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I am having a dug and dammed pond excavated and the topsoil has been stripped. We have found gravel at various depths below a workable clay cap, the shallowest being 2-1/2 feet down, have not reached it in other areas. The comment is often made that 1 ft of good clay will be enough to seal a pond at 10' and we are not planning on going more than 8 or 9. My questions are

If we are careful about leaving a good 1' layer of clay and compact it with a sheep's foot roller, are we safe? Anything else we should be doing, or can do to ensure a good seal?

Thought this could be an issue in some of the core trench area. They ran a test hole 50' out and the gravel went down a ways. We dug about 10-12' down in one area and more gravel, but that area had 5' of clay. In the end they were able to get to clay through the 4' deep core trench. Don't know if there is gravel below but hope any gravel below will not be an issue. Good or bad assumption?

Any input would be appreciated.

Last edited by Retired on 40; 05/06/22 06:48 AM.
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I'm no expert, but if you'd have told me I can have 12" of clay throughout my entire pond build, I'd have been ecstatic. I have gravel seams around and below mine, but it'd doing well thus far.

I'd do my compaction properly and roll with it. Unless you wanna spend a bunch of money on mixing in bentonite.

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I am NOT a pond building expert, but I am a geologist.

You have observed gravel layers in close proximity to clay layers. That means your soils are unpredictable. IMO you are going to have to dig at least 12" past your bottom and side slopes in all locations and then backfill to the final grade.

USDS Pond Handbook #590 states:

"Generally, soils with at least 20 percent passing the No. 200 sieve, a Plasticity Index of more than 10 percent, and an undisturbed thickness of at least 3 feet do not have excessive seepage when the water depth is less than 10 feet."

("Passing the No. 200 sieve" means 20% clay content.)

Basically, if you leave your bottom in undisturbed clay but it is only 6" thick above the next gravel layer, then your pond will probably leak.

The other consideration is that soils in place are already compacted!

Some of your clay areas almost certainly have thin layers of sand or even gravel situated within the bulk clay. These layers will allow water to leak out of your pond.

Generally, you cannot use compaction to get rid of these permeable layers. The soil must be "disrupted" and then well mixed prior to compaction.

"Scarify the soil to a depth of 16 to 18 inches with a disk, rototiller, pulverizer, or similar equipment. Remove all rocks and tree roots. Roll the loosened soil under optimum moisture conditions in a dense, tight layer with four to six passes of a sheepsfoot roller in the same manner as for compacting earth embankments."

Can you spoil the gravel outside of the pond?

If so, then I would stockpile clay in the bottom of the pond while working to get the gravel out. Cover everything in a 6" lift of material with decent clay content and compact. Then do the same thing again with another 6" lift of material.

You DO NOT need pure clay for these sealing blankets! However, 20% clay content is the minimum. You should be able to moisten the blanket material and roll a thin "worm" between your palms.

Hope that helps. Good luck on your new pond project!

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I work with a lot of sand/gravel in NE - it’s our sealing nemesis. Happy to share my strategies but much of it is reflected here compacting thin lifts of clay (4-6”) depending on plasticity and often we will incorporate a blanket of bentonite or polymer as insurance. Building a pond correctly costs a fraction of chasing leak rehab later. Feel free to contact me if you’d like to review - as a volunteer here my time is free to the Pond Boss family.

Tj@hudlandmgmt.com


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All good advice so far.

A compacted clay blanket at least 12-inches thick is necessary with suspect soils as you describe. Mixing or "processing" clay soils is a must, with proper moisture content to get adequate compaction.

Here's the analogy I use all the time, "What holds water better than anything on the planet?....a bathtub, right? Except for that little-bitty hole down at one end." Make sure your pond doesn't have any of those "little bitty holes" and you'll be successful.

The Finger Lakes are notorious for mixed soils due to glacial movement 10,000 years ago. Drumlins, round rocks, mixed soil types is common there. But, there are some fantastic manmade lakes and ponds all over that part of the country, too.


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roymunson, FishinRod, teehjaeh, and Pond Boss,

Thank you for the insights. More work today excavating areas where the gravel is not present, compacting as they go. The gravel areas are mostly at the bottom of a broad swale, and the edges are free from this feature being "pure" clay (clay-gravel mix that compacts easily in your hand, but the pencil test will not work with this material due to the gravel portion). The clay compacts and turns to concrete when dry (I know from experience). The soils are Appleton in the low area and Conesus along the sides, both of which are supposed to be good for pond builds. The soil has a high moisture content and is on the edge of being too wet if we get any rain.

There don't seem to be distinct layers of gravel or sand within the clay, certainly not after pushed by the bulldozers. I have seen these in previous excavations and these "seams" appear more prevalent below the apparently dense top clay layer or at transitions between soils of different compositions.

The crew is well aware of the 1' minimum but will have to talk with them on Monday about how to ensure the clay is 1' thick over the gravel areas and mixed well.

They didn't run into gravel when digging the core trench but there are areas out there. As Bob stated, "The Finger Lakes are notorious for mixed soils" and much of the area they have dug so far seems OK. As to removing the gravel, that is a big unknown and would be an expense of indeterminate amount since we do not know how deep it goes. It was certainly at least 7' thick in one area we tested and the joke was that we could turn the pond into a gravel pit and sell the contents, then make it a pit pond. We found fine sand and 18" rocks down there, and who know what else. I don't want a failed pond, but I can't afford that added expense. If we can avoid disrupting most of it, and ensure the layer is >1', that would be preferable.

Keeping to a 1' cap without removing the gravel means we would alter the planned contour and raise the bottom 18" in one area, but dig deeper in an area where there is no gravel to get the depth. I will also discuss with the contractor removing the first foot of gravel and filling back in with clay. Removing the clay "cap" over these areas would ensure the we know more precisely the clay layer thickness and allow the layer to be better mixed.

