Sinkhole

[Reset]

We found a sinkhole in our backyard the other day. After a bit of excavating the collapsed dirt I discovered a structure. It appears to be 4' x 4' cinder block, topped with blue plastic that has torn and and some wood that rotted. Do I need to remove the block and anything else I find or is it okay to just fill it in? i have no clue what this was for.

[pushkins]

What is inside the "structure" does it look like an old fire pit ?
You don't want to just throw some more dirt on top if the wood continues to rot, this could become a hazard in a few more years.

[Reset]

So far I haven't got far down. I see three walls at least three blocks high, but not sure if it goes down much further. The fourth side is very close to a slope and not sure if there is a wall or not. I will need to excavate more. Certainly doesn't look like a fire pit, the blocks are clean.

[Speedbump]

It could be a home made septic tank or possibly a well pit. If it's a well pit, there should be a casing sticking up through the bottom. You should plug the well with cement or bentonite grout before covering it.

[Reset]

I did some more digging. There is a pipe coming into it. Around the side of the house there is a drain in the sidewalk, never knew where it went. I tried to run some water through it and see if it comes out here, but it does not seem to drain very fast (clogged?) and I can't tell if it goes here. This is also near the current septic tank, then again I imagine there would be some smell to it. So this is a 4' x 4' block structure, no bottom, about 2' in height with plastic and wood on top that has rotted, no mortar on the blocks. I am thinking it was some part of the drain system. Would I need to do something with the pipe or can I just fill in the hole? This is close to a hill so presumably I could divert the pipe to the woods behind the house. If I filled it, should I dig out the top or can I backfill through the opening? Thanks for the input.

[Speedbump]

It sounds like a drain system of some sort. Filling it in might be ok, it all depends on what you intend to do with that spot in the future. If you plan on digging there for some other reason later on, you might want to remove the blocks first.

[Samus]

Sounds like a drain system to me, too. Any recent news how you handled the thing?

[Reset]

I exposed the pipe from above and found it was a drainage type, the ones with all the holes in it. Probably full of dirt at this point since they did not sleeve it with anything. Since this was behind the house I got several loads of street sweepings from our town and filled it halfway with that, then topped it off with fill dirt. Still need to get some topsoil and seed, but we've had some torrential downpours and doesn't seem to be caving in anymore. I expect it will still settle a bit but should be ok.

The drainage around the house has always been a problem after another. The road floods, so built up in front and the town put in a curb. The basement floods, so last year I installed a sump. Around the side there is a sunken sidewalk away from the house and the raised ground between sidewalk and house retains water, destroying the wall. I don't think they thought the house design through. Thanks for all the input.