Afternoon. Been lurking on and off for a bit. Just joined & wanted to introduce myself. Just relocated from NYS to Maysville, NC. Got about a 1 acre excavation the won't hold water. Silty clay mostly but theres a sand layer about a foot above the bottom that allows the water to drain out. Got a quote today for bentonite thats over $40K. Seems a bit pricey buy I dont really know. Will be seeking advice about how to proceed.
Thanks for letting me in....really look forward to getting input and learning from the folks one here

Welcome to Pond Boss, John!!

Welcome John!

I am not a pond building expert, but here is my two cents anyway.

That sand layer has got to go, or has to be sealed away from the pond water!

How thick is the sand layer? If only a foot or two, then I would think your least expensive option would be to excavate and remove it. (Perhaps stockpile the clean sand on shore and later use it for a family swimming beach?)

When churning around in the bottom of your pond site with heavy equipment, you never quite know exactly what is 1" below your disturbed material. Therefore, you usually excavate a little past your total designed depth to ensure you are still in clay, and then pack the disturbed material back on top in a nice compacted sealing blanket. The other option is to heavily scarify (disc) the bottom of the pond. If that is all clay, then you uniformly compact that material back into place.

If the sand layer is very thick, you must seal it away. (Another option would be to sell it if there is a decent market for sand in your area.) If your pond is close to an acceptable depth, and you still have some silty-clay soil in the basin, then you can still seal off the sand - probably for much less that $40,000.

You need to put in some survey stakes in all of the sandy areas. After pounding them in, mark the 6" line and the 12" line. You then need to make a lift of clay that (when compacted) covers to the 6" mark. You then need to do another lift of compacted clay that finishes at the 12" mark. If your pond is deeper than 10', then this sealing blanket probably needs another 6" lift to be sure.

To properly make a compacted lift with silty-clay material, you need to wet the material to the optimum level and use proper compaction equipment. (Just driving over with the dozer treads does NOT create sufficient compaction.)

The bentonite (or other commercial products) can work well when there are some leaks in a sand-silt-clay bottom pond. However, the large pore spaces in a clean, coarse sand in a discrete layer are very difficult to seal.

I hope that gives you some ideas to perform some more evaluations at your pond site and determine which options might be feasible.

Good luck on turning your new hole into a new pond!

I would start over and get rid of the sand layer personally. I know that's not always practical.

Thanks for the welcome and for the advice. I wasn't very clear in my description. The bottom holds water but about a foot up there's a sand layer that prevents the runoff from filling the pond. I'd attach a sketch of a cross section but cant figure out how to do that.
Anyhow, since the original post I found a local geotechnical engineer who had spec'd clay for a detention pond at Camp Lejeune. Spec was for permeability no greater than 0.00003 ft/s. The material supplier is right near by & the lady tells me they're still mining that pit. I expect to get pricing for enough to line the whole bottom come Monday when I talk to the boss man.
Thanks again.

I think I figured it out. Maybe you can see the sketch now.

That is even easier to fix than sand in the bottom!

If you still have silty-clay material on site, or you still have good material in the bottom of the pond, then you just need clay blankets to seal that sand layer.

For a water depth of 10' or less, two six-inch lifts of compacted clay should be sufficient.



This type of compactor can easily roll up and back down your pond side slopes. Rental in my area is only $1,500/day. (The transport fee would be around $200 each way for me.)

Cat CP44 Vibratory Soil Compactor

You might be able to do the entirety of a 1-acre pond in two days? You would also need a small-medium dozer pushing material onto the side slopes. It could perhaps be accomplished in a single day if the sand layer is only present in part of the pond slopes.

You also need to wet the clay blanket material to the correct moisture content for optimal compaction. Is there still water in the bottom of your pond basin? You can use that to spray the dozed material after each load.

I would definitely ask the geo engineer what moisture content was their target for the detention pond, and if there is an easy way for you to estimate moisture content on your project.

Finally, do you have a small tractor with a disc attachment? After sealing the sand layer, I would recommend discing the other side slopes and the bottom of the pond in a few passes with different orientations. Wet that as needed, and then roll the heck out of that while your compaction equipment is still on site. Hopefully, that will give you a pretty tightly sealed pond.

Hi John

Welcome to the forum. I help people with Pond Construction, rehab, and leak abatement nationwide, and I’m happy to share my experiences and provide some insights moving forward. I’m a volunteer moderator here and my time is always free to fellow Pond boss members. Feel free to reach out anytime, happy to help in anyway I can.

tj@hudlandmgmt.com