How to Build a Clay-Lined Pond
If you have loam soil like I do, with very little clay, you can still build a pond, but it is certainly more difficult. You can simply use a liner, mix in sodium bentonite, or bring in clay. I have tried all three of those options, and if you have a relatively inexpensive source of clay, and you can get a truck to the pond site, I think bringing in clay is the best option.
1. The first thing to do is to plan the site. Make sure you figure out the levels of the site. A laser level is a must have here. This project was a small contour dam to be used to enhance the diversity of my site, along with storing water and rehydrating the land. Make sure to locate utilities if needed, and arrange for the materials and equipment you need.
2. Dig your hole deeper than your final depth. Ideally, you have 3 feet of clay lining the bowl. For my small 2 foot deep pond, I only used 1 foot of clay to line the pond. So I dug the pond 3 feet deep, and added in 1 foot of clay.
3. Build up your walls with clay, making sure to have at least a 3 foot wide compacted clay keyway at least 3 feet below the original grade. If you are building a valley dam, this may mean that you have to dig down before you start building up your wall. This is extremely important as you want your wall locked into the landscape. If it is simply on grade, it may slide down the hill.
4. As you are building up the walls, you can also bring in and compact the clay. It is important to bring in no more than 6 inches of loose clay, which should compact to 3 inches. Also, it is extremely important that the moisture level of the clay is correct BEFORE applying to your pond site. If it is too dry, water the pile, using an excavator to mix it up as you water it. Do not simply water the top or the inside will just remain dry. On the other hand don’t soak the pile too much, because clay that is too wet will not compact either. If you can squeeze water out of the clay it is too wet. If you can make a solid ball with the clay, but it won’t stick to your boots, it is probably about right.
5. Get the spillway right. Do not run your spillway over your wall. You can run it around the wall if possible. This was not possible for me, so I had to run a pipe through the wall. If you do this, make sure to enclose the pipe in a baffle to stop water from traversing along the pipe. I would also recommend mixing some sodium bentonite with the clay around the pipe when you hand compact around the pipe.
6. Dress the walls with compost and plant a groundcover with a shallow root system. I prefer clover.
7. If possible, fill it up right away. This will help to avoid the clay drying out and cracking.
See the construction of a clay lined pond. This pond is approximately 300 square feet, 2 feet deep designed to add diversity to the permaculture site. The catchment comes mostly via the driveway through a swale, and overflows through a pipe with a baffle into another swale.