Compacted Fill

Unlike undisturbed soil, fill has to be engineered to hold weight: placed and compacted in thin layers, or lifts, at the right moisture content. Fill that's dumped in and skipped over rather than compacted properly will keep settling unevenly under load for months or years afterward, which is a common cause of sinking patios, cracked slabs, and retaining walls that shift out of position after the fact.

Compacted aggregate base is the standard, correctly done, foundation under paver patios and behind retaining walls, where it provides both stability and drainage. Fill under a footing is different territory though, since a footing generally needs to reach true undisturbed soil, or be carried on piers, rather than bear directly on loose or recently placed fill, no matter how solid that fill feels underfoot.