Grant County PUD has said that a mathematical error during the pre-construction design of Wanapum Dam may be partly to blame for the fracture developing within the dam’s spillway.
Following an extensive 11-week investigation by Grant PUD and its consultants, preliminary results suggest a miscalculation by the original designers was the primary contributing factor to the fracture forming on spillway monolith No. 4. This may have been further exacerbated by a weak construction joint.
A team of engineers discovered the mathematical error while examining the original design calculations of the Wanapum Dam 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 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 Grant PUD engineers known of the design miscalculation earlier, the spillway could have been reinforced prior to a fracture forming, the utility said in a statement.
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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 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. Moving forward, this section of the dam will be monitored and evaluated throughout the repair phase and as the dam resumes normal operations. Grant PUD is committed to following all FERC-approved dam safety and monitoring programs, it said.
Additional steel reinforcement is to be installed through the concrete structure into bedrock to repair the spillway pier No 4. The remaining 12 spillway pier monoliths have a design similar to monolith No. 4 and additional steel reinforcement will likely be necessary to secure all of the spillway pier monoliths into bedrock. The independent board of consultants will review and FERC must approve any repairs prior to implementation by Grant PUD.
Image: Simplified explanation diagram, courtesy of Grant Count PUD
