SPOKANE — Preliminary results of what caused a big crack in Wanapum Dam point to a mathematical error during design of the structure more than 50 years ago, the Grant County Public Utility District said Tuesday.
The underwater crack formed across a spillway pier of the huge dam and may have been exacerbated by a weak construction joint, the utility said.
The preliminary findings will be submitted to the utility’s Board of Consultants for final review.
The utility says a team of engineers discovered the mathematical error while examining the original design calculations of the Wanapum Dam spillway. When they recalculated the design formulas, they found that additional concrete and/or reinforced steel should have been used in the construction.
The dam is located on the Columbia River a few miles south of Vantage.
A crack that was 2 inches wide and 65 feet long was discovered in February on a spillway pier of the 8,000-foot dam. The reservoir behind the dam was lowered by 26 feet, relieving pressure and causing the crack to close itself.
The utility has been working for months to learn the cause of the crack, spokesman Thomas Stredwick said.
They believe the original designers of the dam mistakenly believed that the weight of the spillway’s concrete provided enough strength to resist the force of the water pushing against it, the utility said in a press release.
“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,” the utility said.
Had engineers known of the miscalculation earlier, the spillway could have been reinforced, the utility said.
Investigators believe the crack may have originated a number of years ago and spread gradually, ultimately allowing enough water into the fracture to push the upper portion of the spillway pier out of place, revealing the problem.
Repairing the pier will require that additional steel be installed through the concrete structure and into the bedrock, the utility said.
The remaining 12 spillway piers have a design similar to the one that cracked, and steel reinforcement will likely be necessary there as well, the utility said.
The federal government must approve any repairs, the utility said.
Repairs are likely to take all summer, and the reservoir won’t be raised until the fourth quarter of the year, the utility said.