From sinkhole to pond

Old Guy in Tenn · Aug 6, 2020

We had a sinkhole in the field just up from the house. It always seemed almost like a ready-made pond, and we talked often about finishing it 'some day'. We are right now frantically trying to get the house livable by the end of the year, so I was very surprised when my wife suggested we get the pond scoured out now. For the last 2 days Trevor has been pushing dirt with his Deere 450G dozer. It started out as we expected with a few inches of soil then gravelly red clay. As he got deeper into the hole it changed over to deep pure topsoil, apparently washed into the hole a grain at a time for the last few thousand years.

It is funny, a few months ago I paid to have a load of topsoil delivered, now I have hundreds of similar loads in a big pile next to the ever-growing hole. A few hours ago the topsoil started changing to a more clay colored and sticky texture, so it is probably time to stop digging. After lunch he is planning to shape the hole a bit and cover it with some of our red gravelly soil. He will get that from a hill close to the house, scraping a low slope water access for the kids and their kids.

It has to fill from the watershed above it, augmented by what we can spare from our spring. Will probably take a year or more to fill. I hope it holds water!

ovrszd · Aug 6, 2020

Interesting project. I'll be curious how it turns out. Keep us posted. With pics please. Thanks.

Old Guy in Tenn · Aug 7, 2020

He finished late yesterday. Planned water level is against virgin soil, freeboard added to about 3 feet where needed. Overflows will be added sometime next year, it is expected to take over a year to fill (we missed most of this year's good rain).

Getting the topsoil out was tough. It was like a big funnel, and the topsoil was pretty deep in the center. Finally got to where the soil turned into a sticky clay mess, still darker than surrounding red clay but no longer loam. Scraped red clay from around the top to bury the center under 3 - 5 feet of clay. Should be enough. Pushed up some of the fill to level the road along the woods to the west. Made a shallow slope into water on the side closest to house for easy access by kids of all ages.

Pics came in out of order, so... The dozer on top of loam pile is deceiving. That pile is a good 40 feet wide. The pic where he has pushed up just a little soil is fun. 2 hours later he pushed the soil pile 50 feet back and then it just kept growing. Took the DONE pic from upstairs. We shot the expected water line with a transit. Pond is 12 feet deep in the center, and 95 feet across at the high water level.

ovrszd · Aug 7, 2020

Thanks for the pics and update!!

Tinhack · Aug 7, 2020

Man, someone has bigger ones than I do!

I would NEVER take a dozer into a sinkhole. It sank for a reason and more often than not, there's a cavern under it. You may be waiting more than a year for it to fill.

newbury · Aug 7, 2020

Yup, sinkholes are not a safe place. A pool of water above a sinkhole in karst topagraphy may collapse at any time.

Clairborne County has some big ones. Tennessee Landforms -- Sinkholes

Old Guy in Tenn · Aug 7, 2020

Good feedback. Triggered me to do some more research on sinkholes, and I expect we will be ok. We have a very thick (100 feet) ledge below us. It is exposed in several areas on the hillsides. It seems that we have a subsidence sinkhole, where surface soil is carried by water through cracks in the ledge to fill a void, or be carried to daylight, somewhere below it. The topsoil 'funnel' in the center of the sink lends credibility to that. We capped the funnel with several feet of good clay, so the water-powered soil conveyor should be gone. 7 years ago this was young forest, apparently grown up in a long-abandoned pasture or farm. The trees in the sinkhole were no different in any fashion recognizable to us from those around it.

Time will tell. If we sealed the pond well it should slowly fill over the next year or 2 and then stay full. If we did not seal it well it may indeed never fill, and go back to being a sinkhole. If we sealed it well and the ground below subsides, we may have a surprise ending to our pond. Time will tell.

MikeFarm · Aug 8, 2020

Hi

newbury said:
Yup, sinkholes are not a safe place. A pool of water above a sinkhole in karst topagraphy may collapse at any time.

Clairborne County has some big ones. Tennessee Landforms -- Sinkholes

Newbury is spot on. By putting a pond above it you you may well be setting yourself up for a very big future problem. Also check your legislation as to whether you can go modifying sinkholes on a Karst landscape. In your first pic those rocks at the front did look like limestone. Some land owners do even worst things though - they use them as rubbish tips.The water that goes into sinkholes comes out somewhere else; either on your property or someone else's property some miles away, sometimes as a spring.

Mike
PS. I even used "miles" instead of "kilometres" to bridge the cultural gap