Kabby said: ...Our last major flood in 1990 left 3 ft of sand over most of the yard. One section was pure mud, the clay side. I'm sure there is some silt mixed in. ...
Wow, that's like the Nile Valley where floods drop new topsoil every year!
Kabby said: ... I had read not to mix sand with clay, that it would set up like concrete. 
I take it that's not true. ...
Many people say that, but I didn't find it to be true.
Clay with insufficient compost or organic matter sets up like concrete. The sand doesn't hurt any in that case, and it can't help very much.
In my garden, sand and crushed rock and other gritty amendments like screened bark have always helped clay (with added compost). But clay needs compost! Lots!
To see
much improvement from sand, first the clay needs almost as much compost as it really needs, and then quite a bit of sand, like 20% or more. 50% would be great. 70% sand might be even better.
Since most people CAN'T buy or carry that much sand, it got a bad rap from people who did not add enough sand OR enough compost.
But look at "The Soil Triangle". Clay plus 50% sand is sandy clay.
Clay plus 70% sand is sandy clay loam. Those ain't concrete.
But the soil triangle doesn't mention organic matter, it is only about "texture".
Even more than every other soil type, clay needs organic matter.
