Photo Coutesy Grant County Public Utility district
Crew bores pilot hole in preparation for steel-strand tendons that will be anchored in bedrock.

Early this year, workers at the Wanapum Dam in central Washington state discovered that a 50-year-old mathematical error, made during dam design, had caused a 65-ft-long fracture.

The 8,367-ft-long Wanapum Dam, six miles downstream of Vantage, Wash., generates 1,092 MW of power.

On Feb. 24, a hydroelectric mechanic walking the small road on the dam's spillway deck noticed a bowed curb, says Thomas Stredwick, spokesperson for the dam owner, the Grant County Public Utility District. Three days later divers found a fracture, which had opened 2 in. wide roughly 50-75 ft below the water's surface on spillway pier monolith No. 4.

The PUD went into emergency mode and reduced the pressure on the concrete structure, which originally opened in July 1963, by reducing the reservoir level behind the 185-ft-tall dam by 26 ft.

"That stabilized the dam and allowed us to get divers back into the water to start investigating this fracture," Stredwick says. Pinpointing the fracture also allowed the investigation to enter full forensic analysis to find the cause.

"After reviewing original design calculations during pre-construction [from] the design phase in the late '50s we noticed a mathematical error specifically used to determine the ratio of concrete to rebar in this section," Stredwick says.

Typical 1950s dam designers used mass concrete pours in spillways, believing the sheer weight would be sufficient, but the hand-done miscalculation created a "weak spot." Deeper diving into records and interviews in Chicago with those still alive who worked on the project showed this exact trouble spot was poured over a Fourth of July weekend that saw temperatures rise into the 90s.

"[The concrete] may not have been allowed to cure properly, which created a weak spot in a dam that already had a mathematical error," Stredwick says.

Over time, water pressure caused the concrete to fracture.

With the historical error discovered, the contracting team of Kuney-Goebel JV, a joint venture of Spokane companies Max J. Kuney Co. and Robert B. Goebel General Contractor Inc., along with Cuddy, Pa.-based deep foundation subcontractor Nicholson Construction, mobilized on site in early March, and crews found that the miscalculation placed the dam into tension, rather than compression. Crews, with the help of an independent review panel, devised a scheme to bring the dam back into compression, a key component in a $61-million Wanapum Dam repair plan.