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I live in Florida (aka the land of sink holes). Due to the flat nature of my area, it is common place for housing developments (and any developments really) to dig large drainage areas to give water a place to go other than the newly constructed buildings. In the case of my neighborhood that means that a large depression was dug to drain water from the land where all the surrounding houses were built. The drainage area is roughly square, probably 50 meters by 50 meters in size, and about 4-5 feet deep (I'm pretty sure this is a common practice, even in less-flat areas). In our case houses are built surrounding it in a big "block", and my house backs up to it.

It's covered in grass and well maintained, so I take my kids back there to run around semi-regularly. We bought our house about 6 months ago, and about 3 months ago a large sink hole opened up in this drainage area. Large in this case means that it is probably 2 meters deep at the deepest point and has an overall diameter of about 5 meters. It's on the other side of the drainage area so it isn't right next to my house, but it's certainly not far. We also recently found a second (and much smaller) hole - approximately 20 centimeters in diameter and 20 centimeters deep. This small one is much closer to my back yard.

Of course I'm definitely interested in this sink hole as it pertains to my house (aka does this imply that this area may be prone to sink holes), but I'm also curious about these things generally. To keep it to a simple question, I'm wondering if this is a sink hole that opened up like all other sink holes in Florida - aka a collapsed underground hole/cave that just happened to miss all the surrounding houses? Or is it possible that this is a local affect, aka something about these drainage areas can lead to sink hole formation, implying no immediate concern to the surrounding houses? I realize this is probably not an answerable question for my specific sink hole, but I thought people here might have some insight into whether or not drainage areas might cause their own sink holes.

In case it isn't clear, this drainage area has no drain point itself. During heavy rain, water flows into the drainage area and then slowly evaporates or percolates into the ground. In case it matters, when it does rain, water typically collects on the opposite side of the drainage area than the side which contains the sink hole. I.e. the sinkhole is on the "high" side of the drainage area (although the elevation difference probably isn't more than a dozen centimeters).

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  • $\begingroup$ To give an answer one would really need a lot more information. The largest bit of information is what type of bedrock is this development on. The true sinkholes vs holes due to erosion and settling only occur over rocks that are soluble, for example limestone and sometimes gypsum based strata. Even if the development is over limestone the issue could still be simple erosion or subsistence. Given the size of the hole developing that you describe I would suggest it is time to get a professional onsite opinion. $\endgroup$ – Friddy Jan 7 at 16:57
  • $\begingroup$ @Friddy It's Florida, so I can answer that with certainty: limestone. Pretty much the entire state is built on top of limestone. There's no real bedrock in Florida. en.wikipedia.org/wiki/Geology_of_Florida That's what makes us the land of sinkholes, springs, and cave diving! $\endgroup$ – conman Jan 7 at 17:11
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    $\begingroup$ I'm agreeing with @Friddy: Get a professional on site. Whether what you have seen is true sink hole or a hole resulting from erosion is a technical & moot point. There is a significant sized hole in the drainage area & another one potentially forming. It needs to be investigated by a geotechnical professional. $\endgroup$ – Fred Jan 7 at 17:19
  • $\begingroup$ @Fred The HOA put a makeshift fence around the larger hole. I'm pretty sure that should discourage further sink holes from forming. $\endgroup$ – conman Jan 7 at 17:20
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Having had some time to do some digging (hahaha), I have as close to an answer as I believe is possible.

Small update just because: one of the smaller sinkholes has dangerously grown, in a way that I wouldn't believe if I didn't see it. It has widened slightly to about 45-60 cm, but grew substantially deeper. With a tape measure I clocked it in at 3.3m. No joke. It's pretty much a death trap to small animals or children. I do not take my children back there anymore nor does anyone else seem to play there, fortunately.

Onto answering my question. This is Florida, the land of sinkholes, and it seems that the assumption made by all parties involved is that this is an actual sink hole, and not some obscure side effect of the drainage area. After querying my HOA I learned that the area is owned/managed by the local county government. It doesn't sound like they bother doing geological studies (because sink holes are so common around here), but instead just fill sink holes in as they appear to avoid liability. They are actually aware of the larger hole and put it on an "official" sink hole map (which will probably have a minor effect on my insurance rates). They will also fill it in. However, they are being quite slow about actually fixing it (which is a surprise because of the potential liability issues).

From what I have read on various government sites, sink holes around here are assumed to be fairly independent, i.e. a sink hole within 50 feet of your home doesn't imply that your house is any more likely to experience damage from a sink hole than any other house in the general area. However, since the organization responsible for filling the hole is not going to be performing a geological survey, nor are they likely to bother consulting professionals, a best guess is the best I'm going to get.

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