Looking at the many photos online of the Oroville Spillway collpase that has been in the news, there’s one major component of concrete that should be there, but is blatantly absent:
REBAR
Rebar (short for reinforcing bar), collectively known as reinforcing steel and reinforcement steel, is a steel bar or mesh of steel wires used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and hold the concrete in tension. Rebar’s surface is often patterned to form a better bond with the concrete. Source
If there was REBAR in the spillway concrete, you’d see a mesh lattice of it left behind in the hole, or at least a few sticking out at odd angles. here are several photos, I don’t see any REBAR, do you?
ADDED: Here is a closeup view from our local newspaper, clearly no REBAR visible:
(Upon magnification of this image it appears there is some in the debris, so the question is now, was it enough, or was the problem due to some other factor)
Source: http://media.chicoer.com/2017/02/08/photos-oroville-dam-spillway-dwr/#7 (h/t to commenter TonyL)
If REBAR was present, we likely would not see such a dramatic collapse as it would have prevented water pressure cavitation from eroding more and more concrete. Concrete is a material that is very strong in compression, but relatively weak in tension. To compensate for this imbalance in concrete’s behavior, rebar is cast into it to carry the tensile loads. This means concrete pulls apart much easier than it is crushable, but with REBAR the tensile force required to pull the concrete apart is greatly increased.
One wonders if that lack of REBAR on the spillway was by design, accident, negligence, or some cost-cutting measure like the lack of life-boats and cheap steel on the Titanic. REBAR in concrete was invented in 1849. It seems incredible to me that it seems to be missing from this very important structure.
From AP:
OROVILLE, Calif. (AP) — State engineers on Thursday discovered new damage to the Oroville Dam spillway in Northern California, the tallest in the United States.
Earlier this week, chunks of concrete went flying off the emergency spillway, creating a 200-foot-long, 30-foot-deep hole.
Department of Water Resources spokesman Doug Carlson said officials will ramp up the outflow from the damaged site Thursday so officials can drain Lake Oroville.
Meanwhile, reservoir levels continued to climb behind the critical flood-control structure. Officials said it is at 90 percent of its capacity.
They said the dam is still safe and doesn’t threaten communities downstream.
“The integrity of the dam is not jeopardized in any way because the problem is with the spillway and not the dam,” said Eric See, a spokesman for the Department of Water Resources.
As a contingency, state officials are preparing to use the emergency spillway at the dam.
Crews have been clearing trees, rocks, and other debris from the hillside near the dam where water will flow.
Lake Oroville would naturally flow over this ungated concrete crest, into a mostly unlined emergency spillway if the reservoir reaches 901 feet elevation. This would be the first time the spillway has been used in the dam’s 48-year history although the reservoir came within 1 foot of flowing over in January 1997.
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Here is a good close-up.
http://media.chicoer.com/2017/02/08/photos-oroville-dam-spillway-dwr/#7
The pic will not embed so click through.
There are a lot of steel rods visible, but I can not tell if they are a proper reinforcing mesh. Others here can comment better than I.
One thing that did strike me is the great difference in thickness between slabs in the lower right, and those two slabs in the center.
Much better!
There’s rebar there. Now whether it is correct and enough who knows. Thanks for the pic.
OK, now I can see the rebar!
Tony L, pretty amazing pictures in the gallery. Pictures show the failure in progress; water got in underneath the slab and washed out the fill rather than primarily cavitation breaking up the concrete.
TonyL February 9, 2017 at 6:07 pm
Good evening Tony and all.
Thanks for the photo group. First I am not a engineer. Now to those who are look at things other then the hole. First in #10 look at the over flow entries on ether side of the barrier wall note the water flow. Of course at the failed area their is none. Now look at 11#, see the retaining wall just above the failed area at 90-70 degrees from the spillway? If the spillway was never used did the side wall run-off holes become blocked and that area erode away? If water backed up under that area did it manage to rust the supports rebar?
Someone is going to have there hands full on this. Thankfully no one hurt and and chance for all to learn.
Michael
I’m sorry, you have to have a Ph.D in geotechnical engineering to have an opinion on this. They have sophisticated computer modelling that proves the cause of it and no, you can’t see how it works. it’s proprietary and you might find something wrong with it.
PiperPaul February 9, 2017 at 8:12 pm
before I retired, it this point I would simple toss someone bringing such news into a CNC chip conveyor (juicing them) now I can only pout.
michael
“The steel used for the Titanic was far inferior to the steel typically used today, and was much more brittle and not nearly as impact resistant. However, it was certainly the best steel that could be produced at the time. Leighly concluded that flaws in the design of the ship were a much bigger factor of the sinking of the Titanic than the actual steel used in production.” – from http://www.capitalsteel.net/news/blog/steel-titanic
The dam was built in 1961….they would have used grade 60 rebar….same as today
An alternative to rebar or mesh could be the tying-in of adjoining slabs with starter bars; short lengths of rebar cast into the slabs.
