The investigations by Grant County Public Utility District (Grant PUD) in Washington and its consultants found that the fracture forming on one of the spillway monoliths may have been exacerbated by a weak construction joint.
A team of engineers discovered the mathematical error while examining the original design calculations of the spillway. When 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.
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. Over time this weak point on the spillway pier succumbed to the force of the water pushing against it until after approximately 50 years of operation, the fracture formed. Had Grant PUD engineers known of the design miscalculation earlier, the spillway could have been reinforced prior to a fracture forming, they said.
The investigation indicates that 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. Grant PUD’s monitoring programmes 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. Moving forward, this section of the dam will be monitored and evaluated throughout the repair phase and as the dam resumes normal operations.
Additional steel reinforcement will be installed through the concrete structure into bedrock. The remaining 12 spillway pier monoliths have a design similar to the one where the fracture has been found and additional steel reinforcement will likely be necessary to secure them too.
The water level will be kept low while repairs to the spillway are carried out this summer.
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