[Zeke]

On that design for that "abuse," I'd order the benches with uprights hollow from the bottom, or modify them. I'd then slip the uprights over pipe cemented into the ground about 18" and bolt then from the side. No bolt anchor is going to not work loose over time with a 2 post design like that. 4 post would be much better. At least a gusseted "T" at the bottom to spread the load out.

If you decide to use the 2 legs and 4 bolts per, I'd sink all 4 into a common poured base at least a foot wide and 18" deep.