Pond Boss Forums POND CONSTRUCTION Building a dam Questions on core trench/keyway

I know the core/keyway must be 3 feet or to good clay, whichever is deeper, below the creek bottom. Does that depth set the plane of the keyway for the entire width of the dam? or is the bottom of the core allowed to arch upward to the sides of the dam, assuming good clay is still found?

The reason is ask, the valley I am looking at has one side which is a typical slope out into a pasture,but the other side is quite steep going up into a woodlot. How would a good contractor handle putting the keyway into that hillside? I'd rather not have to loose any more trees than necessary on the hillside above the dam.

I'm not sure I can give you a good answer because I don't fully understand what you are asking.

But all a core trench does is extend the sealing area of the dam down to a level to soil that is sealed so water can not go under the dam. In the case of my recent build of my RES specific pond, I had good clay up each side of the ravine I was blocking off with a dam. Only in the very middle of the ravine did I find topsoil fairly deep that might not be water tight. So in the area with good clay I just dug a slight trench to provide a keyway seal (so there was not a flat area that might create a 'layer' where water could seep through) and then thouroughly compacted the existing clay bank. Where there was topsoil I dug it out down to good clay, then layered in clay and compacted it in 6" layers. The whole idea of the core trench is to provide a seal. It needs to be deep enough to contact soil types that will not leak. That could be very shallow or quite deep. If it is too deep to reach such soil, then the dam has to have a layer of clay that covers all the face to the bottom in a continous seal. That is the whole thing you are doing with a core trench. Providing a seal, and perhaps in some cases structural strength of the dam if the material is of poor quality on each side of the trench.

In my main pond, that the dam followed a small creek bank, I had to go to 6 feet nearly the full length of the dam because of the deep top soil. In my RES pond, only a short distance needed much core.

It all depends. The whole point is to have proper soil and compaction with any area that the water in the pond contacts so you will not have a leaky pond.

Red, if it were me, and it's not, I would buy Mile Otto's book "Just Add Water". It is the best writing I've ever seen re pond building and costs a LOT less than an hour of dozer work.

I have both of the popular books Just Add Water and Perfect Pond.

That is a huge step forward.

To answer your question, the core trench isn't necessarily set by the lowest point. The key (forgive the pun) to a keyway/core trench, is to create the best "waterproof" barrier of clay as you build the dam. That foundation can rise, undulate, or fall, and likely does, since nothing about soils sits on a flat plane. I've seen a few cases where the core trench was simple, until the earthmover figured out a creek bed was way deeper than it should be. Another was a field that had been manipulated decades ago by straightening a creek channel. Under that field sat the old creek channel, winding beneath that field. That core trench went up and down, all over.