Cracks Detected in New Oroville Dam Spillway

November 29, 2017
Ironworker ties rebar together for the transitional slab on the lower chute of a Lake Oroville flood control spillway

California's Department of Water Resources (DWR) hired Kiewit Corp., to rebuild and reinforce the Oroville Dam spillway after last February's close call when 188,000 residents were evacuated when the spillway failed and the dam itself was looking like it too would fail.

Above: Ironworker ties rebar together for the transitional slab on the lower chute of a Lake Oroville flood control spillway

Although Kiewit successfully brought in the first phase of the spillway project by November 1, on time for California's rainy season. However, tiny hairline cracks were detected in October when the curing coverings placed over the erosion resistant concrete slabs were removed.

In a letter to Ted Craddock, project manager for the DWR's spillway project, the Federal Energy Regulatory Commission (FERC) requested DWR to investigate the cracks and provide remedies.

On November 7, Craddock sent a reply to FERC with this explanation:

"The Department of Water Resources (DWR) has performed an assessment of the concrete slabs and concludes the hairline cracks are a result of some of the design elements included to restrain the slabs and produce a robust and durable structure. These design elements include: anchoring the slabs to the foundation; placing the slabs on a layer of leveling concrete; and interlocking the slabs with keyways and a continuous bottom layer of reinforcing steel. With the inclusion of these design elements, the presence of hairline cracks was anticipated and is not expected to affect the integrity of the slabs."

DWR spokesperson Erin Mellon said Monday that hairline cracks are “something you expect to see” in concrete slabs as massive as the those in the rebuilt spillway. “These cracks are not abnormal, nor do they cause a concern,” she said.

DWR will continue to monitor the affected concrete and is working with contractors to refine the concrete mixture to minimize cracking.

But not everyone agrees with the DWR's assessment that the cracks just standard operating procedure.

KQED News reports that a technical memorandum accompanying Craddock's response to FERC was classified and couldn't be viewed by the press or public without signing a nondisclosure agreement.

In light of the engineering problems discovered when the Oroville Spillway failed in February, KQED's editor, Dan Grekke, questioned the DWR conclusions and spoke with a civil engineering expert. Read what Grekke was told in his article Feds Ask State to Explain Cracks in New Oroville Spillway Concrete

image: Ken James/ California Department of Water Resources