I'm putting in a large rectangular water cistern half into a hillside. Needing some solid recommendations on slab and wall sizing.

Water box is 8x8x40. Going up two more feet on back and sides with filled block for retaining wall. Rough overestimate of footing load with living roof and 50 psf snow load is 200k lbs.

Going to use 5k PSI concrete with 7% air entrainment, fiber reinforced, two control joints.

Monoslab- Thinking 6" with #6 rebar every 16". 16" footings 24" deep sloping up to slab.

Walls- Thinking 6" with #6 rebar every 12" with one full height connecting wall in the middle.

Interested in thoughts about what is potentially way undersized or oversized. I'd rather work with #5 bar, if so what spacing?

From what I've pieced together the walls are the biggest concern with lateral loads of 20k lbs. But the recommendations were to keep it long and narrow like this.

Appreciate your time.