Sinkholes seem to appear out of nowhere and swallow everything above them without discrimination. But actually, they are the painstakingly slow work of water.
Grady, from Practical Engineering, explains how sinkholes work and why they appear seemingly out of thin air in this really informative video.
Grady made some great models to illustrate the phenomenon which he demonstrates in the video. First, he explains we have to understand that almost all complex civil engineering problems come from the interaction of water and the ground. The fundamental lesson in understanding sinkholes is that erosion doesn’t just happen on the surface of the earth. But they can actually happen under the ground in through a process called “internal erosion”.
The second lesson is that sinkholes are most likely to occur in areas with a lot of carbonate rocks, such as limestone. Over long periods of time groundwater erodes these softer types of rocks and forms tunnels or caves. This changes the conditions the groundwater can move, allowing it to move faster or pool in certain areas. Critically it also means that there is somewhere for the soil to move to. And this movement of soil is what causes sinkholes.
But sinkholes aren’t always caused by this ‘natural’ flow of water. Many of the most famous sinkholes happen in urbanized areas due to faulty pipework.
Under almost all urban areas there is a maze of pipework delivering water and removing sewage. If any of these start to leak due to a disruption or wear, then the addition of water into the larger area can create the conditions for a sinkhole. The video is worth a watch to get the full picture with the informative models.