workforce peaked at around 200 workers. This included skilled workers (such as drilling rig operators, grout plant operators, and concrete plant operators), as well as local trades and labour. “By using our local building trades through the union, we were able to find skilled workers in the area to support the work,” Holbrook said.

1. For much of the drilling, crews worked 24 hours a day, five or six days a week on a pad atop the embankment, which was modified to accommodate several pieces of heavy equipment. Courtesy: VAT

A focus on safety

Safety was a top priority for project managers. Holbrook explained that TVA requires all staff working on-site to complete an Occupational Safety and Health Administration (OSHA) ’10-hour training’, which gives supervisors and tradespeople a solid safety foundation before starting. work. Holbrook also mentioned a long list of additional courses that everyone had to take, and even more that were assigned based on job duties.

“TVA and the management of our contractors have strongly promoted and encouraged safety involvement and awareness, which has created a lot of enthusiasm among the craftsmen to participate,” he said. The effort paid off, as the project logged 2,068,530 hours of work without lost time injuries.

The COVID-19 pandemic caused some disruption, but the project managed the situation successfully. “TVA’s top priority was to ensure the health and safety of everyone supporting the project. Therefore, we have taken deliberate measures like holding virtual meetings for workers who are not directly involved in construction, social distancing, continuous deep cleaning and encouraging good hygiene,” said said Lawanda Hayes, TVA project manager. POWERFUL.

Saucier noted that the project has installed more than 300 instruments to monitor dam safety during construction, and most of the instruments will remain available for future use, providing a continuous real-time dam health monitoring system. The instruments will complement TVA’s current dam safety inspection and monitoring program, which is routinely conducted by a combination of TVA employees from the Dam Safety, Plant Power Generation and construction. “While TVA’s dam safety program is consistently recognized as among the best in the world, these instruments mark a continuous improvement in our efforts to keep operating dams safe,” he said.

Cross the finish line

Boone Dam’s hydrogen units consist of three turbines capable of delivering 81 MW of clean power. Saucier explained that one of the generating units was temporarily shut down during the construction of the project because a temporary dam safety protection device blocked the discharge tube of this generating unit. After the excavation wall was completed, the temporary protection measure was cleared from the river channel and the generator set was returned to service. Although no major upgrades were made to the unit while it was out of service, the downtime provided an opportunity to perform regular maintenance activities to ensure safe long-term operation. term in the future.

The Boone Dam project was completed in May 2022. “Ultimately, the project did not lose a single shift of production due to COVID and was able to complete ahead of schedule,” Saucier said. In fact, the project ended 17 months earlier than originally planned and under budget. “TVA went to great lengths to negotiate contracts to ensure we got the best rates and were good stewards of the resources allocated to complete the project,” said Hayes.

In addition to one POWERFUL Top Plant Award, the Boone Dam project has won several technical awards from international professional societies dedicated to the practice of dam engineering and construction. “Boone Dam is truly a gem, both to us who live in the area and to the world far beyond our borders,” Saucier said.

Aaron Larson is the editor of POWER.

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