Repair of Oroville dam’s main spillway in 2009 – 7,000 square feet

Damaged Dam Safety
Part of DWR’s 7,000 square foot spillway repair    October 7, 2009

Kevin Dossey, the Senior Civil Engineer for the Department of Water Resources, Oroville Field Division, indicated in a press conference on Friday, February 10, that Oroville’s main spillway has never had a major repair.

But a May 2009 letter from DWR Deputy Director Raphael Torres to the Federal Energy Regulatory Commission (FERC) shows that DWR needed to repair 7,000 square feet of the main spillway, including large cracks on the spillway surface. Torres stated:

In April 2008, personnel from DWR’s Division of Operations and Maintenance inspected Oroville Dam’s spillway invert and walls for concrete deficiencies. While the overall condition of the spillway is good, the detailed visual and sounding inspection found areas of concrete spalling and delamination. DWR’s Division of Engineering is preparing a construction contract for maintenance repairs that will cover approximately 7,000 square feet of the spillway. The scope of the maintenance repair may include cutting, removing and replacing damaged concrete, sand blasting exposed steel rebar, injecting non-shrink grout into voids, repairing or replacing joint seals as necessary, and placing epoxy adhesive to repair large cracks on the spillway surface. To meet the California Environmental Quality Act requirements a Notice of Exemption will be filed. We anticipate work will begin in August, and should be complete by November 1, 2009. (underlining added.)

According to the U.S. Bureau of Reclamation’s Guide to Concrete Repair, epoxy resins are brittle and don’t withstand movement:

It is often very difficult to attain successful repair of cracks … It is better to leave most types of concrete cracking unrepaired than to perform inadequate or improper repairs (figures 71 and 72). The selection of methods for repairing cracked concrete depends on the cause of the cracking. If improper methods are used, cracks will usually reoccur in the repair material or adjacent concrete, and it may make the initial damage worse.

For some cracks, epoxy or polyurethane resin injection (section 38) can be used…. Epoxy resin injection can sometimes be used to seal low volume water leakage, while structurally rebonding cracked concrete members. Epoxy resins cure to form hard, brittle materials that will not withstand movement of the injected cracks.

It is currently unknown if more recent repairs to the main spillway also used epoxy resins for repair of cracks.   If the crack repairs failed during high flows, it could have caused the large hole in the spillway.   According to a U.S. Bureau of Reclamation document on Dam Safety, “Cavitation Induced Damage of Spillways,” opening of cracks or joints in concrete spillways can exacerbate cavitation, potentially causing spillway failure.

Paul Rogers and Matthias Gafney also reported in the Mercury News that cavitation was a possible failure mode in  Oroville Dam: What Made the Spillway Collapse?    The article quoted  Paul Tullis, a professor emeritus of civil engineering at Utah State University, as saying,  “It’s like a big grinder. It causes concrete to be torn apart.”

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