CA Department of of Water Resources says: “don’t worry”, it’s “robust”.
Readers may recall that in February 2017, the spillway at Oroville dam eroded and collapsed due to the massive amount of water from El Nino weather pattern induced storms eroding concrete that had cracks in it that had not been properly maintained. The state DWR agency spent an estimated $500 million to rush a new spillway replacement into service by November 1st. Now it appears this rush job has cracks in the steel reinforced concrete, repeating the same start of the scenario that caused the collapse in the first place.
From ChicoER: Small cracks have appeared in the new concrete in the main spillway at Oroville Dam, but the state says they were expected and nothing to worry about.
An explanation of the “hairline cracks” was called for in a letter the Federal Energy Regulatory Commission wrote to the Department of Water Resources in October. KQED radio in San Francisco first reported the previously undisclosed letter Tuesday.
The Oct. 2 letter from FERC said it understood DWR was concerned when the cracks were discovered after concrete curing covers were removed from some of the slabs poured this year.
It called for a report within 30 days that mapped the cracks, identified potential causes and spelled out remedies, “if necessary.”
The state has been rebuilding the spillway after most of it eroded away in February.
KQED also reported Tuesday on the cover letter to the DWR report submitted Nov. 7 in response to the FERC letter. The actual report was not available.
In the cover letter, DWR said it had evaluated the cracks and determined they were caused by efforts to create “a robust and durable structure.”
…
“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 said.
Yeah, sure. Full story here.
From KQED:
UC Berkeley civil engineering professor Robert Bea, a veteran analyst of structure failures, said cracking in high-strength reinforced concrete structures is never expected.
“Cracking in high-strength reinforced concrete structures is never ‘to be expected,’ ” Bea said in an email. Even small cracks could increase stresses in the concrete when it is under “service loading” — for instance, when large volumes of water hurtle down the structure at speeds approaching 90 mph.
The cracking “develops paths for water to reach the steel elements embedded in the concrete and accelerate corrosion,” Bea wrote in an email. “Such corrosion was responsible for the degradation and ultimate failure of the steel reinforcing in parts of the original gated spillway.”
DWR inspection and repair records going back to the 1970s documented widespread cracking of the 3,000-foot-long spillway chute — largely because of the thinness of the concrete covering the drainage system below the concrete slab. On at least two occasions, sections of the steel rebar inside the slab were found to be corroded and in need of replacement. After the spillway failed last February, some of the steel rods intended to anchor the slab to underlying rock were also found to be corroded.
more at KQED here at their website
And here is the audio report:
Documents:
https://assets.documentcloud.org/documents/4311073/FERC-DWR-Oroville-Concrete.pdf
https://www.documentcloud.org/documents/4311072-DWR-FERC-Concrete171107.html
When I moved to California, I noticed that they just add a layer of black paint (and white stripes) to asphalt parking lots instead of repaving them. It covers up the ugly dirt that accumulates for lack of rain and those ‘minor’ cracks. Some places seem to need a dose every year.
Maybe the magic paint comes in white for concrete?
Professor Bea may be out of his Emeritus, but the State of California denied for decades that there was a problem. They deserve the skepticism that they are receiving.
Cure cracks, The more water used the bigger they are.
As a structural engineer, I find the professor’s comments very odd. Concrete has to crack in order for the reinforcing bars to provide resistance. (no strain, no force) Ideally, these cracks are very fine and not noticeable. In a large slab, cracks will occur due to the concrete shrinking during curing. This volume change effect is very difficult to eliminate (there are special additives and post-tension techniques, but these are not cure-alls), so we provide joints to provide strain relief. Essentially, joints are straight cracks that we caulk.
The reinforcing in the pictures is epoxy-coated – the same technique is used in concrete bridges. There can be issues with the coating chipping – but if installed properly, the rebar will last for a very long time. In the spillway condition, I would expect the bars to essentially last forever – because in order to corrode you need water and an electrolyte. In bridges, the electrolyte is the deicing salt. I would not imagine they would deice the spillway very often. Concrete has corrosion inhibition properties until the electrolyte (typically chloride ion) loading reaches a critical point.
One issue with cracks in concrete is water infiltration that freezes – freeze-thaw. As ice expands, it can spall the surface of the concrete over many cycles. Modern mix designs incorporate air entertainment of closed cell bubbles that allow a bit of “spring” inside the concrete – this allows the concrete to accept the strain from the ice without as much damage. Combined with higher strength mix designs, these concretes are very durable. I do not have the specifics on the concrete mix they are using but this standard practice for exposed concrete.
Yes, you can provide a coating to waterproof the surface. But you will have to maintain it; most coatings need replacement in under 10 years due to UV degradation. However, by using epoxy rebar, modern concrete mix designs for durability and minimizing cracks, that should not be necessary for long term durability.
