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Feds Question Cracks in New Oroville Spillway

Thursday, November 30, 2017

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California officials are downplaying concerns about cracks in the newly constructed concrete spillway at Oroville Dam, despite questions from federal regulators in a recently disclosed memo from October.

Oroville spillway
Photos: Ken James / California DWR

Workers are still putting the finishing touches on the rebuilt sections of the Oroville Dam spillway, but newly disclosed letters show that federal officials are concerned about cracking in the new concrete.

KQED News reported Monday (Nov. 27) that a letter dated Oct. 2, sent by an engineer with the Federal Energy Regulatory Commission, questioned the presence of “a number of small cracks in the upper surface of the concrete slabs” of the newly reconstructed Oroville spillway.

Cracks Questioned

The letter was addressed to the state’s Department of Water Resources, which oversees the dam. In it, FERC engineer Frank L. Blackett wrote, “We understand that DWR has at least initially expressed some concern about these cracks and has taken a number of actions to adjust the concrete mix design and contact some concrete material experts to identify possible actions.”

Blackett asked that the DWR work to determine the cause of the cracking and establish a plan to address it.

Oroville dam workers

DWR officials say the cracking was "anticipated" and is "not expected to affect the integrity of the slabs."

In a reply to FERC dated Nov. 7, KQED reports, the DWR said that the cracking was “a result of some of the design elements included to restrain the slabs and produce a robust and durable structure.” Elements including anchoring the slabs to the foundation, putting the slabs on a layer of leveling concrete, and interlocking the slabs. The cracks were “anticipated” and “not expected to affect the integrity of the slabs.”

A DWR spokesperson told the Sacramento Bee that the integrity of the new spillway “is not in question.”

Phase One Complete

The DWR announced on Nov. 1 that first-year operations as part of the two-year repair project had wrapped up on deadline ahead of the state’s rainy season. Kiewit Infrastructure, the general contractor on the reconstruction job, rebuilt 2,270 feet of the 3,000-foot main spillway during the first year.

The second year will involve the reconstruction of the remainder of the main spillway and work on the auxiliary spillway, which had not been paved previously.

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.

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.

   

Tagged categories: concrete; Infrastructure; Locks and dams; Rehabilitation/Repair

Comment from peter gibson, (11/30/2017, 11:53 AM)

Can't get a concrete mix right in this day and age.


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