drdawg said:... Perhaps you would like to research "Yazoo Clay".
It is impressive that there are mechanical engineering studies of your clay!
"The Yazoo Clay is a calcareous fossiliferous mudrock"
" Caliche (calcium carbonate deposits) and gypsum (calcium sulfate) are common."
Only a few spots in my yard are "pure" blue-gray clay so the rest might range from 'clay" to "silty clay". I don't think I have any Caliche gluing it together.
If I wet down the surface and leave some water standing overnight, my clay will soften 6-18" deep by morning, so SOME water does perk through it. A hole with standing water appears to evaporate more than it perks, but I can't be sure. I can treat it as "almost waterproof" for most purposes.
When soft, a pick does dig it up easily, so I have to defer: your clay IS worse than my clay!
