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.
Also, there’s the problem of ‘Durchstanzen’ (punching shear?).
If a supporting pillar is too thin or not enough rebar installed, the concrete and the rebar may shear off. Then probably no rebar would be visible. This happened to a parking lot in Switzerland a few years ago. They built the lot, put about a metre of soil on top for a playground/park. All went well, until a car burnt out in the parking lot and the rebar melted.
iRill, in the fifth image you see the chain link fence in front. If you zoom in, you can clearly see some wirelike bent things coming out of the concrete. If the concrete is 10 inch thick, then those rebar wires would be maybe 1/4 inch, which is much too thin for this load.
No one has mentioned the most probable cause. This spillway was poured using concrete. This was done in sections or slabs. This means that between each section or slab , there would be expansion joints (as you would have on a freeway or patio) In this case, these gaps would be filled with a caulking compound which is able to stretch and compress with the expansion and contraction of the concrete floor. If honest inspections are done,I am certain they will find that there was a gap in the slabs which allowed water to rush underneath and undermine the soil supporting the underside of this chute. Once a cavity was washed away; the concrete was required to do what it was not designed to do ( with or without rebar) It had to span a large distance without support underneath and with tons of water to hold up.
I suspect the experts know this and will remain silent about it as it would indicate a lack of simple maintenance. The before images show area’s of patching, etc , but this could have been avoided. I.M.O.
Am I the only one who thinks that the concrete doesn’t look that thick to start with?
Look at the water spraying out of the side walls in some of the photos. It appears that there is water pressure (several feet of head) UNDER to slab due to the fact that the earth on the side graded to go away from the wall. It looks like the original failure was erosion from water underneath the slab. The water that is high enough on the sides to come out of those high weep holes would erode the supporting earth under the slab and lead to failures. The fact that the walls stayed up for some time longer than the slab indicates that there is much more steel in the walls and the first few feet of slab adjacent to the walls but very little or no steel in the slab. Once the soil is eroded from under the slab the slab fails. The “Repair” will have to be replacement of the whole slab or a new layer of reinforced concretes on top of the slab.
CORRECTION: Look at the water spraying out of the side walls in some of the photos. It appears that there is water pressure (several feet of head) UNDER to slab due to the fact that the earth on the side IS NOT graded to drain water away from the wall. It looks like the original failure was erosion from water underneath the slab. The water that is high enough on the sides to come out of those high weep holes would also erode the supporting earth under the slab and lead to failures. The fact that the walls stayed up for some time longer than the slab indicates that there is much more steel in the walls and the first few feet of slab adjacent to the walls but very little or no steel in the slab. Once the soil is eroded from under the slab the slab fails. The “Repair” will have to be replacement of the whole slab or a new layer of reinforced concrete on top of the slab.
In my opinion the spillway would be better if is was cleaned out down to the bedrock, rebar installed and tied on top of elevation blocks to to elevate it from the bed rock and a minimum of 3′ of concrete for the water bed surface. It is hard to believe that the engineers built this spillway without reinforcing steel bars with such a shallow pour of concrete. The smallest crack would start undermining the surface with the force created by the water as it cascades down the spillway. Ballast should also be placed about half way up the side rails and about up to 8′ outboard to support and stabilize the structure.
my driveway has steel mesh in it and seems to about as thick as the spillway the amount of money they spend on this stuff is jaw dropping they could have built a one time use state of the art steel plant on site just so they could have lined the spill way with stainless steel and still been under budget on has to ask were does the money go its not in labor or the supplies not much of that was used maybe it all went downstream with the water funny but on top of tax dollars being spent to build these things they also collect from what people pay to have running water
As a civil engineer that was my first observation “where is the reinforcing steel? It should be sticking out of the remaining concrete. It dies not appear to be there….
What a terribly misinformed garbage article by a sensationalist. All this does is attract droves of armchair structural engineers and construction experts, none of whom actually have been on the site in question, let alone had their hands on the contract documents and drawings.
There is reinforcement in the primary spillway in question, noticeably visible in a number of photographs. Whether it was enough to mitigate some of the damage that resulted, or was even intended for such a precautionary purpose in the first place, the author clearly doesn’t know. But he forges ahead in spite of this ignorance. It’s just irresponsible.
And, no, I’m not at all defending those who were involved in the design of the Oroville Dam and its related components. Clearly, some things have occurred that were either poorly predicted or unsatisfactorily addressed prior to February 2017.
