Holt Services Inc. has won a US$74,825 contract from the U.S. Army Corps of Engineers to perform drilling to replace inclinometers monitoring what the agency called a "massive landslide" on the left bank at Chief Joseph Dam.
The Corps' Seattle District said in February that the left bank has been "creeping since monitoring began over 40 years ago" as shown by 14 inclinometers installed between 1973 and 1983.
The agency said the highest rate of movement is concentrated toward the toe at the southern end of the mass, nearest the dam, causing two critical inclinometers to move so much as to disable their ability to be read.
"The possibility of a catastrophic movement affecting dam safety is not known, and a comprehensive study is under way to determine if measures need to be taken to avoid disaster, or if the landslide is a non-critical issue," the Corps said. "These two inclinometers need to be replaced for this effort."
Per the award, Holt Services will drill two 6-inch diameter holes, one 135 feet deep and another 195 feet deep, with an option for a third hole to 145 feet deep. Work is also to include installation of 85-millimeter-diameter ABS plastic grooved inclinometer casing. Soil samples are to be recovered for geological characterization by a field geologist. Standard penetration tests of soil units are to be performed while drilling.
Located on the Columbia River, the 2,620-MW Chief Joseph hydroelectric plant is also currently undergoing refurbishment of 10 Francis turbines. The Corps awarded contracts in 2013 for dissolved gas and temperature monitoring and 2012 for replacement excitation systems at Chief Joseph.