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Work on Upper Stillwater Dam Nearing Completion

Upper Stillwater Dam isn't dainty. It's called a gravity dam, meaning it's so heavy that gravity holds it in place. It contains about a million and a half million cubic yards of material. And that material consists of concrete made from Pre-Cambrian sandstone and argillites with most of the length of the dam resting on hard sandstone. That helps the dam weighs more than five and a half billion pounds, or about 1.8 million PT Cruisers.

Being made from the same rock as the surrounding Rock Creek drainage helps the dam be materially congruent with that rock. So, when it was decided to do some repair work on the dam in 2003, engineers knew exactly what they were working with.

photo: Upper Stillwater Dam

"What we've accomplished from that date to the present is grouting of 14 cracks with polyurethane grout," says Supervising Engineer Jan Ringel. He works out of the Provo Area Office and knows Upper Stillwater Dam better than anyone. He helped build it between 1983 and 1987. So when Area Manager Bruce Barrett was looking for someone to help fix the dam's notorious problems with superficial cracks, it was Ringel he called.

Shortly after the dam was completed, it began to show signs of cracking caused by problems related to the release of heat. Concrete heats up as it cures, and the technique for building Upper Stillwater Dam, called "Roller Compaction" layers concrete like a cake, and can sometimes not allow heat to dissipate fast enough. So cracks began to appear on the downstream face. Although they don't significantly affect the dam's ability to hold water, they are unsightly and allow relatively small amounts of water to seep through. So, Ringel's job was to stop the seeping and fix the cracks.

The main task to address both issues was the installation of three stainless steel membranes. By drilling holes large enough to lower them into the dam behind the cracks and sealing them with flexible asphalt, Ringel and Reclamation Technical Center is Denver is confident the problem will be solved. But the devil is in the details.

"The first membrane is 90% complete," he says. "The second is about 90% complete and the third slot for inserting that membrane is being drilled right now." The problem, according to Ringel isn't the work, but the rising water behind the dam. "To get the [third] slot completed, we're going to be into high water before we get it drilled. So there's going to be a period of time where we're going to lose access because it's on the other side of the spillway and we have the ramps going down into the spillway. So at some point in time, the water will probably stop us from proceeding."

And then, there's the drilling itself. Many people might consider the idea of drilling holes in a dam counter intuitive. But the fix requires membranes that are fitted into custom slots cut from the top of the dam and extending almost to the foundation. The membrane is surrounded with asphalt while the cracks are grouted from the inside and caulked from the outside.

The technical challenges were formidable. For example, for most people, the straightest hole they'll ever have to make might be in a wall for hanging a picture. But imagine making a hole nearly than two-hundred forty feet deep and needing it to be no more than two inches off.

They're the kind of challenges engineers live for. "This is exciting stuff," says Ringel. But there were other, equally exciting challenges to consider when doing the fix. For instance, the stainless steel membrane was made with anchors that would be secured to concrete at the top and the bottom of the slot. But, adding the boiling asphalt sealant could've caused a change in temperature that would've created enough contraction to rip the steel apart.

Another issue was combining the hot asphalt with the water at the bottom of the dam. "When you put hot asphalt in water, it's like throwing water in a frying pan full of grease," says Ringel. Water boils at 212 degrees, and this asphalt is at 400 degrees. If you put water on top of the asphalt, you get an explosion." So what the contractor did was mix asphalt and cement together to make it heavier than water and pumped beneath it. "Since the mix is heavier than water, it stays under the water while pushing the water up. So we avoid any explosions."

"And while you're drilling, you're wondering "Where are all of the cuttings going?" Ringel says there might be several cubic yards of powdered material at the bottom of the holes. It has to be removed as the hole is drilled, otherwise the asphalt and concrete mix can't be applied. "So, we drop another pipe that's connected to a vacuum truck that's sucking material from the bottom of the shaft."

Another contractor will grout and caulk cracks on the upstream face of the dam after this contract with ASI-RCC of Buena Vista, California is complete. And they may face some of the same challenges with weather as did the previous contractor. Ringel and his group have worked for two winters on Upper Stillwater and it gets cold in the upper Uintas. "The colder the temperature, the less productivity you get. We drilled all winter long," he says, "but the productivity goes down."

Still, after 34 years as an engineer, Ringel is glad to be back working on a dam he helped build. "This type of work has never been done before. When you first read how they were going to do this fix, you'd go, 'This is amazing that they even consider this.'"

 

Last updated: January 22, 2007