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Oroville Engineers Consider Cavitation Risk

Wednesday, August 16, 2017

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The board of engineers keeping an eye on repairs to the damaged Oroville Dam spillways in California expressed concerns about the effect of cavitation on concrete being used on the job, in two memos made public last week.

Roller compacted concrete at Oroville
Images: Dale Kolke/Department of Water Resources

According to the California Department of Water Resources, the Oroville project will use 800,000 cubic yards of roller compacted concrete.

The Board of Consultants, first convened in early March, has issued 10 memos that have been made public to this point. In the most recent pair of memos, issued July 19 and July 25 respectively, the board questioned whether roller compacted concrete being used on the lower part of the flood control outlet chute will be strong enough to withstand potential water flow this year.

Roller Compacted Concrete Section

The lower chute will be paved with structural concrete, with steel reinforcement, next year by the end of the reconstruction job, but the 2017 portion of the work plan includes covering parts with RCC only. “The final layers of the RCC (layers at the top of the RCC) that could potentially experience flow will need to have higher strength,” the board notes in Memo No. 9.

The memos discuss potential courses of action to strengthen the top layers of the RCC in order to prevent damage, including from cavitation; the board notes that RCC typically exhibits less resistance to cavitation damage than structural concrete.

According to the Portland Cement Association, RCC is a drier mix than traditional concrete, and is generally constructed without joints, and without the use of forms, dowels or rebar.

Oroville Dam August 2017

The goal of the dam repair plan is to have the spillway functional by Nov. 1 in order to handle water releases if necessary during California’s rainy season.

Cavitation is a phenomenon involving water vapor bubbles that can form in high-velocity columns of water. The bubbles implode with great force—enough to damage concrete. Cavitation was blamed for serious damage to Glen Canyon Dam, at the Utah-Arizona border, in 1983.

Memo No. 10 notes that downstream of station 32+00, the roughness of the surface of the chute measures such that “there would be potential for cavitation damage” if the flow is not aerated. The board says in the memo that models for air vents are being considered by the design team, potentially placed at the wall where the chute transitions from structural concrete to RCC.

In Memo 10, the board says that overall, the RCC operation “is off to a good start.”

The Board of Consultants currently consists of John J. Cassidy (who holds a Ph. D. in mechanics and hydraulics), Eric B. Kollgaard (a civil engineer), Faiz Makdisi (whose Ph.D. is in geotechnical and geoenvironmental engineering) and Kerry Cato (Ph.D. in engineering geology).

Project Overview

According to the California Department of Water Resources, the project will use 800,000 cubic yards of RCC, as well as 146,000 cubic yards of structural concrete, and 55,000 feet of drainage pipe.

The goal of the dam repair plan is to have the spillway functional by Nov. 1 in order to handle water releases if necessary during California’s rainy season. More comprehensive, permanent repairs will be finished in 2018.

   

Tagged categories: concrete; Contractors; Engineers; Locks and dams; NA; North America; Quality control; Quality Control

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