Bad math, bad concrete joint caused Wanapum Dam fractureMay 13 - McClatchy-Tribune Regional News - Christine Pratt The Wenatchee World, Wash.
A combination of bad mathematics and faulty craftsmanship caused the 65-foot crack that today crosses part of the Wanapum Dam spillway, Grant County PUD officials announced today. The findings follow 11 weeks of analysis. They were released, along with a proposed repair plan, to PUD commissioners today. Final approval is still needed from the board of independent experts who are overseeing the damage analysis, and federal dam regulators, PUD spokesman Tom Stredwick said. "A team of engineers discovered the mathematical error while examining the original design calculations of the Wanapum Dam spillway," Stredwick said in a news release. "When (today's) engineers recalculated the original design formulas they found that additional concrete and/or reinforced steel should have been included in the construction of the monolith (concrete spillway section)." The miscalculation led the dam's original design firm, Harza Designers, to believe that the size and weight of the massive concrete section would be enough to withstand the force of the water pressing on it. Hazra is now part of MWH Global, a Colorado-based transnational construction firm. Compounding the math error was a faulty "construction joint" -- the "seam" along the width of the concrete structure where one pouring of concrete meets the one above it, Stredwick said. This was likely caused by concrete being poured on a day when weather or other conditions were unsuitable, he said. Over the dam's 50 years in operation, water forced its way into this weakened joint, pressing it open and causing a crack that continues in an arching pattern through the spillway and down to the bedrock. The crack that formed across the No. 4 spillway pier following the seam between the pourings on the upstream side of the dam, Stredwick said. Once the board of consultants and federal regulators approve a plan to repair the dam, crews will work throughout the summer. The repair will involve anchoring the dam more firmly to the bedrock using "steel-strand" cable anchors that run from the spillway's deck, down through its concrete structure and into the bedrock. Other steel-strand anchors will pass diagonally through the spillway's curved, downriver face and into the bedrock below. This anchoring procedure will be performed on each of the dam's 12 spillway sections, Stredwick said. The damaged portion -- No. 4 -- will receive more of these anchors than the rest. PUD officials are still studying whether they can raise there reservoir to an intermediate level after repairing the damaged spillway section, or if they'll need to keep the river at its current, 26-foot drawdown level until all 12 spillway sections are anchored, Stredwick said. They're currently estimating that the water will remain low and the shoreline closed until at least October, Stredwick said. The PUD estimates the total repair will cost $61 million. Of that, about 30 to 35 percent will be paid for by the utility's contracted power purchasers. They're not currently anticipating having to raise electric rates to cover the cost of the repair, but nor have they ruled it out. Reach Christine Pratt at 509-665-1173 or pratt@wenatcheeworld.com. Follow her on Twitter at @CPrattWW. http://www.energycentral.com/functional/news/news_detail.cfm?did=32478192& |