What are sinkholes?

You may have seen recent footage of a sinkhole appearing on a large football field in Illinois. The hole - which is 100 feet wide and 50 feet deep - swallowed the flood lights, benches and even the football pitch’s turf. But what exactly is a sinkhole? Here’s Vanessa Banks from the British Geological Survey…

Vanessa - A depression in the ground that doesn't have a surface drainage outlet, and therefore this depression is draining only from its space. The depression in the ground may have formed because of the lowering of the ground surface, because at that point there are soluble rocks. And a soluble rock is one that dissolves a bit like sugar in water when you pour slightly acidic water over it for prolonged periods of time. And as the hollow forms, then more and more water is focused to that point, and the dissolution extends to greater depth. And then we have something that we might call a dissolution sinkhole.

So, can anything be done to prevent them from swallowing up the ground and might they actually have a practical purpose in some cases? We put in a call to civil engineer and science populariser Grady Hillhouse, who is best known for his youtube channel Practical Engineering…

Grady - There are lots of areas across the world that have the specific geology that's conducive to sinkholes and there's really not much you can do to stop the problem. Right. It's very insidious. It's wide scale. So in that way it's kind of threatening. But also if you zoom out and look, we don't see that many sinkholes that cause real problems in the world. And so they're kind of rare, but also, you know, it's a widespread problem. <laugh>

Chris - So dramatic when it happens, but thankfully fairly infrequent at the moment. <laugh>. When it does happen, is there anything from an engineering point of view we can do that either we know will make the problem worse, so we know what to avoid doing? Or if we build buildings, build roads, put in infrastructure in a certain way, even if there is a sinkhole in the vicinity, we're a bit more resilient?

Grady - Yeah, definitely. The first step of that is just identifying the hazard. And so a big part of engineering in places that experience sinkhole is just characterising the hazard, right? And usually that involves drilling bore holes to take a look at the subsurface, see if there are caverns, see if there's limestone or gypsum or salt or human made things like mines that could pose a hazard to a structure. And then once you understand that hazard, then you can make a decision about what to do about it. Maybe that means relocating a structure from where you originally intended to put it, or it could also mean building stronger foundations that you know, can withstand a little bit of movement, a little bit of settlement, or even bridge a gap that might form over time.

Chris - So if we know there's a risk in a certain area and we have some plans to do some building there, is there anything we can do to almost put sensors in or anything like that so we can tell how fast things are moving, when things are on the move, and therefore when to watch out?

Grady - Yes and no. There's definitely opportunities to instrument buildings and structures and we do it all the time, right, to keep track of how a structure's performing over time, whether it be movement or expansion or contraction or cracks or how groundwater is flowing. So we do that, not just for sinkholes, but for all kinds of things. But sinkholes are a little bit different in that a lot of times it's a very, very slow process that culminates in this really dramatic collapse. And so it's much harder to kind of predict that very last part of it.

Chris - I suppose one of the issues here is that most big cities sprung up because there was an abundance of mineral resources or something else people could get from the vicinity where that city grew from. And often that was stuff underground, which means most big cities, certainly from days gone by, were on top of resources that people mined out. So we kind of made a rod to beat our own back, haven't we? Because <laugh>, we've got a city there because it was in an area that was plentiful for something we wanted. But the legacy of that is we are now building on land that we've turned into a honeycomb.

Grady - Yeah, I definitely think that's the case. In some places. There are also human made causes of sinkholes. For example,drainage and sewer pipes can get a crack and start eroding soil into the inside. And so oftentimes you'll see a sinkhole that opens up in the middle of a road that has nothing to do with geology at all, right? It's all about a failure of a sewer or water pipe that's eroding the soil and moving it away from the subsurface.

Chris - Is anyone trying to do anything practical with a sinkhole? So if we know that one is coming or one has happened in an area, it sort of saves us from making a big hole in the ground. It strikes me. So is there anything that people are saying, well, this could actually be useful?

Grady - That's a good question. There are definitely examples where natural drainage will flow into a sinkhole and that can be useful to recharge an aquifer that may be used as a water supply. I'm trying to remember the Arecibo telescope in Puerto Rico that was formed in a big basin and I can't remember if they would call that a sinkhole or not, but that was an example of using the natural topography to the advantage of creating a big radio reflector.