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All walls should include drainage stone, even if they don’t require a drain pipe. Install drainage stone at the back of the retaining wall and extend 12-in behind the blocks. Start the drainage stone near the base of the wall and extend up to within 6-inches of the top of the wall.
If you need to purchase backfill, look for processed fill, gravel, sand, or screenings. The base material, wall rock, and drainage stone all require a quarried backfill. Although you could use different materials for each, I recommend sticking with just one material for all three zones.
Filter fabric, AKA geotextile underlayment fabric, is a permeable fabric commonly used as a filter between soil and gravel surfaces. It keeps dirt and debris from clogging the gravel, and therefore preserves retaining wall construction.
The appropriate size of gravel for retaining wall drainage is typically ranging from 0.25 to 1.5 inches (5 mm to 38 mm) and containing less than 10% fines. This size allows for adequate drainage while preventing the gravel from shifting or compacting over time.
A common size of gravel used for drainage behind retaining walls is ¾-inch gravel. This size is ideal because it allows for good water flow and is also easy to install. Another option is pea gravel, which is smaller in size and provides better drainage, but may require a bit more maintenance.
In order to provide proper drainage, at least 12 inches of granular backfill (gravel or a similar aggregate) should be installed directly behind the wall. Compacted native soil can be used to backfill the rest of the space behind the wall.
Make sure to include backfill, the 12 inches of space behind a retaining wall. Fill it with crushed stone or gravel. All retaining walls should include drainage stone even if a draining pipe is not required. Place filter fabric above the drainage stone and below the topsoil.
To help you determine which gravel is best for your project, we’ve put together this list of the best types of gravel for drainage, and we’ve even included a pros and cons list for each.
Drainage gravel backfill is meant to surround and add weight to hollow core SRW blocks, securing them in place. Adding filter fabric between the blocks and drainage stone compromises the wall’s vital compaction, leaving dangerous spaces that make it prone to movement from settling and erosion.
Installing Drainage Systems for a Retaining Wall. Effective drainage is crucial to prevent water pressure buildup behind the wall: Lay landscape fabric in a C-shape, with the open end facing downhill. Add a layer of gravel sloped one inch for every four feet to direct water away. Install a four-inch perforated PVC drainpipe at the base of the wall.