I am willing to mix in bentonite over the suspect areas, that would be a good insurance policy. At what rate would you apply?

On the up side, I plan on tapping a 33 acre water shed that feeds an agricultural drainage ditch, with a weir and turnout planned, and we can get upwards of 2 million gallons from the runoff of just one 4" rain (almost enough to fill the pond) so hopefully have some cushion if we don't get if perfectly right. We get over 3' of rain each year.

If I hear you right,
If possible, remove the clay from the gravel areas
Mix all the clay well to remove stria and layer on to ensure you have 1'
Possibly use Bentonite mixed into the clay over any gravelly areas as insurance.

Let me know if I got this wrong and thanks again for your insights.

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Mike Otto, Bobs partner, and a pro pond builder in the PB Mag, once said something like “ Dirt leaks. Accept it.”.
We can mix it with clay to cut down on porosity but it’s always a crap shoot.


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Originally Posted by Retired on 40
Keeping to a 1' cap without removing the gravel means we would alter the planned contour and raise the bottom 18" in one area, but dig deeper in an area where there is no gravel to get the depth. I will also discuss with the contractor removing the first foot of gravel and filling back in with clay. Removing the clay "cap" over these areas would ensure the we know more precisely the clay layer thickness and allow the layer to be better mixed.

When you have highly variable soils, you might have to deviate from the pond design plan and just take what the ground gives you in some spots.

I like your comment above. If one of your planned "deep areas" is foot after foot of gravel, then quit digging and cover it with a good clay blanket. If you do find an area of good clay, then keep digging there and make that your new "deep area" of the pond. You can then use this extra good clay where you need it.

I do not like that your clay/gravel mixture cannot roll a pencil. It is impossible to compact a clay/gravel mixture once the gravel content gets too high. As you compact, the individual pieces of gravel start to come in contact with each other. Eventually you achieve enough point contacts that the gravel creates a framework of support (like the steel girders in a modern office building). No amount of additional compaction will seal this material because you cannot crush the gravel framework. Even though the gravel pieces are touching, water can still flow through that type of network.

Keep watching your crew while they are working and grab some samples (when you safely can). Most dozer guys don't like to continually get out of the cab to check samples. However, the good ones can "feel" what they are working in with their blade.

Keep interacting with the boss, he wants to get it right the first time too!

Have fun watching your pond get built!

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Keep in mind the 1' layer is not "place 1' of clay and compact"... 3 of the 6-8" lifts each compacted under proper conditions will "bind" best and be in the 1 foot thick range when done-don't stop there-when your seal is completed in reference to depth, go back and place a blanket of 8-12" soil and lightly compact. This gives soil for several types of aquatic plants to root in without going too far into seal and it protects moisture content IN seal from drying out too fast if it doesn't fill as quick as you hope.
Any sand or gravel you could stockpile you can use to make gravel beds in what you believe will be 2-5' water. Say a few areas 12X30'.
I'd highly recommend you get in touch with TJ.. He's very good as this stuff.

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This is a perfect scenario for blanket of polymer compacted followed by compacted clay lifts. Feel free to reach out to discuss.


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Thanks again for the insights.

They noticed the gravel areas on thursday sand spent Friday working in the good clay. I will need to talk wit the team tomorrow about how we go forward, and we are likely to get a clear picture of the full extent of the issue in a couple days.

Snipe, you are suggesting 3 lifts of 6-8" instead of 2, that seems a safer bet in any case.

teehjaeh, if we get 3 good lifts, do we need the polymer or are you thinking of that as an insurance policy? I'll know better this week, but that would seem to depend on the total area where we cut through the clay and into the gravel below. I am hoping that we can keep that to a minimum and not have broad areas to cap. But, we will see this week, and then polymer, (or Bentonite) would seem like good insurance.

FishinRod, good comments as usual. I think a deviation from initial plan is needed to avoid cutting into the gravel in that area. When I said it wouldn't necessarily pass the pencil test it was because of the size of some of the gravel, not the general amount. The soil is variable, as you said, but I successfully made some pencils, while at other times my rolling of the clay is limited by the aggregate size. When I have some gravel the size of a pencil diameter in even 50% clay, then it tends to be a weak link and limits my "pencil."

I am hoping we can agree on a plan of action tomorrow.

Thanks again!

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"Snipe, you are suggesting 3 lifts of 6-8" instead of 2, that seems a safer bet in any case."
If I was the contractor I would go with 3 lifts.. But I'm not. If you work with TJ, follow his advice, he has many years behind him with much success so work with him if you can.

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Just to emphasize the high variability in our area, just 1 mile from my pond in seemingly the same geology is a sand and gravel pit.(Bloomfield, NY). That sand and gravel goes seemingly on forever in all directions. Actually there are several pits right along rt5&20 very near me we order our driveway stone from. Somehow, at my house there is barely enough clay/pulverized limestone to dig a pond. It wasn't close to the house where I would have liked, but I got really lucky it held water at all further away (and yet I still complained that it leaked!).

The top of the hill near the house, the previous owners had a test hole dug, and I am told it was nothing but mixed cobbles under 2ft of light gravel mixed soils. a 5gal bucket of water would leave a wet spot the size of the rim of the bucket.

Anyhow good luck. TJ made a lot of useful suggestions with Soilfloc which slowed my seeps considerably when I applied it. Take advantage of his great advice!

If you have a variety of glacial stone types in your basin, quick get yourself a small 365nm UV flashlight with a filter for rock hounding. There are soda-stones (sodalite) in our glacial debris, and is actually worth a few bucks to rock hounds. Quarter-sized stones are selling for $45. Easy to find as they glow very brightly orange with UV, very cool rocks and the time to do it is at night after a light rain.


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