Pond Boss Forums INTRO Questions & Observations Value of a Test Hole?

I had a test hole dug a few weeks ago in the area that I would like to eventually build a larger tank. The soil is black dirt and clay to about 4 feet, and then a lot of white rock below that. I mainly dug the hole to see how well it would hold water, but it raised a few questions in my mind about what can really be learned from it.

There are many other tanks in the area built on the same soil, most of which seem to hold water very well. The farmer that runs our place has a tank on his property built on almost solid rock that's been there for 20 years and never gone dry. When I talked to him about the construction techniques of his tank, it seems that he broke just about every rule I've read in the ag handbook or on this site, yet he has a nice looking tank that holds water and that doesn't appear to leak. He told the dozer man to put the ripper down until he got the depth that we wanted, didn't pack dirt on it, didn't key the dam...but it worked. I realize that his case is probably more luck than anything else, but it illustrates why I question what I can tell from the test hole that I now have.

Below are a couple of pictures of the hole when it was dug, and about 2 weeks later after a rain.




It just seems that there are so many variables involved, that you can't really tell from a test hole whether or not your tank will hold. The test hole may seem to hold water, but what about when you build a larger tank, with more surface area and much greater hydraulic pressure? I asked the dozer man whether or not he thought it would hold water and he kind of shrugged and said, "Maybe. Maybe not." Honestly, I kind of appreciated that answer because it seems that he's exactly right.

So...the question is...how do you correlate what you see in a test hole vs. what you might expect with a larger tank?

D. I'd like to think the reason nobody is helping you is because there is no good answer. I would expect you need many test holes, and your question as posed in the last sentence mentions only one test hole. It could be consistant or it could be the only one like it.

The more test holes the better. Also, the neighbors poorly built pond that works could be a good sign.

Good luck.

Glad you popped this up, PI.

IME test holes are dug to determine what soils are actually located at the dam location (as opposed to interpolating this from soil maps based on very large area samples), and perhaps over the rest of the pond. While I am used to several small holes, your one big one would seem to serve the same purpose (with the added encouraging bonus of a visible pool of water).

One thing I would want to know if I was in your shoes is the makeup of the rock 4 feet down. Does it seem to be solid, nonporous, and unfractured? Those would be good indications. Is it porous sandstone, easily dissolvable limestone, or all full of cracks? Those would be bad; in the presence of such problems I'd want to keep my pond water away from the rock with extreme prejudice by compacting some of that clay on top over it really well. Based on experiences posted here - I'd want 2 feet of clay compacted over "bad" rock.

There are undoubtedly many different bedrocks in the Great State of Texas. Perhaps one of the TX geology types can help identify what rock you have to contend with.

I'm not real qualified to answer you but your dirt might be similar to mine. Most ponds here hold water well. I have seven ponds dug by someone else (one is a dairy lagoon and one ins a cooling pond for cows) and I have dug two and deepened out a third. Two of the ones dug by someone else and the one I deepened were not constructed properly. You can see that they didn't move the black topsoil away or core the dam first. They leaked slowly.
With such good soils in your area you probably just have to make sure that you move the black dirt away first before you start the dam and compact it some with the dozer.
What I did with my ponds was move the black dirt and then when I started the dam I built it toward the hole so the top of my dam went straight up instead of the top getting farther from the hole. Does that make sense?
In the pond that I dug deeper, you could clearly see that the dozer guy pushed the clay up, over, and behind what he previously just pushed and it left a black seem. I put about two foot of clay on top of that seam which essentially moved the dam in. I have yet to get enough rain to see results.
As far as the rock, two of my ponds have rock bottom and even if there are cracks, they are surrounded by the same clay that holds water.
I think you'll be okay but I can't guarantee it.

Thanks for the replies. I do see the benefit in doing some test holes closer to the dam location to see what's underneath. I think the depth of the rock is pretty consistent around the area, but I'll do some checking to see if one spot is better than another.

Brian, I have the same feeling about the rock bottom. A creek that runs through our property has several pools with solid rock bottoms and they stay full all through the summer (long after the creek has stopped running). I know it's probably less than ideal, but there's not a whole lot that can be done about it short of a liner.

We got quite a bit more rain down there in the last few days, so I'm anxious to get another look at it. I'll try to post some updated pics after next weekend.

Please call my office at 940.665.2258 or email me at mikeotto@ottosdirtservice.com and I can help you with this problem. There is someone at my office mon thru friday from 8 to 12.

Did you call Otto? I'm curious on what he has to say. Keep us posted on your project.

The reason to dig test holes is to understand what kinds of soils you have. Test holes are normally dug straight down with a backhoe to look at layering. When a dam is built, the earth mover uses different soils for different purposes. Soils are graded based on porosity. Least porous soils are "processed" (mixed, pulverized and compacted) and used for backfilling a core trench and for the middle and front side slope of a dam. A good earthmover can figure out what types of soils you have, and work with you to use those soils appropriately. Most of the time, especially in north Texas, the biggest problems are having too much good clay or not nearly enough. Building a dam is a combination of talents, from measuring and quantifying the watershed size to calculating hydraulic pressure to figuring out how much clay-based soils are needed to build a dam to match that site, and then actually putting all that to work building the dam. I have watched Otto take a site which shouldn't have a pond on it, take layers of porous soils, mix, pulverize, crumble. move in short lifts, add clay with his scraper from hundreds of yards away, pack, and do it over and over to build a dam that doesn't leak. He did it at my house. Test holes showed crumbly, eroding soils that could not possibly be used for a dam. So, we moved around and found great clay in two different places. He got rid of the "bad" dirt and replaced it with clay, mixed with moderate soils nearby and built a great dam.

I haven't had a chance to talk to Otto yet, but hopefully I can catch up with him this week. I went down to the farm this weekend to dig a few test holes with a backhoe that I rented. The results were a little disappointing, but pretty much what I expected. It seems the rock layer is consistently the same depth anywhere you dig. I was hoping to find a spot to place the dam where the rock was maybe 6 or 7 feet down instead of 4, but no such luck. I was trying to use my dog for scale in the picture, but I couldn't convince him to jump in the hole:



Here's another one in a different location:



The good news, though, is that's it's been raining and the test hole is nearly full: