Your questions answered: how do sinkholes develop?

Occasionally I’ll pick up the newspaper and there’s a story about a sinkhole that’s “big enough to swallow a car”…or a truck…or a house. Incidents like this tend to cause a bit of concern for those of us living in sinkhole prone areas. After one such article, my grandmother called me to ask if she should stop mowing over a depression in her yard. No, Grammy, you can keep mowing over it…or maybe just get Grampy to do it.

To answer questions like this, I think it’s helpful to understand what causes a sinkhole to develop. So here goes…but first, a refresher on Pennsylvania’s geology…

Bedrock

Underneath the grass and the pavement and the soil and the swimming pools is a layer of bedrock – and the depth of this bedrock varies. Sometimes this bedrock is way down there (40 feet deep or so) and sometimes it’s poking right up through the surface. Bedrock can be made up of all different types of rock – in Pennsylvania, we have plenty of limestone and dolomite bedrock which is often called “carbonate bedrock”.

FracturesCracked mug

Carbonate bedrock is a sedimentary rock. Reach back into third grade science class and you might remember that this type of rock is made up of grains of sediment that have become “glued” together over time. If the bedrock hasn’t been disturbed, you’d see nice flat layers of sediment that were deposited over time and lithified (turned to rock) by chemical reactions taking place between the different types of sediment. But in central and eastern Pennsylvania, the layers aren’t usually nice and level – they’ve been tilted and twisted and turned over time, causing breaks and fractures in the rocks. These fractures allow water to pass through – just like a cracked mug can leak coffee onto your tie.

Dissolution

This type of bedrock is “basic” and is able to be dissolved by acids. If you were to put a drop of hydrocloric acid on a limestone rock, it would fizz. So it would NOT be a good idea to pour acid into your fractured carbonate bedrock coffee mug. (Seriously…don’t do that.) But if you did (you shouldn’t), over time the acid would eat away at the cracks and make them larger. Eventually your coffee mug would act more like a sieve than a mug. That’s exactly what happens to fractured carbonate bedrock. Rainwater in PA has an average pH of about 4.5, so it’s considered acidic. Over thousands of years, the rainwater has percolated through the soil, into the fractures within the carbonate bedrock and has slowly make the fractures bigger, until the bedrock acts like a big underground sieve.

The bathtub modelBathtub model

DCNR has published a nice guide called “Sinkholes in Pennsylvania” and much of the material in this article has been lifted (I mean researched) from that document. In that document, the author, William Kochanov,┬áprovides a handy model for understanding how sinkholes develop from this underground network of cracks and fractures. Let’s say you have a bathtub filled with soil – but it has no stopper over the drain. (The drain represents a fracture in the bedrock). If we added water to the soil, it would slowly find its way down to and through the drain. And it would take a little bit of the soil with it. It would take the soil right around the drain, leaving a small void. If we added water over and over, it would keep migrating to the drain, taking some soil with it and making that void larger and larger. Eventually that void would get big enough that the soil wouldn’t “bridge” the gap over the drain and we’d see a sinkhole at the surface – probably large enough to swallow a large rubber ducky.

Back to sinkholes

The same thing happens with our soil. When there’s a fracture in the rock, that’s been enlarged by dissolution caused by acid rain, water that’s percolating through the soil finds its way down to that fracture and takes some soil with it. Over time a void can develop underground…and it stays hidden until it becomes so large that the soil above it is no longer able to bridge the gap. That’s when Grammy falls into the hole with her lawn mower (I love you dearly, Grammy, and desperately hope that never happens). Much of our bedrock is made up of this carbonate rock so it’s no surprise that sinkholes are a fairly common occurrence in our area. Engineers and geologists pay special attention to sinkhole concerns any time we design a stormwater management facility – especially a facility┬áthat is designed to infiltrate water down through the soil as many of them are today.

There’s lots more that can be said about sinkholes, how they form and how to fix them, but I think that’s quite enough for one day. If you find that you have questions or need more info, please do not hesitate to contact us and we’ll be happy to help out if we can.

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