Where's my dirt going?

I've lived in this place for 21 yrs. When I first moved in, there was a huge indentation in the back yard, approx. 30'50'x8" deep. I brought in 40 yards of topsoil to fill the spot in. My yard isn't flat, but has a slight slope all the way around.
About 6 yrs. ago, I brought in another 10 yards, because of a the dip reappearing.
Now, I have the same area of 30'x50, but it's about 24" deep. It's looks like a sinkhole!
All utilities including sewer run out front, so I kind of figure it's not from that.
As a side note, about 10 yrs. ago, there were some properties about 20 miles from here, which the houses fell into some old mineshafts. The shafts were never charted, so no one knew they were even there. Because of this, I had mine insurance added as a rider to my policy.
It's now gotten to the point of getting me spooked.
Who do I contact, or what's the first steps, getting in touch with the proper agencies?
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Where you at? I got loads of soil and a pickup truck, take all you want (but bring the truck back...)
nate
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...

There's any number of things it could be. Could simply be a poor infill job on a bit of swampy ground, or an old basement that wasn't backfilled properly.
It's not unusual for a depression to reappear after filling. The dirt you add to get rid of it compacts down. It's particularly evident in "new" driveways in rural areas, where surprising amounts of rock fill are sometimes needed to make it approximately stable, and every so often you have to add more until it finally stabilizes.
Our area is _very_ fine sand over relatively shallow bedrock with marshy areas. There's one road over 100 years old that _still_ has to be raised several feet every time it's repaved. The white oak logs they laid down in the late 1800s to keep it from sinking are finally rotting out...
We have "lines" of sinkholes (usually no more than 4' wide and 6' deep) running across our property, some of which are completely covered with vegetation, others that have only tiny holes at ground level. A certain caution is required driving the tractor near them....
You'd expect the compacting down to lessen each iteration tho...
This depression appears to be increasing each iteration significantly... That's not good, and needs to be investigated.
Contact your local govt. They _should_ have maps of such things and might be able to tell you what was there, but more importantly, they'll have the contacts with the appropriate agencies and can advise you what to do/who to chase.
It may just be that the infill was too fine, and is sitting on top of bedrock with a horizontal water flow along the bedrock washing it away. This is what our sinkholes are... Digging it out, laying loads of gravel, and then recovering with the dirt might be what it needs.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
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If'n it were my house, I'd call my town hall first. Not because I'd think it was their fault, but because they would send someone over for free to take a look and advise. I don't know if all towns or cities would do this, but my town is pretty good at keeping us happy and ensuring that property issues are handled before things get out of hand. Can't hurt to drop that dime.
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From my experience in two different houses: 1) A sewer pipe 5 ft down has a hole and every spring when the soil gets saturated it flows down the pipe to the tune of 1 inch settling per year. Village needed to be called.
2) I live in a forest and I can only presume that when they leveled the lot for the house they laid the big trees down in a bunch and leveled the lot over them. They are 3 ft down. I am talking 2 ft diameter trees. Every year I have to find 3 inches of soil to fill the hole in as they decay. The house is 40 yrs old and it still settles . I had some well work done with a back hoe that found them.

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Nate Zigler wrote:

Make sure your insurance is in force and then call your city's building department or engineering department.
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Nate Zigler wrote:

OK, what's the topography - flat land, on the side of a hill, base rock chalk/ limestone? Others have made comment about the possibility of sewers (could be an issue in a chalky/ limestone area). Water mains? Unlikely in that the supplier is likely to note a loss resulting in two foot of soil loss in 6 years, (at least here in the UK).
How old is the property? My first thought when I read the start of your thread was former mining - which you referred to later on. What do you know of the former use of the area?
Just thoughts; not a solution I'm afraid.
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Nate Zigler wrote:

Southern Indiana and Kentucky have that happen pretty often, in hilly country. Underground water flow makes caves by dissolving limestone. If a hole pokes up to the dirt layer, it can carry dirt away with it. Most of these are rather tiny, of course, and the caves too small for human exploration. But some of them are huge. Cheap developers have been known to put subdivisions on top of a layer of fill, on rotten substrate like this. So, sometimes people get a hell of bonus room in the basement.
aem sends...
aem sends...
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wrote:

I can not think of the name (word) these companies use. But when you buy property, they handle the titles, or deeds, or abstracts (whichever you use in your area). Anyhow, I had to go by my local one some years ago, over a property line dispute. When I got there, it was late in the day and the guy was going to be leaving in a half hour, and said he was just sitting around since his work was done for the day. I asked him for the measurements of this "line", and he printed a map and wrote in the measurements. While he was doing that, I was looking at a map on the wall that showed all the property owners and creeks and elevations around my place. When he finished my map copy, I asked him about the creek behind my farm, and where it goes. Little did I know that I'd get to see an entire map of the whole creek all the way to a large river, some 30 miles away. He told me about a hidden spring that is located under the county's equipment parking lot, which was capped and fed into culverts, and these culverts go under an entire nearby small town. He told me it first appears behind one of the restaurants in town, then goes back into culverts, and come out again by a gravel company. It was extremely interesting, and I think he was glad to have someone to talk to before leaving for the day. He told me about some of the iron and other metal deposits in the region and that there was actually a mine in this town in the 1800's, which was later filled in, and that is where the small lake came from in the town parks, and that lake is fed by these culverts. I would have never known any of this.
Anyhow, that is the place you need to go. Look for a company that advertises land abstracts, or something like that. They know most everything about the land.
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Nate Zigler wrote:

You don't mention your general geographical location, and aioe doesn't provide any clue in your headers. That makes it difficult to say what is probable versus merely possible.

In addition to the possibilities already mentioned, it could be a collapsing cistern or old septic system. Has your home been on city water and sewers from the day it was built?

Very wise move.
--
Tony Sivori


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...

It'd have to be a hellaciously huge cistern or septic tank to cause a 30' by 50' depression. Short of some sort of abandoned municipal-sized water supply/sewage treatment I don't think so.
I can't think of scenarios where a residential-sized unit could cause an indentation much larger than, say, 10'x10'.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
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