Three Gorges Dam deformed but safe, say operators

Peripheral structures buckle when record flooding from western provinces puts feat of engineering to the test

In a rare revelation, Beijing has admitted that its 2.4-kilometer Three Gorges Dam spanning the Yangtze River in Hubei province “deformed slightly” after record flooding.

The official Xinhua News Agency quoted the operator of the the world’s largest hydroelectric gravity dam as saying that some nonstructural, peripheral parts of the dam had buckled.

https://asiatimes.com/2020/07/...-safe-say-operators/

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The official Xinhua News Agency quoted the operator of the the world’s largest hydroelectric gravity dam as saying that some nonstructural, peripheral parts of the dam had buckled.

Water will find the path of least resistance. And exploit it.

And then eat its way in or out, depending on where the resistance lies.

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The official Xinhua News Agency quoted the operator of the the world’s largest hydroelectric gravity dam as saying that some nonstructural, peripheral parts of the dam had buckled.

Not a dam expert, but I can’t think that ANY component of or in a dam would be non-structural or “peripheral”. Does not sound good.

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Water will find the path of least resistance. And exploit it.

Mercilessly.

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Originally posted by Georgeair:
Even if you could, the load on the existing dam is now MUCH greater due to the expanded area and the impact on the entire dam structure.

You also can't just build a small simple dam to stop that "leak". If you were building to bring up level by one foot that's pressure from that one foot over an enormous area all coming to bear on that spot. I think - I'm a little less certain on the physics of that. Probably actually much less at surface of water than as you progress downward, butit's not zero.

The load/pressure on the face of the dam is only a function of water depth.
If you make a little sand dam on the edge of a stream or the edge of the ocean, and the water at the base of the dam is 2 inches deep, the pressure at that depth is 14.7 psi (air pressure) plus (0.036 lb/in^3 * 2in) = 14.7psi + 0.07 psi = 14.77 psi.
It doesn't matter if the water extends back a foot from your dam or 1000 miles, in terms of pressure on the face of the dam.

If the base of the dam is 500 ft deep, now you're looking at:
14.7 + 2166.7 = 2181.4 psi
Every square inch of the dam face at 500 ft depth is seeing 2181.4 lb of load.
But that square inch that is two inches from the top is still only seeing 14.77 lb of load.

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Every square inch of the dam face at 500 ft depth is seeing 2181.4 lb of load.
But that square inch that is two inches from the top is still only seeing 14.77 lb of load.

So what you are saying is that the majority of the pressure is at or near the bottom?
And when the water level rises, there's a lot more pressure at the bottom?
So if there is a breach, it's likely to be near the bottom?

The pressure is linear. The higher the depth, the higher the pressure at any given depth.

If a dam is supposed to handle 500ft of water at it's worst case scenario, and there's 550' of water in the dam, the additional pressure at all depths surpasses the maximum. The dam will eventually fail at a weak point where the cement didn't cure correctly or the design/implementation was flawed.

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Originally posted by chellim1:

So what you are saying is that the majority of the pressure is at or near the bottom?
And when the water level rises, there's a lot more pressure at the bottom?
So if there is a breach, it's likely to be near the bottom?

I can't speculate on where a failure might occur, all I'm saying is the pressure on the dam only depends on the water height. It doesn't matter how far upstream the water stretches, only the height matters.

Remember Operation Chastise.

The bombs were designed to sink to the base, where the extant pressure was the greatest, then detonate. Did much more damage than the same bomb detonating at surface level because of the assistance of the water.

Well, at least the concerns about the seismic fault line the dam is built on are pointless. If this weight of water doesn't trigger an earthquake, nothing will.

JoseyWales2 and Shaql described the principle of hydrostatic pressure, except the numbers are off by a factor of 10. Pressure at the 500 ft would be 218.1 psi. In terms of total load on the dam, each foot of length of the dam would be resisting 7,800,000 pounds of pressure. Keep in mind, though, that the concrete in the dam is almost three times the density of the water, so there is sufficient mass available to resist that pressure.

That doesn't mean that it can't fail, however. That water pressure at 500 feet not only acts against the dam, but it can also provide uplift on the base of the dam that reduces the contact pressure of the dam with the foundation material. That makes it more susceptible to sliding. How much uplift depends on the nature of the foundation material and the design of the dam. Even with that uplift pressure, there should still be sufficient weight to hold everything together, assuming that the foundation material remains solid.

Which brings us to the biggest risk. All of that pressure is also acting on the natural materials beneath the dam and around it. If there is a weak zone of fractured or highly weathered rock or, heaven forbid, soil that wasn't identified during construction, the seepage of water through those earthen materials can reach a high enough velocity to begin to dislodge particles and cause internal erosion (called piping). Once that begins, it's very hard to stop. It gets progressively worse as more material is removed until it collapses. Most dams fail not at the concrete structure, but in the embankment and foundation materials.

Dams go through a rigorous potential failure mode analysis (PFMA) to determine what the most likely failure mechanism is. Floods are just one of many ways a dam can fail. The PFMA for a large dam can get very detailed and cover just about every contingency imaginable. Everything we are seeing has been analyzed time and time again. But, poor construction practices can render all of the assumptions in the models moot. Just because it works on paper doesn't mean they built it right. Still, my speculation is that, reports of shoddy construction notwithstanding, the dam will hold up.

I wish I had more time right now to dive into the Three Gorges design and construction to give a better analysis.

Thanks for your insight jaaron.

