Drainage drilling is finished and monitoring devices are now being installed
EPHRATA —Drilling for drainage and inspection in the spillway monoliths at Priest Rapids Dam is complete, as around 230 core samples have been extracted from the dam’s grout gallery. The discovery of excess leakage due to a disbonded lift joint earlier this year, resulted in the need for the drilling.
Crews are now installing additional instrumentation that will help detect monolith movements within the spillway. Thirty-seven new instruments are scheduled to be installed. The devices are intended to measure vertical movements of the monoliths as well as any displacements across the disbonded joint or at the foundation of the structure.
Installation of these devices is anticipated to last until early September. The placement of the new equipment, coupled with the existing surveillance and monitoring instruments and activities, will allow for additional data collection across the spillway.
Following the installation of the monitoring equipment, Grant PUD will seek Federal Energy Regulatory Commission (FERC) approval to remove the lower operating-level restriction that is currently in place for the Priest Rapids reservoir.
The elevation of the reservoir was lowered by approximately three feet, still within the normal operating range, while inspection work took place. Priest Rapids Reservoir continues to be held at operating elevations between 484.5 to 481.5 feet above sea level. The maximum normal reservoir elevation is 488.0 feet.
Once FERC makes its decision, Grant PUD officials will determine what, if any, other remedies are needed beyond the drilling. Engineering staff will also examine factors that caused and led to the disbonded lift joint.
The drainage drilling reduced the pressure from the water inflow at the disbonded lift joint, while also allowing crews to determine the extent of leakage throughout the spillway monoliths.
In March, Grant PUD officials declared a non-failure emergency at the dam when inspection drilling revealed leaking in the spillway monoliths. The incident has never presented a safety risk to the public or Grant PUD employees. Priest Rapids Dam continues to generate electricity and operations continue as usual.