Water & Drought

Oroville Dam spillway built on crumbling rock, warned contractor that built it

An investigation into last winter’s near catastrophe at Oroville Dam uncovered a litany of problems with how the dam was built and maintained, but one of them stands out: Even as workers built the dam, they were raising alarms about the eroded, crumbling rock on which they were directed to lay concrete for the 3,000-foot-long main flood control spillway.

Construction reports from the fall of 1966 showed an abundance of loose clay, “shot rock” and “very little solid rock.” The surface was so crumbly, according to a state engineer overseeing the work, that a laborer at one point refused to do any more prep work until he got clearance from his boss. The contractor told the California Department of Water Resources it needed to dig deeper to find stronger rock.

But DWR limited the additional excavation work proposed by the contractor, a decision that investigators now say might have been motivated by money.

Nearly four decades later, in 2005, DWR officials took a similar stance when a coalition of environmental groups urged it to reinforce the weathered earthen hillside below another crucial component of the dam: its emergency spillway. This spillway – never tested – was intended to allow water to escape if the main spillway couldn’t release water quickly enough in a megastorm.

In 2005, DWR dismissed out of hand the notion that the emergency spillway needed strengthening. Critics said that decision also was driven by costs.

The warnings proved prophetic. Last Feb. 7, the main spillway fractured in two – the result of being built atop a foundation riddled with “erodible materials,” a team of forensic investigators has concluded. Five days later, with the reservoir brimming, water poured over the emergency spillway for the first time ever. The unlined hillside quickly began washing away, triggering flood fears and an emergency evacuation of 188,000 downstream residents.

In addition to the erosion issues, the forensic team hired by DWR to investigate the Oroville emergency found that the main spillway was designed by an inexperienced engineer, plagued by a woefully inadequate drainage system and poorly maintained in the years that followed its 1968 completion.

The team also revealed that DWR had studies in its files, from well before the dam was built, that showed both spillways rested atop a foundation of wildly varying quality, some of it dangerously weak.

“That sort of set the physical stage for what happened 50 years later,” forensic team leader John France said in an interview.

The investigators concluded the eroded rock and a poor drainage system below the main spillway allowed water to gather beneath its concrete chute. The voids made the spillway vulnerable to “uplift forces” that eventually caused it to crack open.

“The decision to leave the weathered material in place underneath the chute (was) a big decision, a significant decision,” France said. “It is certainly a big part of why we said the causes of this really go back to the original design and construction.”

DWR officials have said they’ve now addressed the underlying problem. As the general contractor has repaired the fracture to the main spillway, it’s scoured deep into portions of the foundation to find stronger rock. That’s been costly: Excavating and then filling in the foundation has helped drive the repair cost to around $500 million, nearly double the original estimate.

The forensic team’s findings are seen as crucial to DWR’s efforts to reinvent itself under its new director, Karla Nemeth, following the Oroville crisis. Stung by the investigators’ report, the department has pledged to revamp its inspection methods by incorporating blueprints and construction records to get a better handle on the dangers lurking at the 1,249 dams it oversees.

In Oroville’s case, there was no shortage of archival material. In its 584-page report released Jan. 5, the forensic team cited a series of geological surveys revealing spotty conditions where the spillways were built. A study conducted in 1948, when the site was being scouted for dam construction, showed multiple instances of weathering to the amphibolite, greenstone and ophiolite rock. Three follow-up reports conducted between 1962 and 1965 also spoke to the uneven foundation quality.

Once construction got started, daily reports by the general contractor and DWR officials provided further evidence of flaws in the foundation. In one report Oct. 31, 1966, a DWR engineer noted that one spot consisted of “loose clay ... on the surface, with very little good rock.”

By 1967, the contractor building the main spillway was telling DWR that it needed to excavate more of the hillside to find solid ground. The company, a joint venture between Oro Pacific Constructors and George Farnsworth Construction Corp., said it would dig deeper into the bedrock and “backfill” the excavated sites to create a stronger foundation.

The daily reports “support the contractor’s claim of poor foundation conditions,” the forensic team said.

But DWR wasn’t eager to allow more excavation, and a dispute arose between the agency and the contractor.

