Workers at a drill rig at Boone Dam, where work has entered the design stage.
By Jeff Keeling
They may look like typical job site trailers, but the simple boxes set up near Boone Dam’s repair site are hubs of high-tech data analysis. It’s all part of the five-to-seven-year journey toward what the Tennessee Valley Authority hopes will be a top-notch repair job – one currently expected to cost up to $300 million.
TVA officials conducted a media tour of the repair site Thursday. Sam Vinson, the project manager for the Boone Dam remediation, covered a broad range of topics with the help of Public Relations Manager Jim Hopson.
The pair revealed a busy job site that’s about to get busier, but one where safety is paramount and workers already have logged 170,000 man hours without a safety incident. The completion of an environmental assessment in early January has propelled TVA from the project’s planning phase to the design phase. Long-term, the project aims to repair the earthen portion of the dam, where the karst, limestone-based topography that dominates the area has undergone structural changes creating sinkholes and other weak spots.
In simple terms, crews will drill holes into various spots in the earth wall that extends out from the dam’s concrete portion, some of them hundreds of feet deep, and inject a special grout material into the holes. The cement-like grout should help strengthen the underground drainage systems that have developed over time.
Around 115 workers were on site Thursday, and Vinson said the drilling and grouting work is about to move to 24 hours a day, five days a week, with two days off for equipment maintenance and logistical planning. The goal, he said, is to get around 17 holes drilled, at depths up to 350 feet, grout them in a similar fashion to how they’ll be done in the repair stage, “and monitor the dam to see how it responds.”
Hence the engineers and scientists in those trailers, who are monitoring all the data, which also is fed to TVA headquarters in Chattanooga. “That allows us to gather information to feed into the design,” Vinson said. “‘How deep does the repair have to be? What’s the rock look like actually beneath where we’re going to put this composite wall?’”
TVA has a request for proposals out, and expects to select a general contractor that will oversee the multi-year repair phase by late spring.
Meanwhile, Boone Lake’s pool elevation appears destined to remain between 1,350 and 1,355 feet above sea level. That’s what TVA deems a safe level considering the issues being repaired, but one roughly 30 feet below the normal summer pool of 1,382 feet. To that end, Hopson said, TVA continues to work with the Tennessee Wildlife Resources Agency (TWRA) and local citizens to provide as many recreational opportunities as possible given the low lake levels.
Sam Vinson
Permits have been approved for three boat ramps that will be either built new or extended. Hopson said TWRA is partnering with TVA on the Pickens Bridge and Devault Bridge boat ramps. “We have worked with them to help design and fund those, and their crews are going to be building those this spring with the expectation to have those available by summer,” he said.
In addition to those existing ramp sites, TVA itself will create a public swimming beach and a new boat ramp a few hundred yards northeast of the current public beach, which won’t be accessible during the repair work. Hopson said up to 5,000 people visited the swimming beach some weekends.
“Our expectation is to have that facility completed and available by early summer as well,” Hopson said.
TVA and TWRA also continue working together to monitor fish habitats in the anomalous conditions created by the drawdown.
Synclines, anticlines, and hard rock: Why it’s always tougher in Northeast Tennessee
The Boone Dam repair isn’t TVA’s first rodeo, nor even its first experience fortifying a dam structure in a karst environment. Vinson mentioned the Wolf Creek Dam in Kentucky and Center Hill Dam in Tennessee as projects used to benchmark this one. They took longer than the five-to-seven-year target for Boone, but, Vinson said, “we’re kind of standing on their shoulders, using their lessons learned.”
Both dams overlie karsts, so some of the risks that were mitigated are similar, Vinson said. “Sinkholes, crevices, same type of seepage.” But here, he said, “the rocks are not flat line.”
Instead, they’re folded, something any astute observer has noticed in looking at large rock faces in the area, where layers of rock often stand at angles. Those are the synclines and anticlines, or upward sloping and downward sloping rock strata, that characterize this area.
And that characteristic could make the repair process trickier than it was at the other dams. Throw in the fact that the rock here is harder, and it’s clear TVA’s positive outlook isn’t because the job won’t be challenging.
“We just have a little more complex geology,” Vinson said. “And the rock itself is harder. Everything is more difficult here. It’s definitely harder and more abrasive.”
