Grant County PUD
DESERT AIRE, Wash. — Initial analysis shows a loss of bonding in a lift joint is causing greater than normal leaking at Priest Rapids Dam, Grant County Public Utility District officials say.
A lift joint is an area between two concrete blocks poured at different times. Investigative drilling has shown a loss of bonding resulting in a flow of 3 to 4 gallons per minute through a lift joint near the base of one of the dam’s 22 spillway monoliths.
Monoliths are large expanses of concrete supporting piers holding spillway gates. Concrete is porous so there always is some leaking.
The monoliths are stable but merit further investigation, the PUD said in a news release. The dam continues to generate electricity and operate normally. There is no threat to property or people, the PUD said.
Many holes from the investigative drilling show no leakage at all and others have very low flow, said Christine Pratt, PUD spokeswoman.
“As we continue to drill additional holes, we’ve found that measurable water pressure and flow in the lift joint has been reduced significantly. This is a good sign because it indicates that water that was once backed up in the lift joint is getting an escape route through the new holes, relieving the pressure inside the structure,” Pratt said.
It also indicates that the separation at the lift joint probably doesn’t extend all the way through the monolith, she said.
During construction, from 1956 to 1961, the dam was poured in different sections at different times. The river ran over some lift joints for a year before the next lift joint was poured on top of it, Pratt said.
Engineers suspect that surfaces may have been worn smooth, not properly cleaned and not roughed up prior to the next pour on top of them, causing poor adhesion, she said.
Inspection drilling has occurred through about half the spillway and will continue throughout remaining monoliths into May. The PUD officials will determine if remedies, beside the drilling, are needed.
Leaking was detected by inspection drilling the morning of March 28 and the PUD declared a non-failure emergency. Reservoir water behind the dam was lowered approximately 3 feet. The reservoir will remain at the lower level of 481.5 to 484.5 feet above sea level until inspection work is completed. Maximum elevation is 488 feet.