Money may have been a factor. The additional work would have added $1.2 million to the spillway’s cost, according to the forensic report. That’s about $9 million in today’s dollars, inflating the cost of the spillway by nearly 10 percent.

DWR eventually agreed to the idea, but only partially, allowing some of the weathered rock to stay in place, the forensic team said. It appeared that DWR was trying “to prevent significant cost overruns,” according to the forensic report.

France, in the interview with The Sacramento Bee, said there was no definitive proof that cost played a role in DWR’s decision; the investigators weren’t able to find full documentation of the dialogue between DWR and the contractor. He added that time constraints may have factored into DWR’s deliberations as well.

“The more you have to excavate and replace (the rock), that takes time and it delays construction,” France said.

As it was, DWR was wrestling with a host of other problems at Oroville, which was the centerpiece of the State Water Project – Gov. Pat Brown’s almost messianic vision for delivering water as far south as San Diego. The dam’s construction was plagued with labor strikes, fatal accidents and other woes.

However, there is no indication in the forensic report that pressure from the Governor’s Office influenced DWR’s decision on the spillway foundation. By 1967, when the dispute with the contractor came to a head, Ronald Reagan was governor.

In any event, Ron Stork of the Sacramento environmental group Friends of the River, said it’s troubling that DWR stuck to its beliefs that the bedrock was basically sound, when the agency’s reports clearly showed it wasn’t.

“There wasn’t due diligence to dig into the underlying basis of the claims and beliefs in the geology that DWR was developing,” said Stork, whose group has tangled with state officials for years over Oroville safety issues. “That’s bad science, and bad science has consequences.”

Completion of the dam and its spillways in 1968 appeared to reinforce the mindset within DWR that all was well. As the years passed, the warnings from the early 1960s seemed to fade from DWR’s institutional memory, according to the forensic team.

“Actual bedrock conditions, and the implications of those conditions, were well documented prior to and during construction, but there was no post-construction recognition of the weathering potential of the rock types present at Oroville,” the investigators wrote. “Detailed and accurate information was not properly accessed in subsequent years; rather, inaccurate and incomplete summaries of information were passed on through generations of DWR personnel.”

This habit manifested itself in a big way in 2005, when Oroville Dam’s operating license was up for renewal at the Federal Energy Regulatory Commission. Friends of the River and two other groups petitioned FERC. Their argument: The emergency spillway – a concrete lip perched on an unlined hillside – was a potential danger zone. They said the hillside should be reinforced with concrete to prevent it from washing away.

FERC asked DWR for a response to the coalition’s demand on Nov. 29, 2005. It took DWR less than a day to respond. DWR’s geology branch concluded the hillside was “composed of solid, essentially non-erodible bedrock with an insignificant layer of top soil that will erode,” according to the forensic report.

The forensic team said DWR’s reply was a rush job, based on a cursory look at the 1960s geological studies. “It is evident that very little actual research, if any, was conducted,” the investigators wrote. “This was merely an exercise in document gathering.”

For its part, the federal government didn’t appear to be interested in scrutinizing DWR’s findings too closely, either. DWR’s response was “obviously not critically reviewed by FERC,” the forensic team wrote.

Along similar lines, the forensic team found that precious little scientific evaluation went into a lengthy memo to FERC, opposing the environmentalists’ demands, by the Metropolitan Water District of Southern California and other so-called State Water Contractors. These water agencies, which would have to pay for lining the hillside with concrete, said in the memo that there was no evidence that erosion would be a major problem.

According to the forensic team, the water agencies didn’t perform any research; they simply assumed that the dam and its spillways had been designed safely in the first place. “There was no actual technical questioning by the (State Water Contractors) of the geologic conditions,” the forensic team wrote.

Environmentalists said the resistance to reinforcing the hillside was driven by cost considerations – a notion that has been flatly denied by officials with the Metropolitan Water District.

Ultimately, the federal agency rejected the demand to have the hillside lined with concrete – an oversight that is now being partially corrected. As part of the estimated $500 million repair work at Oroville, millions of dollars are being spent installing concrete slabs that will extend several hundred feet down the hill. State officials hope the bulk of the repair costs will be reimbursed by the U.S. government, with the rest covered by the State Water Contractors.

Dale Kasler: 916-321-1066, @dakasler

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