SWEET HOME — Contractors have been drilling near Foster Dam this week, testing the composition of embankment soils as part of long-term monitoring to ensure dam structural safety in the event of an earthquake, said Tom Conning of the U.S. Corps of Engineers.
Testing began Monday and will wrap up Friday. Samples will be sent to a laboratory in Canada. It could take up to 18 months to get final results.
Corps of Engineers Geotechnical engineer Silas Sanderson said the process is being repeated at dams across the United States.
"This geotechnical investigation is done every 10 years," he said.
Testing is done on embankment soils, not the concrete in the dam, Sanderson said.
"We want to evaluate the characteristics of the dam's foundation," he said.
Sanderson said this type of testing is especially important in an area like Oregon, where seismic conditions exist. He called it a "seismic active area."
"The dam is safe, we don't have any concerns, but we want to research how the dam might perform under a seismic loading or other natural disaster," he said.
Similar research has recently been completed at Hills Creek and Cottage Grove dams.
Sanderson said the dams undergo annual inspections as part of what he called a "rigorous safety program," adding, "Our nationwide goal is to reduce risk as much as possible."
Contractors from Shannon & Wilson Inc. and Holt Services have been drilling into the embankment with equipment resembling a well drilling rig and pulling numerous core samples every five feet apart.
They were working about 80 feet deep on Wednesday.
"We want to know soil types and strength parameters," Sanderson said.
Results so far indicate the embankment soil is primarily dense silt, according to Sanderson.
"We aren't finding any loose sands, or other sensitive soil classes that might lose strength when moved," he said. "We haven't found any bad stuff so far."
If unfavorable soil conditions are found, he added, there are materials that can be used to change the density of them.
"There are lots of options," Sanderson said.
Matt Chase, Portland District Dam Safety Program Manager said, "Drilling is not going to impact the integrity of the dam."
The study also includes Emergency Action Plan exercises.
The road over Foster Dam has remained open during testing.
Foster and Green Peter dams were constructed east of Sweet Home in the mid-1960s and mitigate valley flooding, as well store water for agricultural irrigation and recreational opportunities.
Foster Dam is one of 13 dams operated by the U.S Army Corps of Engineers in the Willamette River Basin.