Oroville Dam: Formerly classified memo describing spillway cracks now public
AGENDA 21 RADIO
Oroville – The previously secret state Department of Water Resources memorandumexplaining the hairline cracks in the Oroville Dam spillway is now public.
The document provides more details on how Kiewit Infrastructure West Co., the contractor for spillway reconstruction, tried to reduce shrinkage, which leads to cracking in concrete. It also contains photos documenting the cracks. After initially being classified as Critical Energy Infrastructure Information, only one attachment in the 15-page report was redacted.
Officials with the department maintain cracks were anticipated because the spillway is highly restrained and do not pose a threat to its structural integrity or require a fix at this time. DWR plans to continue monitoring the small gaps throughout the reconstruction process.
“The features that produce the high level of restraint also produce a highly robust and durable spillway structure,” the memo reads. “By design, location, spacing and amount of reinforcement was intended to distribute cracking and keep it at a hairline level.”
The just-published memo dated Nov. 2 was written by Dale Brown, DWR’s deputy project manager, and addressed to Ted Craddock, the department’s project manager. Craddock then sent the letter to the Federal Energy Regulatory Commission, which agreed with DWR’s analysis that the spillway was safe and no immediate fixes were warranted.
The hairline cracks were noticed through inspections in August over the 14-day period the concrete was curing, according to the memorandum. After some were discovered, DWR directed the contractor to lower the amount of cement in the concrete mix from 800 pounds per cubic yard to 660 pounds per cubic yard, but cracks continued to appear.
Brown wrote that most of the cracking was likely caused by the vast amount of restraints, including the underlying leveling concrete and slab anchors. That theory was supported by mapping which showed most of the cracks between anchors, he said.
Thermal shrinkage could also have been a contributor, though not likely the main reason because the difference in temperature between the top and interior of concrete panels was negligible, according to the report.
Cracking could also have been a result of plastic shrinkage, “usually associated with the rapid loss of moisture caused by a combination of factors that include high air and concrete temperatures, low relative humidity, and high wind velocity at the surface of the concrete.”
In that case, what begins as shallow cracks can turn into deep cracks later, according to the American Concrete Institute, which Brown cited in the report. At the end of the memo, it states that the department would expand its investigation into the cracks and look into modifying the curing process or making changes to the concrete mix, such as using shrinkage-reducing admixtures, which Assistant Professor Feraidon Ataie, director of Chico State University’s concrete industry management program, suggested in a previous interview with this newspaper.
Professor Robert Bea with U.C. Berkeley’s Center for Catastrophic Risk management said on Monday he found two of the pictures, figures one and three in the report which show thin and long breaks in the concrete, significant and “very scary.”
“I hope no one will believe that this type of cracking — not the crazing at the surface of the concrete — is ‘to be expected,’” Bea wrote in an email. “I have helped design, construct, operate, and maintain some very large reinforced concrete structures. These structures had many more ‘restraints’ than the re-constructed spillway. Significant cracking was not tolerated. If found, it was immediately ‘fixed’ and then monitored to be sure that the ‘fix’ had been effective.”
A DWR spokesperson could not be reached for comment before the deadline for this story.
