How deep to go
| Use | Published figure | Source |
|---|---|---|
| Compacted wall-rock base | 6 in minimum | allanblock.com |
| Wall rock directly behind the block | 12 in | allanblock.com |
| Wall rock size | 0.25 to 1 in | allanblock.com |
Allan Block's best-practice manual bases the trench on a minimum 6-inch compacted wall-rock base and a 12-inch column of wall rock directly behind the block facing. Its garden wall planner describes ideal wall rock as crushed or smooth stone from 0.25 to 1 inch; ask your block dealer which size they stock.
Working it out
Base = length × base width × base depth. Drainage zone = length × wall height × 12 in. Convert inches to feet and divide by 27 for cubic yards.
Example: a 30 ft wall, 3 ft high, with a 24-inch-wide, 6-inch-deep base and 12 inches of rock behind it needs 1.22 cubic yards for the base and 3.67 for the drainage zone, with 10% extra. At 1.29 tons per cubic yard of No. 57 that is 6.30 tons.
Rochester Concrete Products' install guide adds 10% for compaction and uneven ground.
Common questions
How much gravel do I need for a retaining wall gravel?
Measure the area, choose a depth from the table above and enter both in the calculator. It gives cubic yards, tons and bags.
Should I order extra?
Rochester Concrete Products' install guide adds 10% for compaction and uneven ground. Loose stone also compacts, so a compacted layer needs more loose material than its finished depth suggests.
Related calculators
Sources
- https://allanblock.com/docs/Best_Practices/2-typical-wall-construction.html
- https://www.allanblock.com/retaining-walls/garden-wall-plan.aspx
- https://www.fhwa.dot.gov/publications/research/infrastructure/structures/bridge/15034/003.cfm