Tension from water pressure was a factor in creating the fracture discovered on the Wanapum dam's spillway pier monolith No. 4, Grant County PUD has announced.
The team of engineers and analysts engaged at the dam, located in Washington State, has determined that the fracture was not caused by seismic activity, foundation settlement or uplift, activities at the US Army's Yakima Training Center, or operation of the spillway gates. Data instead is showing that water pressure on the spillway face was a contributing factor, and Grant PUD is now working to determine how the tension caused the fracture.
The fracture was detected by divers on February 27, three days after a worker at the dam noticed that the top of the spillway pier had shifted slightly downstream. When the reservoir behind the dam was drawn down, the pressure on the spillway was reduced and the fracture closed, stabilizing the monolith.
Grant PUD will continue to evaluate and examine the spillway. Six holes have been drilled into the monolith and more holes are planned to determine the geometry of the fracture. The drilling has been temporarily suspended while the utility assesses the best way to complete the task. When finished, the drilling will identify the reach of the fracture into the monolith.
The drilling and the root cause analysis are expected to be completed by June, after which necessary repairs will be made to stabilize the spillway. Grant PUD will not raise the reservoir to an intermediate level until the stabilizing repairs are finished.
