Green Peter Dam has been leaking for several years now, but it only took four days to patch the thing up.
“I’m very happy with the crew and the maintenance foreman and the engineering staff that all worked together to put this together (in four days),” said Mike Pomeroy, operations and maintenance manager for the Army Corps of Engineers at Foster and Green Peter dams.
Pomeroy said the leak was not a major concern, but they took advantage of an opportunity to try to fix it.
“The dam safety engineers have been monitoring it,” he said. “There was never any danger of it causing a dam failure or anything like that, but it’s not something you want, either.”
It was the deep drawdown that allowed the Army Corps of Engineers to access the culprit: a faulty waterstop seal between blocks nine and 10 that has been slowly eroding over a loosely estimated 10-year period.
“Some people have called it a crack. It’s not really a crack; it’s just a leak in between two of the big monolith blocks that make the upstream side of the dam,” Pomeroy explained.
The leaking vertical seam was approximately 200 feet in height. Instead of trying to make any repairs to the leak, they opted for a patch job.
Thanks to the first deep drawdown last year, dam safety engineers and maintenance crews were able to get their first real look at what was taking place on the reservoir-side of the dam, Pomeroy said. Then they spent the rest of the year making plans and preparing to tackle the project during this year’s drawdown while water levels were low enough during a long enough period to complete the patch.
“It was a pretty amazing process, in my opinion,” Pomeroy said. “Last year allowed us to get down there and evaluate it, and then we did quite a bit of prep. A lot of the stuff had to be fabricated in-house, and then this second drawdown allowed us to get down there and get at it and actually make the patch.”
First, staff had to pressure wash years’ worth of debris that had been sucked into the approximately half-inch seam between the blocks, debris that was composed of small sticks, leaves, fishing line and dirt.
Next, they ran a sealant along the seam, similar to a caulking job. They followed that up by attaching a 12-inch wide, half-inch thick rubber patch along the seam, attached by aluminum plating on each side.
“It was pretty labor intensive,” Pomeroy said. “You’re talking about 200 feet of patch, about 430 anchor bolts that all have to be drilled in, 30 tubes of sealant, about 400 feet of aluminum plating, and these guys would kind of rotate through because it’s pretty labor intensive, and they got it done in four days. We were so happy.”
Pomeroy estimated it would cost “a couple million dollars or more” to fully repair the leak, adding that a repair would have involved injecting some kind of grout, but that option caused him to question how effective it would be, given that the dam needs to be able to flex during high- and low-water conditions.
The cost to patch the leak, Pomeroy said, cost just under $61,000 for both labor and material.
Still, Army Corps staff must wait until the reservoir is filled again to see if the patch works.
“There’s no guarantee that it’s gonna work. It really hasn’t ever been done like this before,” Pomeroy said. “The patch job went pretty smooth, but we’re really not gonna know the full effect, how effective it is, until we fill back up.”
