Poured concrete foundation walls that are less than 8 feet tall and have soil outside that is 6 or 7 feet deep against the wall can often be 8 inches thick and function quite well.
Poured concrete retaining wall height.
In general the top of the stem of any cast concrete retaining wall should not be less than 12 inches for the proper placement of concrete.
Backfilling with a clay soil instead of sand can increase the load by double or more while going from an 8 foot to a 10 foot wall just a 25 height increase adds 50 to the lateral pressure.
Gravity poured concrete retaining walls gravity retaining walls depend on their own weight and any soil resting on the concrete in resisting lateral earth forces.
They are generally economical up to 10 feet in height for cast concrete structures.
Concrete retaining wall footing size the depth to the bottom of the base slab should be kept at a minimum of two feet.