Pond Boss Forums POND CONSTRUCTION Selecting a site ran into a cave

Was having a pond redone because it wouldnt hold water. The crew dug down a lot further and we ran into a cave. Tried digging it out since it is full of clay. I can see back about 20 feet since there is about a foot gap between the clay and the ceiling. Ordered a sewer camera to put back in there to see where it goes. Right now its a race to see if it is worth digging out or letting it fill up with water and see what happens.

Ooops

musicplayer,

Does that rock ledge extend laterally to both sides of the "cave"?

Can you tell the difference between a limestone, dolomite, and sandstone? All three types of rock formations are present at the surface in Polk County. The first two are usually continuous units over broad areas. There are some odd Pennsylvanian-age sandstone channels that occur in your area. Some of these are very narrow and winding, and are sometimes called "shoestring sandstones".

Do you still have contact with the original pond owners or the original pond builder? If they encountered that size of a bedrock remnant in the core trench, it would have been too big and solid to remove by normal excavation. They might have just packed in around it with clay. If the "cave" aspect extends through the core trench, that might be the source of your water leak.

Even if you observe a "plug" in the cave using the sewer camera, there is a decent chance that the plug does NOT hold water, especially when the pond approaches the full pool level.

Do you have a nearby source of water? If so, you could rig up a trash pump and some 2" or 3" hoses to pump water into the cave. Do NOT enter the cave. You could probably change over to a 20' piece of solid PVC pipe and run that into the back of the cave. If you can effectively pump water into the cave, you MIGHT be able to determine if it is the source of your pond leak.

The water will probably just run back out into your pond basin. If so, you might need to build a sandbag wall at the upstream end of the solid PVC pipe and form a dam so you can build a little hydrostatic water pressure in the cave. Look on the backside of your actual pond dam (or the pond side near the cave) for any sign of water coming through.

Sorry, for the bad luck on the discovery the cave. Too bad the cave didn't contain a few bags of gold coins to relieve your pain.

Good luck moving forward on your pond project!

Caves are cool. Hoping you can use this as a plus!

When they cored the dam they went down to solid rock and it was all flat. Not sure what rock material it is. Doesn't look like sandstone anywhere. Yea The rock extends on both sides of the cave. Unfortunately I dont have any water near by to use a trash pump. There is water that comes out of the bottom of the clay that is packed in the cave. I'm hoping that whatever the system is, it ascends so the pond will hold water. I'm not too hopefully about things working out though. I guess if I find that the cave goes further than I can see right now, and the pond doesn't hold water I will enjoy exploring a cave.

I bet that either Michael Gray or Mike Otto have dealt with this at some time. M. Gray is in TN but it might be worth sending him an email (mgrayconstruction@yahoo.com). Since he has told many stories of working in leaky rock formations, he might have an idea of how to deal with it. He has an article in the July/Aug PondBoss and lists his email.

Thanks for the advice. I will put the camera in the cave tomorrow to see what I see and reach out to them.

Well if the cave has a system to it and it goes down instead of up I'll probably just dig out the cave and have the guys dig down the spillway to drain any water that gets in there. I've always wanted a cave to be on my property. This seems like a prank from the cosmic joker though to put it in the middle of my pond lol

A cave would be cool to have.

OK, I expected that the rock layer extended to both sides of the cave, and that they probably encountered that layer when excavating the core trench. I don't know how close you are to Crystal Cave, or Fantastic Caverns, but there are many, many small caves in your area besides the famous large caves.

The problem with having a cave in that geological formation on your property is that ALL of the rock layer has been subjected to surface waters that were capable of dissolving limestone/dolostone.

It is quite likely that there a multiple smaller flow paths through this rock layer across your pond site. Typically these paths started as joints (small cracks) in the rocks that have been slightly widened over time by the dissolving fluids.

These water flow paths can be sealed off, typically by installing a well-compacted "clay blanket" at least 12" thick over the entire rock layer. If the water depth is more than 10 feet, then add 2" of clay for each additional foot of water depth.

Hopefully, that is how they covered the rock layer in the core trench. However, they probably needed to put a similar blanket over the rock layer across the entire pond basin. (Do you know if they did that?)

Likewise, you can probably seal the cave by moving clay to the entrance and compacting a "plug" across the entrance.

1.) I suspect this is a common problem in your area. Those geologic formations cover a very broad area of the state. If you (or the previous owner) used an experienced local pond builder (rather than dirt mover) then they probably have dealt with this problem several times. Ask your contractor how they fixed it in the past. However, once you complete a pond like this over a permeable rock layer, it can be nearly impossible to find all of the significant leaks if they didn't get sealed during the initial construction phase.

2.) It is very difficult to perform good geological analysis from 250 miles away and without directly observing the rocks! I am trying to give you some ideas of "how to think about your problem". Consider this advice much less than "rock solid".

Hopefully you can solve this problem and get a good pond.

We talked about packing it full of clay. where the water is seeping out there is a troth cut into the rock like water really gets to moving through there maybe when it rains a lot. Right now my area is in a moderate drought. There was also a big hole in front of the cave along the path of that troth that has water in it which makes me think water has been eating at it for a very long time. They packed the hole full of clay. We are going to see what happens when we get a big rain and see if water makes its way out of there. There is still some rock showing above the cave that I forgot about until you mentioned it. I will have to bring it to their attention just in case they are not aware of it. They are kind of the go to guys for ponds around here. I've only heard good stuff about them. They do admit that they have never encountered anything like this though. I put that sewer camera and it seems to only go about 30 feet and I did not see any other tunnels. That camera is not the best and the light i added to the camera didn't work as well as Id hoped. I appreciate the advice this is definitely out of my realm of expertise.