Repairing a dam that was initially built beginning in the 1940s is a big job under any circumstances. But this job was especially important because the whole community was aware of the possibility that the dam could fail. However, this time new technology exists that makes the engineers more confident in their ability to create a stable, durable structure.
The Wolf Creek Dam is one of the largest dams in the United States, and like most other dams in this country, it’s definitely not aging well. The original Wolf Creek Dam is nearly a mile long, running along the Cumberland River in Kentucky. The dam not only prevents flooding, it also generates hydroelectricity and creates Lake Cumberland, a popular recreation area and tourist attraction.
However, by the 1960s, seepage started to become noticeable. The region’s karst geology created large gaps underneath the porous limestone deposits, which eroded the old dam and allowed water to seep through. Some of the limestone deposits were filled with clay, and as the clay eroded, it created caverns as high as 40 feet tall inside the dam.
Public officials started to worry that the seepage was weakening the entire structure and could result in massive flooding along the banks of the river—a potential disaster that could have cost billions of dollars in property damage. Campers at Lake Cumberland and thousands of residents nearby were in danger if the dam collapsed, so repairing it became an emergency project. Annual visitors to the lake declined because of safety concerns and some of the local tourist businesses reported revenue losses of up to 30 percent.
According to Mike Zoccola, the lead engineer at the Army Corps of Engineers that oversaw the project, they attempted to rebuild the barrier wall in the 1970s to address the seepage problems. But, because that barrier wall only ran about two-thirds of the length of the dam, the problems reappeared. “As the limestone eroded, it created an effect like a sinkhole,” Zoccola said.
Planning the Rebuild
Some construction projects select contractors based on the lowest bids. Because of the serious risks of a failing dam as well as the complexity of the project, the U.S. Army Corps of Engineers chose a different method. “There are only a handful of contractors in the world who can even do this kind of work,” Zoccola said. “We chose a ‘best value’ approach in selecting a contractor to do the job. We looked at their technical skill first, not the lowest price. We wanted to choose a contractor that we knew would be able to get this job done right, because we can’t afford a mistake.”
The engineers also knew that the rebuild had to be much more effective than the old ones. “The 1970s rebuild just wasn’t long enough or deep enough,” Zoccola said.
In 2005, the Army Corps of Engineers made the emergency decision to temporarily lower the lake levels by 40 feet to reduce the risk of property damage and risks to human safety.
The first major contract was awarded in March 2006, primarily for site preparation.
The second phase began in October 2008 to construct the main barrier wall.
Technical Challenges and Solutions
One of the biggest challenges in rebuilding the Wolf Creek Dam was the fact that the contractors needed to fill in the holes in the limestone and build a much larger and deeper wall than ever before. “We knew we were really pushing the envelope on technology,” Zoccola said.
The biggest advantage of rebuilding the dam now is the greater technology available to help engineers and drillers. “Now we had a really good picture of what we were working with, which makes it a lot easier to repair problems that are 300 feet below you in the water,” said Zoccola. In the past, they just had to operate more on estimates based on equipment that was much less precise. “Now we can control verticality and drill with specificity.”
The contractors built a work platform at an elevation of 750 feet.
The concrete wall measures 1,796 feet, with a spillway section of 10 37-foot-by-50-foot tainter gates.
The grouting went 50 feet below the bottom of the wall, and there were two lines of grouting, one upstream and one downstream. They used a Wassara water hammer drill bit to drill cores for sampling. They drilled 5-foot-by-30-foot deep centers.
“We pumped water down there to see how much it could withstand,” Zoccola said. “If there were a lot of openings, we’d add more grout. We were measuring permeability, to make sure that water wouldn’t seep through even at high pressure.”
The contractors used a combined barrier wall construction technique. The wall extends 275 feet from the work platform, with overlapping 50-inch holes filled with concrete. “We had very tight tolerances,” Zoccola said. “The majority of the wall required the secant piles.”
“What we did was to drill a primary pile, skip over, build another primary pile and then fill in between,” Zoccola said of the construction of the wall. “We created 6-foot-by-9-foot panels to create a permanent barrier wall.”
The wall is constructed of a combination of a grout curtain and concrete dam. The panels extend through the embankment into the top of the rock. The embankment is 3,940 feet long and the depth varies from 60 feet to 150 feet.
The completed dam project is now much larger than the old dam, measuring 980,000 total square feet of barrier wall. By the end of the project, a total of 1,197 piles were placed. The team between the Corps and Treviicos-Soletanche Joint Venture was able to complete the work 239 days ahead of schedule.
“We still have one more phase for this project, even though the dam is finished and secure,” said Zoccola. “We still need to do some site restoration, remove the work platform and make it look better in general. But it’s safe for use and will protect the community, which is the important part.”
Because the Wolf Creek Dam repair project was completed several months ahead of schedule, it’s still undecided about whether or not the Corps will allow the lake levels to return to normal for the 2013 summer season. One thing is certain: The Corps was very motivated to finish the dam as quickly as possible, so that large numbers of tourists will return to Lake Cumberland and revive the local businesses and marinas. Now that the dam is secure again, the tourist industry in this beautiful part of the country hopes to be safe to enjoy for many decades to come. This repair was finally done right, and they expect this one to last.