Drainage aggregate is clean, angular crushed stone, largely free of sand and fine particles, placed directly behind a retaining wall so water can move through it freely instead of building up hydrostatic pressure against the wall.
The whole point of drainage aggregate is the open space between the individual stones. Rounded gravel or stone with a lot of fine material packed between the pieces compacts tighter and holds water like a sponge, while clean, angular aggregate leaves gaps that water can travel through toward a weep tile or weep hole instead of sitting still against the wall. Most retaining wall designs call for a defined column of this drainage stone directly behind the wall face, separate from the rest of the backfill, precisely so water has a clear, fast path out rather than working its way through denser soil first.
This detail matters more here than in a lot of places, because the Blue Ridge foothills go through repeated freeze-thaw cycles all winter, and water trapped against a wall doesn't just sit there, it freezes and expands. A wall with proper drainage aggregate behind it sheds that water before it ever gets the chance to freeze in place. A wall without it is holding back ice pressure on top of soil pressure every cold snap, which is a big part of why we don't skip this layer on any wall we build, block or stone, tall or short.
More masonry vocabulary from the glossary.
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