For more breaking news get our mobile app for free from the Apple app store or the Google Play store.

Oroville – The independent consultants reviewing plans for the Oroville Dam main spillway reconstructions expressed some concerns in two recent memos about construction of the center part of the project.

That part of the spillway will get a temporary fix this year with roller compacted concrete (RCC), which isn’t as strong at the structural concrete that will ultimately surface the faces of the spillway chute.

The Board of Consultants’ ninth and 10th memos were posted on the Department of Water Resources’ website Wednesday, and both raised questions about the RCC.

Kiewit, the contractor handling the reconstruction has removed 2,270 feet of the damaged spillway. The top 870 feet and the bottom 350 are currently being filled with leveling concrete, with a layer of structural concrete going on top.

The 1,050 feet between will be filled with RCC this year. Next year it will be shaved back to allow structural concrete to be put on top.

RCC CONCERNS

The memos suggest some concerns that the RCC could suffer erosion.

For example the 10th memo mentions a cutoff wall anchored to bedrock at the place where the structural concrete at the top of the chute transitions to RCC. A cutoff wall is what is being built on the emergency spillway hillside to prevent erosion from cutting uphill toward the critical weir.

An aeration system is also now being recommended for the structural concrete-RCC transition, where there will be a 3-foot vertical drop.

There is also discussion in both memos of a surface treatment for the RCC that is supposed to increase its surface strength. In the 10th memo there are concerns the smoother surface might result in faster flows that could “increase the potential for cavitation.”

Cavitation is erosion that occurs when tiny air bubbles are slammed into a concrete surface by swiftly flowing water.

Reading this on your iPhone or iPad? Check out our new Apple News app channel here and click the + at the top of the page to save to your Apple News favorites.

Both memos indicate the top layer of the RCC will have a higher cement content for added strength.

The ninth memo also has concerns about how the walls of that part of the chute will be built, as the usual way of delivering RCC “is not practical” because of steep drop-offs. Those concerns appear to have been addressed by the 10th memo, however.

The RCC walls will be replaced or resurfaced with structural concrete in 2018.

CURRENT CONDITIONS

During a media conference call Wednesday, DWR and Kiewit officials said the work was still on schedule for the spillway to be usable on Nov. 1.

DWR Director of Public relations Erin Mellon stressed the spillway has only been used in 26 of 49 years, and it’s only been used four times before Jan. 1.

“It’s an aggressive construction schedule but a conservative weather schedule,” she said.

The first slab of structural concrete was poured last week in the bottom section of the main spillway, the media was told. Pouring of leveling concrete was continuing there and in the top section.

Forms for the walls were also under construction.

In the center, about 25,000 cubic yards of RCC have been poured, about a 10th of what will ultimately be used.

On the emergency spillway, a heavy-duty percussion rig has been brought in to drill the holes for the cutoff wall. The wall should be completed in December or early 2018.

Reach City Editor Steve Schoonover at 896-7750.