SPOKANE, Wash. (AP) — Pressure from the water behind Wanapum Dam may have contributed to the big crack that has disrupted operations at the structure, the Grant County Public Utility District said Wednesday.
The cause of the crack remains unknown, but the utility said it had ruled out four possibilities. Ruled out were seismic activity, foundation settlement, operation of spillway gates, and explosions at the nearby Yakima Training Center operated by the U.S. Army.
Chuck Berrie, assistant general manager of the utility, said learning the cause could occur as soon as the next two or three weeks.
The 65-foot-long crack was detected by divers on Feb. 27, three days after a worker at the dam noticed that the top of a spillway pier had shifted slightly. When the reservoir behind the dam was drawn down by 26 feet, the pressure on the spillway was reduced and the fracture closed.
Six holes have been drilled into the pier and more holes are planned to determine the geometry of the fracture and how far it goes into the pier, the utility said.
"We are taking core samples of the concrete and analyzing that" to determine its strength, Berrie told the Northwest Power and Conservation Council on Wednesday.
Drilling has been temporarily suspended while the utility assesses the best way to complete the task.
The utility said it may be June before a repair plan is formulated.
The reservoir shoreline exposed by the drawdown will remain closed, the utility said. The drawdown has left docks and boat launches on the reservoir high and dry.