I guess I’m just calling out the author for his clear attempt to stir the pot in the first place, his success in doing so (evidenced by roughly 150 subsequent replies, mine included) notwithstanding.
REPLY to PHIL LANTON: Mr. Lanton, our local news media was asking the same questions that I was. The question was legitimate, and via this thread and added photographs, we answered the question, as did our local news media. If you don’t like it, that’s fine. You are entitled to your opinion. I’m also entitled to my opinin, and my opinion is that your accusation of “stirring the pot” is way off base.
I put this thread together only AFTER I had contacted my local newspaper and local TV station news editors (I know both) and vetted the issue with them. I asked readers what they saw, I asked readers for help in identifying the issue, and I passed that info on to our local TV station and newspaper, where they were satisfied, as I was.
You call me a “sensationalist”, without knowing one thing about why this story was in place. I call you some some armchair quarterback from North Carolina with nothing more than a long distance opinion, who’s never been on the dam as I have, never interacted with the news media, and only knew about it because some other news outlet “sensationalized” it enough for it to reach you. – Anthony Watts
Another armchair observation. One presumes that piping under the spillway was analyzed and dealt with in the design. Is it possible that this flow under the spillway did take place and the pressure from this piping manesfted itself at the point under the spillway where the concrete blew out? There was an earthquake in the location after the filling of the reservoir which could have created fisures in the underlying rock, through which the piping could have occurred unbeknownst to anyone and defying detection by inspections.
Most likely, we will hear from a team of engineers that investigate this spillway failure. I suspect that severe undermining of the spillway foundation material had been occurring for many years resulting in a void(s) that enlarged to such an extent that the rebar could not support that span once it became loaded by the full engagement of the spillway.
The rebar is intended to keep the slabs from cracking and is also used to pin the slabs together and to the vertical side walls. Any cracks that may have developed due to undermining would lead to an acceleration of the undermining due to the high velocity jets of water scouring out the foundation materials. As the compromised slab(s) deflect and move, projecting edges occur which are then impacted by the extremely high velocity discharges. It all results in a cascading failure.
Unlike a prestressed concrete beam, the slab steel is not intended to support the loaded concrete slab over large voids.
Design of dams has changed over the years. This dam was constructed in 1968. It appears from the clos-up photos that the side walls were independently constructed with a heel and a toe, much like a retaining wall. It’s hard to tell without being there on the ground, but it appears that the slabs were then dowelled into the toe of the side walls, as there is a uniform construction joint along that section of the failure.
The spillway was reportedly in need of maintenance, repair or rehabilitation per previous accounts. A FOIA request of the State’s Dam Safety Inspections would provide that information. I spent 15 years with a state dam safety program and 18 years in the private sector analyzing, designing and repairing new and existing dams. The forensics team will be thorough and may take some time to determine the cause of the chute failure.
However, we may see an accelerated design and repair project due to the critical nature of the dam and the reservoir for both flood prevention; and more importantly, for its water supply.
Extremely interesting project failure. I wish I was on the forensics/design team.
I found some video that will prove to be very instrumental in determining the cause of the failure. Note at the very start of the video where the gates have been closed on the principal spillway (concrete spillway); there is an enormous amount of water exiting the weep holes from the side walls.
Weep holes should not be gushing water like a fire hydrant. Those weep holes are located above (upstream) of the failed slab section. That tells me that there was undermining from massive seepage flows. I would suspect that a very large section of the remaining slab has been undermined to various degrees and may need to be replaced.
Typically the weep holes are the outlets for a drainage system behind the side walls. Normally, one would also design an underdrain that collects and discharges any seepage that may occur under the slabs.
I noted this very early on in the video presentations of the event; however, no mention has been made of whether this high flow seepage condition from the weep holes had been observed in previous inspections of the dam when the lake level was high.
Understanding that the lake had been low for several years (at about 40% capacity); the recent rains and snow melt has resulted in a maximum reservoir level that may not have occurred in a long while or perhaps ever in the life of the dam. There may not have been any recent observations of this seepage condition.
It also occurs to me that discharges over the ogee-crest emergency spillway may have directed flows to one side wall. However, the video shows extremely high weep hole flows from both sides of the principal spillway
Damn, I would sure love to see previous state dam safety reports. They would be fairly enlightening. If you do FOIA them, be sure to ask for earlier inspections when the lake level was high. You can check historical lake levels and tailor your request for those dates or just after.
GTA – grand theft auto.
All that’s left from the ‘american dream’.
chocking all over my spill board.