I have 8" diam pilings; they cracked and are in need of replacing. 4x4 post on them. Am guessing an 8" is fine, but with rebar in them this time. Question: any good reason to go up a size? I'm concerned that there is some sort of shearing going on in the ground. Also: can I / should I backfill with rock around the piling? I dug down to remove the old ones, and it's mostly clay with relatively poor drainage. I'm thinking if I drop crushed stone around it that it might help with shear issues.