Yes, thanks, jaaron11. Always good to learn from people who know what they are talking about.

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Originally posted by jaaron11:

Dams go through a rigorous potential failure mode analysis (PFMA) to determine what the most likely failure mechanism is. Floods are just one of many ways a dam can fail. The PFMA for a large dam can get very detailed and cover just about every contingency imaginable. Everything we are seeing has been analyzed time and time again. But, poor construction practices can render all of the assumptions in the models moot. Just because it works on paper doesn't mean they built it right. Still, my speculation is that, reports of shoddy construction notwithstanding, the dam will hold up.

Thanks for the information. It's very helpful to understand the forces at play.

I'm sure dams in the Western World undergo this rigorous analyses. I'm not too confident China did so. Also, on Page 1 of this thread, StarTraveler mentioned that the second video from the OP stated that the contractor performed their own quality control. I don't think we can confidently take American, European, etc., building practices and automatically assume China performed the same due diligence. It IS China, after all. . .

One of my early structural engineering mentors spent a number of years working in the Far East (Based in Singapore, I think, but doing projects over a wide area). He told me about doing quality control inspections in a reinforced concrete building under construction one day and going back just before the concrete pour early the next morning to find that a large amount of the steel reinforcing bars (rebars) had been replaced or completely removed. He said this was very common practice in the area.

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Originally posted by jaaron11:
JoseyWales2 and Shaql described the principle of hydrostatic pressure, except the numbers are off by a factor of 10. Pressure at the 500 ft would be 218.1 psi.

Ah yes, I misread the decimal point on my depth calculation.
But we both made a mistake. It's not 218.1 psi total. Atm pressure has to be added correctly, but I think everyone here gets the general principle being discussed.

If the base of the dam is 500 ft deep, now you're looking at:
14.7 + 216.67 = 231.37 psi

But atmospheric pressure is the same on both sides of the dam, so it effectively cancels out, leaving the net number jaaron noted. You could also deduct the pressure due to the depth of water on the downstream face, but I think that would usually be taken as zero to give the worst case condition since it will vary significantly over the life of the dam.

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Originally posted by StarTraveler:
But atmospheric pressure is the same on both sides of the dam, so it effectively cancels out, leaving the net number jaaron noted. You could also deduct the pressure due to the depth of water on the downstream face, but I think that would usually be taken as zero to give the worst case condition since it will vary significantly over the life of the dam.

Okay, I don't want to derail this thread too much further but my calc is not the pressure differential on either side of the dam. It's effectively the weight of a square inch column of water and air at a given depth. This is true in the middle of the ocean, your swimming pool or on a dam face. Note that pressure acts equally in all directions.

Correct Calc: 14.7 + 216.67 = 231.37 psi
Incorrect Calc: 14.7 + 2166.7 = 2181.4 psi
Incorrect Calc: 1.47 + 216.67 = 218.1 psi
Only the second addend in the sum needed to be divided by 10, not both. The standard atm pressure term of 14.7 psi at sea level is correct.

Josey, good point for the max pressure. I was considering what the dam as a whole would “feel” and what would be used in the design. My Master’s thesis was on the effects of the membrane weight in the design of inflatable dams, proving that the membrane weight could be neglected in the design calculations. Previous research had already proven that atmospheric pressure at a given point could be neglected on overturning and sliding due to the cancelling effect noted.

The source for this video is sketchy, and little details smack of too-produced/actual-propaganda.

A quick look reveals the Epoch Times (despite being a Trump supporter, QAnon supporter, and other quirks) is the unofficial mouthpiece of Falun Gong, a quasi Buddhist and qigong group who have a 400 acre compound in New York whose "spiritual leader" is a man named Li Hongzhi who believes in aliens.

The PR message of Falun Gong is that of a harmless group who is persecuted by the CCP.

Their spiritual leader, however, had this ludicrous shit to say:

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TIME: Where do they come from?
Li: The aliens come from other planets. The names that I use for these planets are different . Some are from dimensions that human beings have not yet discovered. The key is how they have corrupted mankind. Everyone knows that from the beginning until now, there has never been a development of culture like today. Although it has been several thousand years, it has never been like now.

The aliens have introduced modern machinery like computers and airplanes. They started by teaching mankind about modern science, so people believe more and more science, and spiritually, they are controlled. Everyone thinks that scientists invent on their own when in fact their inspiration is manipulated by the aliens. In terms of culture and spirit, they already control man. Mankind cannot live without science.

The ultimate purpose is to replace humans. If cloning human beings succeeds, the aliens can officially replace humans. Why does a corpse lie dead, even though it is the same as a living body? The difference is the soul, which is the life of the body. If people reproduce a human person, the gods in heaven will not give its body a human soul. The aliens will take that opportunity to replace the human soul and by doing so they will enter earth and become earthlings.

When such people grow up, they will help replace humans with aliens. They will produce more and more clones. There will no longer be humans reproduced by humans. They will act like humans, but they will introduce legislation to stop human reproduction.

The thoughts of a court recognized expert on Cults on Falun Gong.

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Falun Dafa also known as Falun Gong and the teachings of its founder Li Hongzhi.

Falun Dafa, which was founded in 1992, is an idiosyncratic blend of beliefs and practices as assembled by its founder Li Hongzhi. This includes Taoist and Buddhist references predicated upon a belief in extraterrestrials and practiced through a set of prescribed exercises and meditation techniques.

https://culteducation.com/grou...lun-gong-a-cult.html