The long-term stability of your wall depends on a solid foundation, or footing as it's often called. In a stable soil that has the strength to support a lot of weight (and that doesn't expand and contract with changes in moisture content) you have some latitude in the kind and size of footing you use. Less favorable soils may require the help of an engineer to design a solid footing for a long-lasting wall.
Earthen walls are often put on a conventional concrete footing, but they can also be built on stone or other more natural waterproof materials. The foundation should go below the frost line in cold climates and must be strong enough to hold the wall. For a wall under 2 feet in height, a foundation 12 to 18 inches wide and 8 to 12 inches deep is usually adequate. But soil conditions may require larger footings in some cases; consult with a civil engineer if you aren't sure. Also be sure to check local building codes for requirements in your area.
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