Subtitles section Play video Print subtitles 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!