As we build the ABRI on a slope, we had to put the uprights (??) for the roof into the solid, original, soil and not the loose stuff we put on it to level the thing.
They went down some 2 mtrs - or about 6 Ft. 5" in your money..
We did obviously do an overkill - but it makes for a very, very solid building.
In your case, the digger going down should reach good solid soil and that should do.
We're starting late this year on another ABRI (40x6,5x4 mtrs.) with some serious concrete floors and the
uprights go into the ground about a metre.
Have Fun Building !