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  • Some of the most complex civil engineering problems stem from the interaction of water

  • and the ground.

  • It sounds mundane but, there’s a good chance youve seen one of these on the news.

  • How is it possible for the ground to simply open up and indiscriminately swallow anything

  • or anyone that happens to be around?

  • I’m Grady and this is Practical Engineering.

  • On today's episode, were talking about sinkholes.

  • This video is sponsored in part by Blue Apron.

  • More on that later.

  • We all know about erosion.

  • This is the process that takes soil and rock from the earth’s crust and moves it somewhere

  • else.

  • And there’s a lot of ways this can happen: wind, landslides, abrasion, and scour.

  • But here’s the thing, none of it compares to just the movement of water.

  • Water is the great eroder.

  • If you ever find yourself wondering how did this particular feature of the earth come

  • to be here, or why is the ground shaped like so, or just why are things the way that they

  • are, more often than not, the the answer is pretty much just water.

  • The ability of water to move soil or rock depends on several factors.

  • The faster and more turbulent the flow, the more erosive it is.

  • Larger particles like gravel and more resistant to erosion than small particles like silt

  • or clay.

  • Finally, rather than physical erosion, some materials are soluble in water, just like

  • sugar or salt, and can be eroded just by dissolving into the groundwater over time.

  • Most of us think about erosion on the surface of the earth, but erosion can occur in the

  • subsurface as well.

  • In fact, scientist and engineers have a very creative name for just such a process: internal

  • erosion.

  • If just the right factors come together in the subsurface, some very interesting things

  • can occur, including sinkholes.

  • But let’s look at a non-erosive example of groundwater movement first.

  • This is a from a video I made before the channel was even called Practical Engineering.

  • Water is flowing from the left side of the demo under an obstruction and over to the

  • right.

  • Notice two important things: first, the movement of water is slow.

  • There’s not a lot of open space between all that sand, so it takes time for water

  • to flow through it.

  • Second, the sand is confined.

  • Even if it wanted to move, there would be nowhere for it to go.

  • If those two conditions go away, that’s when sinkholes happen.

  • Most natural sinkholes happen in areas with large deposits of carbonate rocks, like limestone.

  • Over long periods of time, groundwater flowing through the subsurface can dissolve the rock,

  • creating voids and open tunnels.

  • In fact, this is how most caves are formed.

  • These tunnels and voids create a significant change the character of groundwater flow.

  • First, they allow water to flow quickly just like it would through a pipe, making it more

  • erosive.

  • Second, they create a space for soil to wash away.

  • With those two conditions, any soil overlying a dissolution feature runs the risk of eroding

  • away from the inside, eventually leading to a sinkhole.

  • But not every sinkhole is formed through natural processes.

  • In fact, many of the most famous sinkholes in recent times have been human-created.

  • Just like a cave dissolved into the bedrock can act like a pipe and allow groundwater

  • to carry away soil, an actual pipe can do the same thing.

  • And actual pipes aren't limited to areas with a specific geology.

  • If you could take a look into the subsurface of any urban area, you'd see miles and miles

  • of water, sewer, and storm water drainage pipes.

  • Unfortunately we can't see into the ground, so I built this demonstration so we can see

  • for ourselves how this works.

  • All it takes is a little bit of settlement or shifting to create an opening in one of

  • these pipes and allow internal erosion to start.

  • I added a gap in my pipe to simulate this effect.

  • Water moving through the pipe is able to dislodge the adjacent soil and carry it away.

  • Notice that there's no signal on the surface that anything is awry.

  • As more soil is washed away, the subsurface void grows.

  • Depending on all those soil properties we talked about earlier, this process can take

  • days to years before anyone notices.

  • Many of our subsurface utilities are placed directly below roadways, and the paving often

  • acts as a final bridge above the sinkhole, hiding the void below.

  • It's only a matter of time before anything above is swallowed up.

  • Sinkholes aren’t the only problem caused by internal erosion.

  • A specific type of internal erosion called piping is the most common cause of failure

  • for earthen levees and dams, including Teton Dam in Idaho which killed 11 people and caused

  • billions of dollars of damage when it failed in 1976.

  • Maybe I’ll build a piping demonstration someday for a separate video.

  • Internal erosion can be a natural process, but sometimes sinkholes can form due to bad

  • decisions, bad construction, or just bad luck with human-made infrastructure as well.

  • It’s just one of the complex failure modes that civil engineers must consider when designing

  • a structure that might interact with water, the great eroder.

  • Thank you for watching, and let me know what you think!

  • Big thanks to Blue Apron for sponsoring this video.

  • Were in the process of moving and just starting to get unpacked in the new house.

  • The last thing on my mind is going out to buy groceries.

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  • It’s essentially just the fun parts of cooking with none of the chore, which is exactly what

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  • Again, thank you for watching, and let me know what you think!

Some of the most complex civil engineering problems stem from the interaction of water

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B1 中級

陥没穴はどのようにして形成されるのか? (How Do Sinkholes Form?)

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    Amy.Lin に公開 2021 年 01 月 14 日
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