Retaining Wall Base Prep (Gravel, Depth & Drainage)
Part of Landscaping & Aggregate
Quick answer
Dig a level trench, compact 4–6 inches of crushed stone for the base, and set the first course of block so it's buried below grade (about one block height, or 1 inch of burial per 8 inches of wall height). Backfill behind the wall with free-draining gravel and run a perforated drain pipe at the base so water escapes instead of pushing the wall over.
Most retaining walls that fail failed at the base and the drainage, not the blocks. Get a flat, compacted gravel pad and good water management behind the wall and the rest is just stacking.
Base prep steps
- Dig a trench wider than the block (about 2 ft) and deep enough to bury the first course plus 4–6 in of base.
- Compact the soil, then add 4–6 in of crushed stone (3/4-inch minus) and compact it dead level.
- Set the first course on the base and level every block front-to-back and side-to-side — this row sets the whole wall.
- Bury the base course below grade: roughly 1 in of burial for every 8 in of wall height.
Drainage behind the wall
- Backfill the first foot behind the wall with free-draining gravel, not soil.
- Lay a perforated drain pipe at the base, sloped to daylight or a drain.
- Use geogrid every few courses for walls over ~3 ft or holding a slope.
- Cap the top and grade the soil to shed water away from the wall.
The retaining wall calculator sizes the compacted gravel base (tons) along with your block and cap count.
FAQs
How deep should a retaining wall base be?
Plan on 4–6 inches of compacted crushed stone, plus enough depth to bury the bottom course below grade — about 1 inch of burial per 8 inches of finished wall height. So a 3-ft wall buries roughly the first 4–5 inches of block.
What gravel goes behind a retaining wall?
Use a clean, angular drainage stone (3/4-inch crushed, no fines) for the backfill directly behind the wall so water drains freely. Compacted 3/4-inch minus is used for the base pad under the first course.