[Bird Doo Head]

I did my in-laws' house with my a spade bit in SDS hammer drill set on Chisel to break the clay, similar to what was suggested earlier. It broke into good size chunks that I could pick up. Unfortunately, it was one 5 gallon bucket at a time to remove the dirt.

Hint: Use a GFI if using an electrical tool in the crawl space. Also use breathing protection if mold or dust is present. And, the obvious- Eye Protection.