How Do Sinkholes Form?
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 sinkhole 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?
We all know about erosion. This is the process that removes 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. Another important soil property is cohesion, or the ability of individual particles to stick to one another. Clay soils have more cohesion than sands, so they are more resistant to erosion. However, some clay soils are dispersive, which means they naturally wash away with water, making them particularly vulnerable to erosion. I love the standard test for dispersive soils, which is literally just to drop a clod of soil into a cup of water and see what happens. 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 sink holes is formed through natural processes. In fact, many of the most famous sinkholes in recent times were human made. 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. 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 the type of soil and the speed of erosion, 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 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!