# Foundations for Retaining Walls
A sturdy and long-lasting retaining wall depends heavily on the foundation. It must be sound and stable to successfully support the wall. Many local building codes allow construction of a retaining wall up to 4 feet without the need for any special engineering, so creating the foundation is straightforward. The foundation for a tall wall requires more detailed work and strict adherence to local building codes, since the additional height can pose a greater threat of failure.
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Function
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A foundation serves as a stable base for a retaining wall. They are typically installed below grade level so that the surrounding soil will lock them firmly in place. Keeping the foundation from shifting around helps to prevent movement of the wall. The foundation is made level so that the wall built upon it remains level. When the foundation is properly constructed, the retaining wall can remain sturdy and intact for decades.
Types
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A foundation made of concrete block is an economical and strong option for supporting retaining walls made of masonry, such as brick or masonry blocks. The concrete blocks are mortared to a concrete footing, upon which the retaining wall material is secured. When using decorative concrete blocks to construct a wall, such as those found at most home improvement stores, the first layer of block can successfully serve as the foundation, when the soil beneath it is sturdy and level.
Footings for Short Walls
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For retaining walls measuring 4 feet high or less, a compacted and level base of leveling sand, soil or gravel is a suitable option for a foundation's footing. Typically the trench must be 12 to 18 inches deep to provide a stable surface for constructing your wall. Alternatively, the trench can be filled with concrete to serve as a continuous footing and a sturdy foundation for the first layer of stones.
Footings for Tall Walls
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For walls taller than 4 feet, many local building codes require the footings to be constructed at least 24 inches below ground level. The dimensions for the footings depend on the thickness of the retaining wall and the condition of the soil. Strict standards for materials and reinforcement are also specified for tall walls. In average soil conditions, the foundation is often secured to a concrete footing and built up to within a few inches of ground level. Firm soil often will not require a separate foundation so the trench for the footing is filled with concrete up to ground level and serves as the foundation.
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