Backfill is the material, usually compacted gravel or crushed stone, used to fill the excavated space behind a retaining wall once it's built, and it does as much structural work as the wall face itself.
What goes into backfill matters as much as the wall in front of it. Native soil dug out during excavation is often full of clay and fine particles that hold water and compact unevenly, so it typically gets replaced with clean, angular crushed stone that compacts to a stable density and lets water move through it instead of pooling against the wall. Backfill goes in and gets compacted in lifts as the wall rises course by course, and it's what geogrid ties into on reinforced walls, so a shortcut on backfill undermines the whole structure even if the visible wall face looks perfect from the yard.
Backfill is where a lot of retaining wall failures start in this climate, even on walls that look properly built from the outside. If the backfill isn't free-draining, water sits behind the wall through the winter, freezes, and expands, adding pressure the wall was never designed to hold on top of the soil pressure it already carries. We use clean crushed stone behind every wall for exactly that reason, so the water that inevitably gets into the soil above a wall has somewhere to go besides straight into the wall itself, one freeze-thaw cycle at a time.
More masonry vocabulary from the glossary.
The services where this term comes up.
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