The reason to dig test holes is to understand what kinds of soils you have. Test holes are normally dug straight down with a backhoe to look at layering. When a dam is built, the earth mover uses different soils for different purposes. Soils are graded based on porosity. Least porous soils are "processed" (mixed, pulverized and compacted) and used for backfilling a core trench and for the middle and front side slope of a dam. A good earthmover can figure out what types of soils you have, and work with you to use those soils appropriately. Most of the time, especially in north Texas, the biggest problems are having too much good clay or not nearly enough. Building a dam is a combination of talents, from measuring and quantifying the watershed size to calculating hydraulic pressure to figuring out how much clay-based soils are needed to build a dam to match that site, and then actually putting all that to work building the dam. I have watched Otto take a site which shouldn't have a pond on it, take layers of porous soils, mix, pulverize, crumble. move in short lifts, add clay with his scraper from hundreds of yards away, pack, and do it over and over to build a dam that doesn't leak. He did it at my house. Test holes showed crumbly, eroding soils that could not possibly be used for a dam. So, we moved around and found great clay in two different places. He got rid of the "bad" dirt and replaced it with clay, mixed with moderate soils nearby and built a great dam.

Teach a man to grow fish...
He can teach to catch fish...