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Oroville Dam First-Year Repairs Complete

Friday, November 3, 2017

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California officials announced Wednesday (Nov. 1) that Phase 1 of the two-part repair to the Oroville Dam spillways has been completed on time, and the spillway is ready to handle rainy-season overflows if necessary.

The main spillway has been repaired to the extent planned for this year, the Department of Water Resources says, and is prepared to handle flows of up to 100,000 cubic feet per second. Kiewit Infrastructure, the contractor hired for the Oroville job, rebuilt more than 2,270 feet of the 3,000-foot-long spillway, according to the DWR.

Oroville Dam spillway
Photos: Kelly M. Grow, Department of Water Resources

The main spillway has been repaired to the extent planned for this year, the Department of Water Resources says, and is prepared to handle flows of up to 100,000 cubic feet per second.

More work on the main spillway and the auxiliary spillway—which had previously not been paved—remains as part of the second phase of the work, scheduled for 2018. When the entire rebuild is complete, the spillway will be able to handle a flow of 270,000 cubic feet per second, the same flow it was originally designed to handle when it was built nearly 50 years ago.

Dam Problems

About 188,000 residents of the Feather River valley were evacuated in February after a series of failures raised the risk of catastrophic flooding. The dam’s main spillway was initially taken out of service due to a massive patch of eroding concrete, and in the midst of heavy rainfall, the DWR began to direct water releases onto the never-before-used auxiliary spillway.

Auxiliary spillway

The construction of a secant wall downhill of the auxiliary spillway is 50 percent complete.

That spillway, though, began to erode badly, as it was never paved over with concrete. Erosion of the earth at the top of the spillway threatened to cause a failure of the concrete weir, which could have sent uncontrolled water into the Feather River.

The DWR averted disaster by returning the main spillway to limited service, and when the rainy season ended in the spring, the massive rehab effort began. In June, the agency noted that the hot, dry spring had sped up the efforts to replace the main spillway and build a proper emergency spillway; by July, the contractor received permission to speed up the job in order to meet the Phase 1 deadline of Nov. 1.

Thicker Slab, More Anchors

Phase 1 of the rehab involved rebuilding two sections of the spillway with structural concrete, and replacing another section with roller-compacted concrete, while a 730-foot section at the top of the spillway was simply repaired. The top section will be replaced completely during Phase 2, and the section paved with roller-compacted concrete will be covered with structural concrete.

In addition to repairing parts of the spillway that eroded, the repairs involve adding to the thickness of the spillway chute, and increasing the number of slab anchors. While the original construction of the spillway in 1968 called for 5,700 slab anchors approximately 5 feet deep, the rehabilitation of the spillway will in the end involve about 7,000 epoxy-coated anchors, at 15 to 25 feet in depth.

Spillway diagram
Department of Water Resources

The new spillway will have about 7,000 epoxy-coated anchors, about 15 to 25 feet deep.

Engineers who have examined the spillway since the incident have concluded that a number of issues played into the original concrete damage, including slabs that weren’t thick enough, insufficient steel reinforcement and ineffective ground anchors.

The new chute will be 7 feet, 6 inches thick on average—nearly 5 feet thicker than the original chute—and the new construction will also involve wider underdrains made of PVC rather than clay. The new slab has two layers of 1-inch epoxy-coated steel rebar, where the original slab had only one layer of 5/8-inch rebar.

Crews are constructing a secant pile wall downhill of the emergency spillway; the DWR says the wall is 50 percent complete now and will be finished in January.

   

Tagged categories: concrete; Contractors; Infrastructure; Locks and dams; NA; North America; Program/Project Management; Rebar

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