I've got a sinkhole in my back yard of a tear down home (torn down from an old duplex from 1950 and rebuilt). The hole is about 5 feet wide and 4 feet deep. The corner of the deck has sunk in the hole and is temporarily hanging in the hole.
I've had a contractor offer to fill it with concrete for $700. They mentioned the possibility of a drainage issue but my drains are newly and correctly installed. A similar post (without an answer) stated that if this issue persisted into something larger we would have a huge boulder that would need to be removed on top of whatever else may be causing the problem.
I also spoke with a few architectural landscapers and they gave me large price points to JUST examine and determine the cause of the issue. One of them said it may be an old septic tank from the original property. I am located in the Atlanta area and have not heard of this possibility. I've heard of debris pits but not sure that's the issue.
My question is what should I do? Has anyone done this type of work before? Is there a good idea of what causes sinkholes in this part of the country (Atlanta area)? I'd appreciate any help. (I currently have it lifted off the ground with some cinder blocks.
UPDATE: I had a landscape architect come out this afternoon and he is adamant that it is an old well. One from before 1950 most likely. He cited his 20 yrs of experience but in the last 2-3 years he has done about 3-4 of them a year as houses in my area (Atlanta, GA) are getting torn down and rebuilt. His plan would be to dig down deeper, about twelve feet or until water is found, and then cap the well with concrete. After that he would fill with dirt to the top of the sinkhole.