PRIEST RAPIDS DAM — Construction should begin by the end of the year on an estimated $50 million project to reinforce the right embankment (the Yakima County side) at Priest Rapids Dam.
The project is designed to address an unlikely – but possible – problem, said Dale Campbell, senior manager for power production engineering for the Grant County PUD. The goal is to keep the bank from crumbling in the case of a big earthquake.
In this case “big” would be a magnitude 6 or greater on the Richter scale, Campbell said. “One of those rare events, but the consequences are very high when it occurs.”
The dam was not built to the bedrock in that location, Campbell said. In the case of a big quake the ground could get very unstable. “The soil becomes like a milkshake,” he said, a phenomenon known as “liquefaction.”
If that happens the whole bank could crumble, he said. It’s not certain that would happen, he said, but it could. And if the whole bank crumbles it could take that end of Priest Rapids Dam with it.
In order to keep that from happening the bank will be excavated down to the bedrock, then a concrete dam will be built to replace the dirt. Even though it’s extensive work, it shouldn’t have any impact on Priest Rapids operation. “Operations as normal, access as normal,” Campbell said.
Crews will excavate about 226,000 cubic yards of dirt and rock, and the new structure will use about 66,000 yards of concrete. What’s currently unknown, Campbell said, is how much – if any – water will seep from the bedrock. If that happens, material will be injected into the bedrock to stop the seepage.
The structure will be downstream from Priest Rapids, and about 55 feet high, with most of that underwater. The concrete is a special mix that doesn’t use reinforcement or forms, Campbell said, something “relatively new” and used in dam construction since the 1990s.
The material is treated more like dirt would be handled, dumped from a truck then compacted with rollers.
The structure is “100 percent designed” and has been approved by a board of engineering consultants. Currently the design is under review by the Federal Energy Regulatory Commission. Utility district administrators have applied for the necessary federal and state permits, Campbell said.
The project is expected to take about 18 months to two years. The contract should be awarded by August, with work beginning in September or October. Some work will go on year-round.
As the work starts at Priest Rapids, PUD officials will be analyzing the left bank (the Grant County side) at Wanapum Dam. Campbell said no one knows yet if mitigation will be needed at Wanapum, and what it will cost if it is needed. “That is a much more expensive fix, if it needs to be fixed.” The report and its conclusions should be available sometime in 2020, Campbell said.
Cheryl Schweizer can be reached via email at firstname.lastname@example.org.