A 50-year-old math error made during the design of the Wanapum Dam is the chief suspect in a 65-foot-long crack that will now cost $61 million to repair, officials have found.
An 11-week investigation by a team of more than 100 engineers identified three likely causes of the massive underwater crack, the Grant County Public Utility District announced in an update Tuesday (May 13).
The primary culprit, officials said: an apparent miscalculation that understated the amount of concrete and/or reinforced steel needed for construction of the dam's monolith.
The fracture was discovered Feb. 27 at the central Washington State dam on the Columbia River.
Photos: Grant County Utility District
The 65-foot-long crack occurred on the upstream side of the No. 4 spillway pier monolith below the water line. Investigators determined that a math error during design had led to insufficient support for the spillway.
The two-inch-wide horizontal crack ran below the water line on the upstream side, prompting district officials to draw down the reservoir elevation by 26 feet to reduce pressure on the structure.
The crack occurred in the fourth of 13 spillway pier monoliths that support the spillway gates. A follow-up inspection March 4 showed that the fracture had closed, the monolith was stable, and the rest of the upstream face showed no fractures, Grant County PUD reported.
The 8,637-foot-long dam was built from 1959 to 1963 at a cost of $93 million.
Doing the Math
The math error came to light when engineers recalculated the original design formulas of the structure from the late 1950s, said dam officials. They did not detail the precise miscalculation at issue.
"The original designers of the dam miscalculated that the weight of the spillway’s concrete provided enough strength to resist the force of the water pushing against it," the district reported.
"Over time, this weak point on the spillway pier No. 4 succumbed to the force of the water pushing against it until after approximately 50 years of operation, the fracture formed."
Had the utility known of the error earlier, it could have reinforced the structure, officials said. But the crack was the first sign of a problem.
The investigation showed the fracture "may have originated a number of years ago and spread gradually over time, ultimately allowing enough water into the fracture to push the upper portion of the structure visibly out of place," authorities said.
An infographic by the Grant County Public Utility District shows the highlights in the incident's timeline.
The utility's regular monitoring programs "never identified this section of the dam as being susceptible to this type of potential failure and did not identify the issue at the time the fracture began," the agency said.
Two other factors played a role in the fracture, the investigation found.
First, the dam's 800-foot-long spillway section is not anchored to bedrock with steel, a lapse that will be corrected during repairs this year.
Second, the concrete cube for the fractured section, poured on a hot July day in 1960, "might not have cured properly," utility district spokesman Thomas Stredwick told the NW News Network.
Officials have said repairs to the dam will cost about $61 million—about two-thirds of the original construction cost. The project will include additional steel reinforcement through the concrete into bedrock below the dam.
Repairs are expected to begin this summer, and the utility hopes it can raise the reservoir by 19 feet to about 561 feet in the fourth quarter of the year.
Meanwhile, public access to the shoreline and boat launches will remain off limits, and the area is being patrolled seven days a week.
The dam is currently operating at between 50 and 60 percent of capacity.
Regular updates on the project are available at www.grantpud.org.