BEVERLY — Drilling more than a dozen holes in the Wanapum Dam’s already damaged concrete might not seem like a repair strategy.
But it’s actually the first step in a tentative plan to fix a 65-foot-long fracture in one of the giant concrete blocks that supports the dam’s spillway.
Such cracks are rare. However, engineers already have a potential solution that’s been used to stabilize scores of dams around the world.
But before repairs can begin, they need to figure out how deep the crack stretches into the concrete and how it happened in the first place. Hence, the drilling now underway.
In late February, divers discovered the crack, which stretches the entire length of the concrete block and was 2 inches wide in places. The entire spillway appeared to be bulging downriver slightly.
The Grant County Public Utility District, which owns the dam, dropped the level of the river behind the dam 26 feet to reduce pressure on the spillway. It worked; the spillway has been stable since early March.
But studying the cause and planning for repairs have moved much more slowly. Crews can only access the area by crane or boat, so windy weather delayed work, said PUD spokesman Chuck Allen.
They’ve been drilling 4-inch-wide holes into the concrete block in a grid pattern to find out how deep the crack reaches into the concrete. When they first hit the crack, water gushed out of it like a geyser.
To keep the drilling crew dry every time they reach the crack, divers have since installed a large sheet of waterproof fabric over the underwater crack.
“It’s like a giant Band-Aid on the face of the fracture,” Allen said. “It’s like a pocket of water now, but it won’t be under pressure.”
So far, crews have drilled six of about 20 holes planned. Allen said they expect to have the exploration and analysis done by early June.
The PUD has determined water pressure caused the crack. It’s unknown if the problem stems from the design, a construction error or material failure. Engineers have ruled out earthquakes, instability in the bedrock beneath the dam and explosions at the Army’s Yakima Training Center, adjacent to the dam’s west side.
“The primary goal of a dam is to withstand water pressure,” Allen said. “We are analyzing the whole spillway, not just the fractured area.”
The entire spillway is 820 feet long, with 12 spill gates, each supported by an individual, 65-foot-wide concrete block, identical to the one with the crack.
The tentative repair plan, Allen said, is a technique known as the anchored tendon approach.
Steel cables, called tendons, will be drilled through the fractured area and anchored to the bedrock below. Tightening down on the cables from the top will pull the crack closed. The drill holes around the rods are then sealed with a specialized grout.
“The steel cables are stretched just like a rubber band,” explained John Wolfhope, an engineer and vice president of the U.S. Society on Dams who has written reviews of the technique. “We’re adding more resistance to hold the concrete in place with these very engineered rubber bands.”
The technique has been used to stabilize concrete dams for decades, Wolfhope said. It’s commonly used to add support to dams and to repair damaged structures.
In fact, it was once used at Wanapum Dam in 1962, the year before the dam began generating power. It was not immediately clear late last week what prompted that work.
It was one of the first places where the technique was used in the country, said Wolfhope, who was not involved in earlier work or the current project.
More recently, the technique was used to improve stability at the Olmos Dam, outside San Antonio, Texas, in 2010. Much smaller than Wanapum, that dam was built in 1928 for flood control.
The $4 million improvement project installed 68 steel tendons. And in 2000, the anchored cables were used to help repair a dam in Corpus Christi, Texas, that had been pushed 4 inches downstream by water pressure, Wolfhope said.
Allen said this technique is currently considered an intermediate repair.
“This could be a temporary repair and it could also possibly be the repair for the long term,” Allen said.
“We’ll install it, bring reservoir up to intermediate level, and see if it’s stable enough to be the long-term solution.”
Studies have shown the tendons can provide long-term structural support if they are properly sealed to prevent corrosion of the steel by exposure to water.
The final design plans have to be reviewed by an independent board of consultants and the Federal Energy Regulatory Committee before the PUD will be authorized to start the work.
Allen said they don’t have an estimate of how much the repair work will cost yet, they are just paying the bills as work gets done.
So far, the PUD has paid out about $700,000 to contractors.
“It seems like a low figure now, but not all the invoices have come in,” Allen said.
The PUD is generating a cost estimate to present to its board of commissioners at its next meeting, he added.