It also looks like the spillway concrete was cast on natural ground.
If the spillway was designed only to prevent erosion, high compressive strength concrete of, say, 40Mpa should provide sufficient abrasion resistance. base failure is another matter.
Let’s hope that they used REBAR in the actual Oroville Dam…
It is an earthen dam.
There appears to be some kind of webbing on the edges of the concrete.
You can see that in the photo….it’s expansion joint filler
Go to the gallery at https://ww2.kqed.org/news/2017/02/07/engineers-assess-spillway-problem-at-oroville-dam/#jp-carousel-11307359, and when viewing the images, click on the button to view full size images. Rebar is visible.
These are much better pictures….you can see the bent rebar in them
It appears that the break coincides with the trail approaching the right side of the spillway. There appears to be water upstream of the break also along the right side of the spillway.
I’m pretty sure this is precast construction… so a lot of huge prefab tiles that are just lifted into place, from the bottom up. Reobar would be in each tile section but would not tie each section together. Rebates are cast into the section edges to lock it all together. ( as can be seen along the sides ) As stated… a geothechnical problem has caused the tiles to be undermined.
This obvious not precast….you wouldn’t use precast for a slab on grade. The tiles are individual poured sections with expansion joints between them.
A Geotechnical problems did not cause the problem….it is highly likely that poor maintenance on the expansion joints ( failure in the sealant) allowed water to enter beneath
And to undermine the slabs.
Looks like the case. Check out TonyL’s pictures above. In the gallery there are pictures of the failure in progress.
More reinforcement or not it wouldn’t have mattered. Concrete on a 25% slope, with no ground support underneath is going to fall down. And with 20,000 cfs of flow through the hole, the failed concrete it is going to be carried away. The spillway wasn’t intended to bridge anything and when the (poor) subgrade finally disappeared, through long term(?) erosion, nothing was going to stop the collapse.
See videos of high flows across the failure … it held up surprisingly well. And where it is supported by native rock it survived completely. No structure anywhere is designed to withstand the hydraulic forces encountered after the collapse.
Also note the last photo above (left side-middle); The water uplift broke and pushed that slab up, where it remained because of the reinforcement. Opposite that slab are a couple of irregular pieces that are hanging on by the reinforcement.
Maybe the rebar lap could have been greater (or tied better?), but it wasn’t being designed as span decking or for uplift forces.
Somebody blew it originally by not removing and replacing the area the eroded away & somebody else blew it by not noticing that the material erosion was running down the hill (my guess is that the erosion started as soon as the project was complete).
The annual inspection is more enjoyable when it is sunny. When it is raining (which is when they could have seen that they were losing material and that the east wall footing base was acting as a sub drain) inspection is less enjoyable.
Just ignore it for now and fix it in the summer. This way they won’t have to demo as much prior to reconstruct. During reconstruct run a “beam” under the spillway every 50 feet … not for support but to intercept subflows and direct to weephole in side walls, and as deadman.
I wonder if the huge rain of 1996/97 was a factor in undermining the spillway?
Not all concrete structures need rebar. Hoover dam has miles of cooling pipes but not rebar. All compression? I am pretty sure the spillways and diversion tunnels, which have three ft of concrete lining, also have no rebar.
Look at the image for Sept. 19, 1933, that looks like rebar to me. Could be wrong though.
http://mashable.com/2015/09/11/hoover-dam-construction/
Incredible project in the depths of a depression.
Inlet towers do have rebar.
Can’t even pour a driveway without a rebar inspection in most counties, and the rebar requirements in any state approved spillway plans would have been massive. Don’t know how such a glaring omission could have escaped notice.
It looks ugly but it isn’t a safety issue. The hill on which that spillway sits is not part of the dam proper. There is no reason to put rebar in there as there is no requirement for structural strength. The concrete is just a path for water to flow on and rebar will not save the pavement once the earth underneath is eroded away as you can see in many street collapses. This looks like water got under the structure on the right side (as looking up from the bottom) and undermined it. It will leave a scar but it isn’t going to hurt anything. Rebar would simply add unnecessary cost.
Concrete cracks as it shrinks, Crosspatch, that’s often why rebar is used tocontrol that cracking! 😉
Can someone tell me what this article has to do with global warming/climate change?
Jerry is laser focused on cooling the planet and took his eye off the infrastructure ball
Like in any other field, e.g. climate science, it helps if you know what you’re talking about.
In this case a bunch of people with no knowledge of structural engineering somehow think they know better, sounds familiar ?.
And there YOU are again with your vile name. “WTH” would get the job done and not be as disgustingly vile. But, then, it wouldn’t be an apt nickname for YOU, would it.
Janice
Falls from high horses can be dangerous.
WTF is a very acceptable and direct from of communication. When communicating on a marine radio it should followed by ‘over’.
WTFO
I guess that’s why you just muck around in the mud at their feet. Glad you enjoy it so much. To each his or her own.
