I have a sinkhole in my backyard along the fence line which began expanding during the last few months. There's a slight depression in the ground about 6' in diameter centered around one of the fence posts which is set in concrete. The fence post at the center of the hole is about 6" lower than the other posts. A hole just opened up on my side of the fence that's about 1' in diameter, and kind of shoots down at a 45° angle into my yard. My neighbor actually pointed it out, because the hole is more noticeable on her side of the fence.
How can I fix this? My neighbor says they've put a truckload of dirt down it without any real results. She told me yesterday that I needed to go stomp on my side of the fence and pack the sinkhole in, which sounded like a very bad idea to me. She also wants to pour concrete down the hole in an attempt to fill it up.
I'm worried that if dirt didn't solve the problem, concrete won't either, and we'll be stuck with an area that we can't excavate because there's a giant man-made concrete boulder in the way. What are the general steps involved with identifying the underlying cause of the sinkhole and the best solution to fill it in?
Follow Up: Our fence got mostly destroyed by high winds from a nearby tornado, so I went ahead and had the fence company repair the sinkhole while they were replacing the fence. They brought in a mini-backhoe (good call @shirlock) and started digging.
The sinkhole was filled with all sorts of construction debris, and something that may have once been part of a port-a-john (gross). Five feet down, it seemed like the debris was thinning out, so they stopped digging and poured a concrete footing to support the fence post.
We'll see if it holds up. I just hope we found the bottom of the sinkhole. It's hard to know since we couldn't excavate on my neighbor's side of the fence as well.