I'm in the process of designing a nature pond to raise some small fish and attract amphibians. Its requirements are the following:
- Contain a round central chamber 2 ft across (diameter) and 4 ft deep
- 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
- Maintain a minimum digging and clay laying area for the benefit of labor, materials and plant roots
- 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.