Houston, is it possible to dig through the clay with a shovel?

I'm doing a drainage system in my backyard. I've dug holes 4 feet deep in some areas to put in some water retention with catch basins. I'm wondering if it's possible to dig past the clay/hardpan. How deep does this crap go? Is there sand below it?

Not my first time digging holes either. In theory the clay doesn't go down for ever. But how deep is this stuff.