The Science of Sinkholes

This week, we heard the tragic story of Florida resident Jeff Bush who was killed when a 60 foot deep sinkhole opened up underneath his bedroom. We wanted to find out a bit more about what these natural phenomena are so were joined by Nigel Woodcock from the faculty of Earth Sciences at Cambridge University. We started by asking him what actually a sinkhole is. 

Nigel:  A sinkhole is any subsidence in the ground surface, usually circular plan,  and much deeper than they are wide. They form by a number of mechanisms. Although usually they're due to soluble rocks underneath the ground, limestones or rock salts. These sorts of things which are easily dissolved especially by acidic ground water and caverns form underground, and then unpredictably, these things can fall in overnight as it tragically happened in Florida.

Chris: Is it limestones chiefly then that go?

Nigel: It's limestones and what we call 'evaporites' which are minerals which develop from evaporating seawater. So mostly, rock salt or gypsum, and all these are very susceptible to solution.

Chris: Could it happen anywhere?

Nigel: No, it couldn't. The estimate is that about 20% of the continental area of the United States is susceptible. It's underlain by these sorts of rocks. In Britain, maybe a little bit less 15%, Australia about 15%. So, the good news is, if you're in the other 80%, 85% of those countries then you're perfectly safe.

Chris: But equally, if you're not, you could be unsafe. How did they manage to miss something as huge as that sitting under a house?

Nigel: It's a little bit like geologists predicting earthquakes. We could define the general zones which are most at risk, but saying exactly where and exactly when these collapses are going to happen, it's extremely difficult, almost impossible. You rely on local observations. Usually, they don't happen overnight. In this case, they seem to have done, but there are usually warning signs and the local information is critical in predicting like this larger substance episodes.

Dan: Do they go very deep? I know it's a simple question, but I understand they never found the man who fell into the hole in Florida.

Nigel: They go frighteningly deep. I gather, the deepest sinkholes known in China are something like 600 meters deep and this one was said to be much less than that, but still deep enough to obviously cause all the damage if houses collapse into them.

Ginny: What do you do once one of these has opened up because I heard that they can be sort of caverns that are connected to other ones? Can you fill them in ?

Nigel: This is the problem. Engineers in these areas which are susceptible try and fill them with concrete sometimes, but their volume is many times...

Chris: With 600 meters deep, that's a lot of concrete, isn't it? That's a lot of Ready-mix, isn't it?

Nigel: And as you say, these fissures are often interconnected in unpredictable ways. So, the concrete just disappears down into much lower levels of the Earth's surface. It's not usually the best plan.

Chris: So, what does this mean for the other people in that street? Can we do surveys now and have a look and say, "Well, we know there's a hot spot here." Can we do ground penetrating radar or something and see what these things are?

Nigel: It would be possible to do that sort of geophysical survey and define where the fissures are, but even then one fissure of a particular width could be stable and another one next door may not be. So, there's a lot of random chance involved here.