[Although less well known than entrainment, air entertainment is much springier… -mod]
UC Berkely Professor Robert Brea earned his Ph.D. is from the Center for Oil & Gas Engineering at the University of Western Australia. The UC Berkely website states his 48 years of expertise is in “engineering and management of design, construction, maintenance, operation, and decommissioning marine systems including offshore platforms, pipelines, and floating facilities.”
This guy is not an expert on concrete structures…
Did you mean Bea instead of Brea or is there a Brea at Berkely?
You are correct…
Maybe they could’ve had Mark Jacobson put in ten times more turbines.
“Unbelievable”
No, it’s not. It’s the Dem stronghold of Caliphornia.
It seems that “robust” is a very popular word these days.
As an principal engineer I can tell you small cracks on high strength concrete is not uncommon – many standards even allow for it! That professor probably hasnt ever worked in the real world. Hair line cracks typically self heal and are not an issue. Concrete shrinks as it cures so small cracks can form especially when working on a exposed site during hot fine weather. However its hard to comment in detail not being familiar with the concrete used specifically. For a spillway you would expect something very durable due to flow velocity and abrasion i.e. super plasticised wire reinforced at a.strength greater than 70mpa. A bigger issue is a tomo forming underneath causing the slab to span a void it wasnt designed to span as happened on the last one!
Despite extolling the design and construction of the repairs, I have to say the slagging of Dr. Bea is unwarranted. He is a very serious guy with very serious credentials. His particular specialty is failure analysis, and he’s one of the top guys in the US in that area, and has been hired to analyze many engineering disaster events. He runs a program at U.C. Berkeley on this topic. His reports on the Oroville Dam are thorough and well-reasoned and well-documented. He might be wrong about some things, but one thing he is not is incompetent.
Second, the issue of the cracks wasn’t raised by a bunch of cranks or trolls. It was the Federal Energy Regulatory Commission, also very serious people.
This structure was cast in hundreds of pours..for the cast portion. Thousands of lifts for roller compacted concrete. Yes, there WILL be curing cracks as I didn’t see any post tension process where steel reinforcement is tensioned after initial cure to put the concrete into compressed state. Post tension is the only way to prevent cure cracking in a structure of this size.
Now, will cure cracks doom this spillway? Not in the areas replaced to bedrock. In other areas ? Aerators and turbulators can be added if cracks will experience cavitation in areas without prep to bedrock.
The original spillway survived many prior uses of equal magnitude. It was lack of PM which led to failure. Most likely, plugged drains which allowed water pressure to lift slabs after reinforcement failure from corrosion.
we know concrete cracks, but as long as they are not builders cracks I think they will be fine.
Like others, I’ve watched this saga closely on a daily basis since the original spillway collapsed. Before people light their hair on fire, let’s wait for the final report. If the cracks are merely due to shrinkage, it’s very different than if the cracks were caused by settling. I will be very surprised if the cracks turn out to be problematic. The structural concrete is laid on a very well-prepared layer of leveling concrete, which is laid on prepared rock. The structural layer is thick and has extensive rebar. This is very different from the original concrete, which was thin, had insufficient rebar and was laid on badly weathered rock.
Correct me if wrong, but I thought the original spillway was undermined.
$500 million for a 900 m spillway ? seriously? 1/2 f***ing million per meter ? for a thing that should be used once per century, or so ?
paqyfelyc
The alternative is loss of the dam during any subsequent period of high snowfall and rains, and complete loss of every town and city down river from the dam between the mountains and the north San Francisco Bay and estuary region. Tens of billions of dollars in damages and tens of thousands of lives. Worse, the loss one year could be repeated each subsequent year. Loss of power, loss of the irrigation and drinking water storage. Loss of the farmland.
The Original spillway was poorly constructed and failed. The high water and high snowfall that over-flowed the spillway, causing the first small cracks and erosion damage that caused the catastrophic/unstoppable damage last spring threaten the dam integrity and utility itself.
Permit me to doubt that 1/2 billion dollar are required for this job done. And, BTW, a perfectly flat surface is just plainly stupid if you want to stand a huge water flow. 1000 m3/s falling 10 m are mgh/s = 98 10^6 W, that’s why water can destroy piers and cliffs. It should be irregular and cascading, to use air and vortex to slow water, turn higher energy water into mist, and introduce bubbles, so as prevent energy to build up in the flow into destructive waves. Piers are done of irregular blocks for a reason. Actually two reasons. 1) it is cheap, much more so than a wall. And 2) the very same reason natural rapids are so irregular: because a flat surface is destroyed and broken apart, where an irregular stands.
As Bob Kelly once told me, “There are two kinds of concrete. Concrete that is cracked and concrete that is gonna crack.”
For the best reporter on Oroville:
Juan Brown —