How do I fix a sinkhole in my backyard?

Question

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.

Answer 1

How old is the house? If it's fairly recent, do you have any idea of what the property was like before being built on? It's possible that the builder put down a lot of fill, and there was a large amount of organic material in that fill, or that he just dumped fill on top of what was there - stumps, trees, etc. Depending on the composition of the fill used, you might be able to get away with just dumping earth or sand into the hole, but it sounds like a large area - which means that digging it up, removing the rotting organic material, and replacing it with fresh fill is really the only way to permanently fix it.

You might want to get a local plumber to come out and "scope" the hole to see if you can tell whether or not this really is your problem.

Answer 2

Sinkholes need to be professionally investigated. The size and depth can vary but it could be part of a complex underground system or just a hole in the ground. To put it in context I know of one sinkhole that was filled with concrete and it took over 145 cubic metres of concrete to fill it. You really shouldn't try and fix it yourself because there is a good chance that you will think you have fixed it, but there remains an underlying problem which is putting your house or yard at risk. Similarly don't try and excavate it without professional advice because there is a good chance you could destabilise it further.

Answer 3

The problem varies depending on where you live. If you live in Florida, for example, the bedrock would be limestone, which tends to have underground rivers and caverns. This type of topology would require professional inspection. I just thought I'd put that out there since you didn't give (roughly) your location.

Answer 4

If you have to guess and fill without digging to find the real cause, I'd try gravel. There's almost certainly some sort of underground water movement that's taking the soil away - if you fill with gravel, there's some chance that the water will move through the affected area without issue. Of course, even if that works, it might only move the sinkhole.

Answer 5

Depending on the cause of the sink hole, filing it with concrete may be a viable solution. If I had to guess (which I am), I would say it is either caused by an under ground spring, or the collapse of a drainage pipe.

What could have happened is the top of the spring/drain pipe collapsed, this caused the soil above to continue to drain down into the spring/pipe and be washed away. Filling the hole with concrete could solve the problem, or make it worse. Once the concrete is cured it will not drain into the spring/pipe so it will not be washed away, however, if the concrete blocks the spring/drain it could cause flooding and/or other problems up/down stream.

The best solution here would be to excavate the area, to determine and fix the underlying cause.

Here is some good information on causes of sink holes from Wikipedia.

Sinkholes also form from human activity, such as the rare but still occasional collapse of abandoned mines in places like West Virginia, USA. More commonly, sinkholes occur in urban areas due to water main breaks or sewer collapses when old pipes give way. They can also occur from the overpumping and extraction of groundwater and subsurface fluids. They can also form when natural water-drainage patterns are changed and new water-diversion systems are developed.

Edit:

The first thing you could try is to disconnect the fence from the post, then try to pull the post and concrete 'root ball' out of the ground (this may require a group of friends, or machinery). Once the post is removed you may be able to see the cause of the sink hole, if not you could at least sure up the hole to reset the post.