Question about a stone and concrete footer

I'm building a wallipini and I'm not concerned about code here as much as what works. I have packed tires around the rim of the underground greenhouse hole and my plan is to pour a bond beam or a footer on top of the tires.. I have 2x8 lumber for the forms and for the sil plate. I would rather line up heavy ~20lb stones as my concrete forms and pour with the two rows of stones. Hopefully with the concrete sticking to the stone but not blowing out either.

I haven't found info on this topic so maybe it's not a thing. Is this a bad idea?

How well that will work depends on the shape and size of the stones, how tall you are trying to make the containment, and the character of the base the stones are sitting on. 20 pounds is not that big for a 7" high containment unless the stones are at least a foot wide perpendicular to the wall (to avoid tipping over) and lodged into/against tire crannies to avoid sliding. Stakes bracing the stones from moving would also help if the stones are not that massive.

My experienced opinion would be to stick with regular form work for your footer. (31 years of concrete work) This bond beam/sill plate will be the foundation of everything above it, so you want to maximize this investment. Irregular faces and angles from stones will create “crack points”. That being said, you could mortar stones together first as wall forms (outside dimensions.. not “in” the footer itself.), and place concrete between them afterwards, although I would still recommend bracing to prevent blowout. This adds steps, and while it may be more aesthetically pleasing, would not necessarily add to the strength of your footer. Whatever you decide, I also recommend a low slump mix with fiber added and NO steel rebar. (Fiberglass and other non steel rebar exists, and is worth researching.) The least water necessary for workability and a long slow cure time will give you the strongest end product. I hope this helps, and best wishes for your endeavor.

p.s.: not trying to be know-it-all, but I know enough to offer you this advice in full confidence.

One of the issues that I see with using stones as the forms is that you won't have a flat surface across which to screed the concrete flat.

And the tires are being cast in the concrete instead of steel reinforcing bar?

Appreciate the inputs. I'm sticking with the footer without the rocks. What's being said here seems right, that the rocks are unessessary and won't really add anything good, only complications. I always get good info here, ty.