How Do Sinkholes Form?

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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 you’ve 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, we’re 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. We’re 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. Blue Apron delivers all the fresh ingredients you need, right to your doorstep, in exactly the right proportions to create delicious recipes at home. It’s essentially just the fun parts of cooking with none of the chore, which is exactly what we needed this week. If that sounds like something you’d be interested, you can find out for free. The first hundred people that click the link in the description will get 3 blue apron meals with no commitment whatsoever. If you like it, you can sign up for a subscription, and if you don’t, hey, you just got three free meals shipped right to your doorstep. Again, thank you for watching, and let me know what you think!
Info
Channel: Practical Engineering
Views: 7,956,325
Rating: 4.8419213 out of 5
Keywords: how do sinkholes form, sinkholes, internal erosion, civil engineering, grady hillhouse
Id: e-DVIQPqS8E
Channel Id: undefined
Length: 6min 40sec (400 seconds)
Published: Wed Jun 28 2017
Reddit Comments

Hey, that's me! Thanks for sharing!

👍︎︎ 205 👤︎︎ u/gradyh 📅︎︎ Jun 28 2017 🗫︎ replies

"this video is sponsored by Blue Apron, more on that later." I love it when YouTubers don't get on their knees and suck the sponsors dick by making the first 30sec-1min of there video half ass promoting their sponsors. Well done mate. Great video.

👍︎︎ 38 👤︎︎ u/houston117 📅︎︎ Jun 29 2017 🗫︎ replies

https://youtu.be/UFVVKqbNLYA

biggest sinkhole recorded I believe

55 meters deep says one of the guys in the video but a wiki article says it was 30 meters

👍︎︎ 29 👤︎︎ u/WhiteHairedWidow 📅︎︎ Jun 28 2017 🗫︎ replies

"It sounds mundane"... hehe. "Mundane" actually means "of the Earth".

👍︎︎ 5 👤︎︎ u/[deleted] 📅︎︎ Jun 28 2017 🗫︎ replies

This video is concise, easy to follow, and fun to watch. He offers nice graphics with examples in real time and in history. Too cool! would love to watch more

👍︎︎ 5 👤︎︎ u/littleguysofly 📅︎︎ Jun 29 2017 🗫︎ replies

it bums me out that this video has less than 100 upvotes, whereas a 9 second video of some fat chick shitting on a public floor has more than 27 thousand upvotes.

can someone point me in the direction of "the new reddit"? reddit as it was, 7-8 years ago? when it was interesting, intriguing, intellectually engaging content? this website is literally garbage now, jesus christ.

👍︎︎ 46 👤︎︎ u/punt_the_dog_0 📅︎︎ Jun 28 2017 🗫︎ replies

I work as an agricultural drainage contractor and what he described is what I have to go and fix all the time. Old clay pipe breaks or settles wrong and starts sucking dirt. Sometimes newer plastic pipe will be installed shallow and will attract mice. Mice find water in pipe and a cavity and chew a hole in the tubing. Nice home for a mouse. Sometimes we dig these up and will see half a dozen or more mice scatter. Good times.

👍︎︎ 2 👤︎︎ u/zaqufant 📅︎︎ Jun 29 2017 🗫︎ replies

That brief clip of some roads in downtown Seattle (near where i live) is unsettling

👍︎︎ 2 👤︎︎ u/Rent_Money 📅︎︎ Jun 29 2017 🗫︎ replies

As a Geotechnical Engineer, this video is very well done. It explains the simple concepts very clearly. On his note of creating a piping model; while attending graduate school at Utah State University other master's students were trying to various piping modes. It may be more difficult than it appears. But there should be some good starting points in their thesis and journal articles.

👍︎︎ 2 👤︎︎ u/elderbio 📅︎︎ Jun 29 2017 🗫︎ replies
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