Sacramento >> In its eighth memorandum released Thursday, the independent board analyzing the redesign of the Oroville Dam spillways commends the construction contractor’s work and makes slight tweaks to former recommendations.

This is the second memorandum issued without any redactions which the state Department of Water Resources previously requested, citing national security concerns.

The board suggested aerators may be necessary in its last memo, in order to prevent cavitation — a process where bubbles can damage a concrete surface. But after further investigation, including calculating the spillway’s cavitation index for a variety of flows, the team changed its mind. The team also noted it could be difficult to complete construction by the deadline, if the contractor, Kiewit, had this added to its plate.

However, the board recommended the state Department of Water Resources research similar spillways without aeration features. It also suggested testing by use of a physical model, if possible, to confirm the board’s analysis.

“If a decision is made to add aerators in the FCO (flood control outlet) Spillway, they should be thoroughly evaluated and tested to make sure that they do not adversely impact the performance of the spillway,” the memo reads.

The board said it was pleased on a site visit June 21 with Kiewit’s demonstration of placing roller-compacted concrete, planned to go into the largest holes of the spillway. All members were in attendance except for Faiz Makdisi who was out of the country, according to the document.

“The BOC observed the RCC (roller-compacted concrete) placement and found the RCC mix to be workable with no segregation, easily compacted, and can be placed within the temperature restrictions, despite high ambient temperatures exceeding 100 degrees Fahrenheit,” the memo reads.

As roller-compacted concrete is going into small, deep crevices, the board noted it may be difficult to fill with the material, so the team suggested using a cement mix called flowable fill or dental concrete for those hard-to-reach places. Cleaning and preparation of the rock foundation of the spillway has been done much better this time around, compared to when the dam was originally constructed, the board said.

“The BOC is favorably impressed with the Contractor’s general approach, use of the job site and the mobilized equipment,” the memo reads.