BZ
http://www.newsmax.com/TheWire/oroville-dam-spillway-sinkhole/2017/02/09/id/772697/
Lucy ju goh som splainin to do……..
I really do see any 5/8in rebar sticking out – at least not in this section
Lindsay is correct – it is a geotechnical issue brought about by the failure to keep water from undermining the spillway proper. The dip and strike of the rock units point to a potential problem. This is exacerbated by the weathering profile of the various rock units. From the photographs, clay is forming above a slate. The clay is stable if kept dry but notoriously unstable if there is water ingress as appears to have happened here.
Eerily similar to the erosion of the Glenn Canyon Dam spillways in 1983. Those were clearly reinforced. According to current engineering notions water should never be able get at the concrete enough to even make reinforcement relevant. Surely the “weight” stress increase is minimal during a storm.
It must be a pothole effect, or like washboarding in a road. A pattern of cavitation once started developing enormous persistance and force.
In the case of Glenn Canyon they could feel and later see chuncks of concrete, and later the rock laterally supporting the dam, bouncing down the spillways and ejecting.
This smacks of perhaps a construction decision because of cost overruns or design incompetence. The way Northern California is getting creamed by successive intense rain systems this can’t end well if there are two or three more of these super soak-er systems and the citizens of Oroville might want re-evaluate where they are living. It doesn’t look to me like they can lower lake level fast enough of the rains don’t soon stop.
There was some patching repair in 2013 just below where this large initial failure occurred. A picture of this 2013 patching depicted several pickup trucks inside of the spillway, one parked next to the the left hand wall. The failure at 35,000 cu ft/sec was during a gradual increase up to the maximum gate capacity of 100,000 cu ft/sec. The gates were closed and engineers many made some sort of analysis. There was movement of heavy equipment, along with truck loads of rip rap staged somewhere nearby, a road was being constructed into the failure site. Today the gates were reopened, to 70,000 cu ft/sec as the dam/lake was at 880 ft crest is 900 ft. None of the rip rap was untilized. There are many pictures showing the initial failure and the gradual increase in collapse. One viewpoint from DWR was a sink hole type of erosion caused the first collapse of surface concrete. The now ongoing release at 70,000 has eroded the right hand outside of spillway natural terrain creating an every growing width and depth trench. The sidewalls of the spillway are massive, my estimate of 16-20 ft tall. One picture phase of this initial failure showed one sidewall suspended for about 150 ft estimated from the DWR claim that the failure was 220 feet long. In the lower edge of the initial failure the slab concrete is on solid rock. Pictures during the engineering analysis the sub surface up slope is deeply eroded with a significant hollowed out cave like undermining. Approximately per-second 4,200,000 pounds moving down hill.
crosspatch has it right. “It looks ugly but it isn’t a safety issue.” One should add ‘not at this time’, as long as the heavy rains stop.
As to Rob Bradley’s comment, this is a great example of differentiating between a weather event and a climate event. Weather events can be catastrophic in the short term. Climate events happen over much longer periods.
There is an access road on the right side (looking up-slope) that looks like water would run down that road and against the side. The washout begins there.
Shows best in the photos of the ChicoER link.
And while some of us look forward to a wonderful future of high tech, information technology, mini cameras and silica fibres supplying and controlling our every need, desire and essential service – some of us are waiting for the day humans can successfully get 2 friggin rocks to remain sitting on top of one another.
Yet we are *so* convinced we are *so* clever
This guy, likely nuts about the explosives, but the erosion could rapidly grow and work back towards the leading edge of the earthen dam and then it’s over.
http://82.221.129.208/baasepagew2.html
” The Oroville dam is going to overflow and there is nothing they can do to stop it.
Right now the inflow is 180,000 cfs and they can only get rid of at most 30,000 cfs. The conditions at the dam were greatly worsened by increased rains. I still believe the spillway was destroyed with explosives to prevent a controlled release. If the spillway was not damaged, it would handle the current inflow without incident. However, now they are going to have to let the water flow over the mountain that forms part of the dam, and that has never been done before. If the mountain cannot take the beating, it is going to be GAME OVER.
At the current inflow rates, once the dam overtops the mountain will have to take, at this time, the full fury of the entire Niagra falls, TIMES TWO, and worse, Niagra falls only has a drop of a little more than 200 feet. This water will do a drop of 900 feet. It is going to be epic, even if it does not end up being an epic disaster.
There is nothing they can do to stop this, because with the spillway as damaged as it is, there will be little difference between letting the dam overflow and trying to use the spillway which is effectively destroyed.
The silence of the government is deafening. They are still telling the public not to worry. I believe that is negligent to the point of criminal. At the current rate of rising water, the dam will overtop tomorrow. If you live downstream it would be a very good idea to pack the car and take a week end vacation.”
DC, won’t be a safety issue even if the entire hill washes away, will cost more to repair, though.