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Corps tests aim to gauge stability of soils under Foster Dam, earthquake resistance


October 31, 2018

NATALIE EHRLICH, a geotechnical engineer with the U.S. Corps of Engineers, and Kyle Romney of Shannon and Wilson, Inc., prepare a sample of silt from below Foster Dam for shipment.

The U.S. Army Corps of Engineers drilled into and through Foster Dam last week as part of a test of underlying soils to determine how well the dam will hold up during an earthquake.

The Corps' Dam Drill Safety Team spent the week, beginning Monday, Oct. 22, conducting tests and removing samples. Corps staff anticipated completing the drilling by the end of October.

The work is part of a larger geotechnical assessment in which data from the borings will provide information on the physical properties of the dam. The dam safety team will then use the data to study how the dam will perform under various conditions. The study also will include Emergency Action Plan exercises, seismic and hydrologic studies.

"We're drilling into the embankment and getting a better idea of the makeup of the dam," said Corps spokesman Tom Conning.

"Drilling is not going to impact the integrity of the dam," said Matt Chase, Portland District Dam Safety Program Manager. "We use these investigations to improve our understanding of the make-up and risks at our dams."

He said the Dam Safety Program's goal is to make Corps dams as safe as possible, and minimize risks to the public.

"We're conducting this investigation in conjunction with our routine dam safety program which includes inspections, performance monitoring, and risk assessments," Chase said.

Prior to construction of Foster Dam, the Corps surveyed the the ground and soils where the dam would be constructed. The main dam is built directly on bedrock, Sanderson said, while the earthen embankments are built on the natural surface.

During construction, the Corps discovered a channel through the bedrock under the dam south of the main dam structure, said Silas Sanderson, a geotechnical engineer with the Portland District. The Corps did not have a good understanding of the characteristics of that channel.

The channel was carved through the bedrock and then filled in with silt material, Sanderson said.

At intervals of a year, five years and 10 years, the Corps conducts different levels of inspections and evaluations on each of its dams, Sanderson said. If officials determine there are issues, the agency will do more in-depth studies. Last week's study is the result of a 10-year evaluation.

When Foster Dam was built, it didn't have a seismic component about how the dams would perform during "seismic loading," Sanderson said.

Geologists began discovering evidence of the Northwest's susceptibility to large subduction zone earthquakes in the late 1980s.

"We're trying to reduce risks as low as possible for people living downstream," Sanderson said, noting that dams, in general, perform well during earthquakes. "They can move around a lot, but they also tolerate a lot of movement. Our expectation is we'll have good performance."

The Corps' understanding about the channel through the bedrock is not complete though, he said, and the Corps wants to eliminate that uncertainty.

The sampling will allow the Corps to determine the strength parameters of the dam's foundations.

After contractor Shannon and Wilson drills down more than 80 feet to reach the material, in one test, the Corps drops a hammer to see how much force is required to drive a weight into the material.

The Corps also is extracting undisturbed samples of the material using a Shelby tube, a hollow elongated cylinder designed to collect a core sample.

Those samples will go to a laboratory where they will be subjected to forces that mimic a seismic event, Sanderson said. That process has taken about two years to produce a final report at other Willamette Valley projects, like Hills Creek Dam.

The Corps is collecting dense silt from the channel, Sanderson said, which he views optimistically.

The Corps worries most about loose sand or sensitive clay that loses strength when it's moved around, Sanderson said, but the Corps hasn't found anything so far "that gives us a lot of concern."

SAM HEUSEN, left, and Aaron Kuper of Holt Drilling prepare to take samples at Foster Dam.

If the Corps were to find something that indicates a problem, Sanderson said, the Corps would begin a modification study to determine the best way to address the deficiency.

Among options are additions to the dam itself or strengthening and increasing the density of the underlying material, Sanderson said.

Foster Dam, located on the South Santiam River is one of 13 dams and reservoirs operated by the Corps of Engineers in the Willamette River Basin.

Each dam contributes to a water resource management system that provides flood risk management, power generation, irrigation, water quality improvement, fish and wildlife habitat and recreation on the Willamette River and many of its tributaries. Since their completion, the dams have cumulatively prevented more than $20 billion in flood damages to the Willamette Valley.


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