How to Build a Nature Pond with Stone & Bentonite Clay

Question

I'm in the process of designing a nature pond to raise some small fish and attract amphibians. Its requirements are the following:

1. Contain a round central chamber 2 ft across (diameter) and 4 ft deep
2. Have an irregular shallows extended from the central chamber 2 ft on either side, sloping from lawn edge down to 1 ft deep at edge of central chamber
3. Maintain a minimum digging and clay laying area for the benefit of labor, materials and plant roots
4. Not use a plastic liner that completely blocks the flow of water or roots

My current idea is to use bentonite clay to seal the pond, in concert with a stone retaining wall.

The problem with bentonite clay is that it is not meant to be sloped more than 26 degrees, even when mixed in with the surrounding soil. If I did it this way it would expand the diameter of the area I have to dig and on which I have to lay clay from 6 ft to 16 ft, and could require up to 800 lbs of clay, instead of hopefully more likely 100 or 200 lbs. This could also interfere with our surrounding garden.

My questions are: will my idea of using a retaining wall to hold in the bentonite work, or must it be sloped at 26 degrees no matter what? Could I add some sort of plastic mesh around the wall to keep the clay in further, and if so how fine would it have to be? How think should the bentonite walls be? Any other thoughts or ideas for this project?

Please see my images below. The top view does not correctly display the slope it should. And the dimensions in neither diagram are necessarily to the scale I mentioned. You can't see the bentonite in the top view except at the very bottom because the rest is covered by a layer of soil.

Answer 1

Using clay is a time honoured tradition in building ponds. It can be done but can prove problematic if there any possibility of subsidence or water infiltration. The other solution is concrete with steel mesh reinforcing.

I live in chilly zone 4 in North America with clay subsoil. If I used bentonite clay after a couple of winters the heave thaw cycle would cause cracks and there would be a very hard to locate leak. If you have access to water that is not chlorinated to top up the pond then this might not be an issue.

I wonder if you would reconsider your desire not to use a plastic liner. Although it is not cheap EPDM pond liner is readily available, stable, non toxic and can be used with shape of pond including yours.

Professional installers will recommend that you put a drain in the bottom of the pond for ease of maintenance.

You can finish off the edges by edging the pond with two layers of heavy flagstone where the liner wraps around the flagstone in a "U" shape.

Answer 2

The reason ponds have plastic containers, rubber liners , and/or concrete is because it works. The only clay lined ponds I have seen are very large ,few to many acres ; where they can be constructed with bulldozers , etc, to compact the clay and may still use rubber liners and very shallow angles on the sides. I have built and a few ponds: For one as small as you want I would use a plastic tub . Alternatively , line it with cinder-block or brick ( vertical sides) and put in a rubber liner. That is how my largest pond is made, brick on the top edge- bring the rubber over the brick- put flagstone (or similar) on the rubber. The rubber is not visible and it looks natural. I suggest reading books on ponds or even the internet. Your size will be too small for cold water fish but small tropicals will be good until the water temperature drops below about 55 F. Frogs and toads will show up by themselves ; I have even had a few turtles show up.