Pond Boss Forums POND CONSTRUCTION Building a dam Dam Help (Clay Core Questions)

I am building a pond and having an engineer do the plans on it, however, he is a ways out on getting to me to get everything calculated and I have an opportunity for some really cheap pure clay that is being removed from a place nearby. I would love to get a general idea on how thick the clay core needs to be for this pond so I can get roughly the correct amount of clay hauled in. The pond is going to be roughly 2 acres in size and 8 feet deep, the pond dam will roughly be 100 feet long and 12 feet tall. I was thinking the core needed to be 3-4' feet but I would love any opinions/guidance on this because I have limited experience with this. Thanks in advance

Welcome to PondBoss. I am no pro on this but I would hazard a guess that the best soils for the core are the width of the equipment being used to compact it. So if your doser is xx' wide.....your core will match that same width.

Doser spreads a lift.....compacted by sheepsfoot and then rinse and repeat.

Have you dug any test holes on site to see if you need to have clay brought in? If you have good soils on site you may not need to bring anything "extra" to the site.

Hopefully somebody else will chime in that has a little more expertise.

According to Mike Otto (one of our old experts), the core trench should be a minimum of 8' wide. As Boondoggle says above, that may be further influenced by the width of your equipment.

Typically, if your core trench is cut significantly below grade, it will be the minimum required width at the bottom - and then actually get a little bit wider over the next several feet of vertical distance. This is because your trench walls will NOT be vertical, but will have to be cut back a little to hold their slope. (Unless of course you are cutting the trench in nearly pure clay that just has sand and gravel streaks.)

The width will then start to taper back closer to the minimum design width of the core trench as you start building material above grade to reach the final dam elevation. For example, with an 8' minimum width, the core trench fill will be 8' wide at the bottom, about 10-14' wide near the original ground level, and then back to 8' wide at the top of the dam.

Also, the core trench does NOT need to be pure clay! Usually, if you have good clay available at the pond location, the good clay is mixed with the material that came out of the pond and core trench excavation, and the resulting mixture is used for the core trench. (No big rocks, organic material, etc. Preferably nothing gravel-sized or larger, but lots of sealed dams have been constructed by good builders where conditions obligated them to include some gravel.)

Pure clay is actually an inferior material. If the water level in your pond goes way down during a drought, there is a chance that a pure clay core trench will dehydrate and large cracks may form.

I believe the minimum clay content is only 20% for a core trench (for well mixed and properly compacted material). The "optimum" content in your area will vary based on clay type, the size distribution of the other particles in your final mixture, etc. I would hope your design engineer has excellent local knowledge in pond construction and is telling you the correct numbers to use.

Good luck on your new pond project!

I’m building a pond that’s almost exactly the same size. Min is spring fed by a year round spring. Spring may flow 10 gallons per minute. Trying to come up with best pipe, and size for my overflow that will go through the dam. My core is 12’ deep and 15’ wide and is red clay soil that has been layered and compacted with a vibratory roller. I’m thinking of installing mine somewhere in the 3 or 4’ depth from future water surface.

The size of the pipe depends on your watershed and the vegetation on the watershed. Your local county NRCS should be able to calculate the pipe size. You need a primary overflow and secondary overflow. NRCS has a 70 to 90ish page manual on how to properly build a pond. You can download it.

If you used a vibratory smooth roller, you possibly could have leaks between the layers. How thick were the layers that you compacted? You have to watch that you don't make those layers too thick or they won't compact properly. Ideally a sheepsfoot roller (vibrating or not) is the preferred compaction device.

Just remember that if the spring that feeds your pond is below the full pool mark, then the spring can also take water out of your pond if the water pressure in the pond is greater than the water pressure in the spring. Springs don't have one way valves on them.

10 gpm is easy for a small outlet pipe.

You need to size for a big rain onto the watershed that feeds into your pond. Can you calculate/estimate that?

Water going over the top of a cored dam is a pond killer. It will find the lowest spot, and then cut down through the dam in minutes once it starts a bit of erosion.

Just assure that it is good sandy loam. Solid loam can crack when dry and sand is porous.

Assure that the dozer guy is a pond builder, not just a dirt pusher. There’s a difference and that difference is important.

Find neighbors with ponds or pond owners in the area. Stop and ask who built it and whether they would use them again.

Get several bids.