Rammed earth and rubble trench...oh, that sounds like a song title!

Hi all, brand new to the forum.
I am hoping there is a genius mind out there that can direct me to where I might find a “formula” for the dimensions of rubble trench to support a rammed earth structure.
I understand there are variables, expansive soil, doesn’t bear well, etc.
I’m really looking for the starting point. If non, I will simply overengineer, sure would prefer not to.
Really enjoy the site and like to see other folks “livin the dream” as well.

P.S. no need to comment on punctuation, I think I went fishing during those periods it was being taught..at least in my mind!

Any thoughts would be appreciated.
All the best,

Various Grit
(Soon to be crafter of nouveau mud hut)