Dave:
I used to work in your part of the world and you are right the clays are a little different down there in good old Texas. I buildt two large dams on the San Gabriel, one at Granger and one at Georgetown Texas. I have first hand experience with those so called gumbo, "expansive" clays. The Dam I built at Granger was two miles long and used 17 million cy. These large dams were cored but we could have skipped the coring because the material used for the the zones outside the cores was such high quality inpervious clay. They were so large skrinkage cracks were not a consideration in the design.
Smaller dams may be a problem using these clays because the skrinkage cracks that form as the clays desicate can be 15 feet deep. I think to build a good small dam in that gumbo you may want to have upstream and downstream zones of clay with a low expansive index to protect the gumbo from desication.
In my part of the world, (upstate New York), we don't get drought conditions like you do and our clays are not near as expansive as your clays.
Here I can scrape the topsoil away and expose the clay subsoil and start building the dam. Typically I have built my personal ponds where the clay strata was thick, so I excavate maybe 10 feet into the ground and built a dam 10 feet high. This results in a 20 deep pond.
I have gotten a little lazy in building ponds for myself due to having great pond building conditions. I can get 90% compaction from my dozer treads so I haven't even been using my sheepsfoot roller. The clay I have been using comes out of the ground a little on the dry side but since the entire embankments are solid clay, I have skipped using any moisture adjusts to the clay. So far I have completed six pond with only one small visable leak in one of them.
