Ask any wall guy why retaining walls fail and you'll get the same answer nine times out of ten: water. Not the wall block, not the mortar, not the caps. Water pressure builds up behind the wall, has nowhere to go, and eventually pushes the wall over or blows it out.

Backfill drainage is what stops that from happening. And backfill drainage means the right aggregate, in the right layer, at the right depth. Virgin stone works. Recycled concrete aggregate works just as well and costs less. Here's how to spec it right.

Why Retaining Walls Need Drainage in the First Place

When water sits behind a wall, it turns the soil into saturated mud. Saturated soil weighs more than dry soil, and it exerts what engineers call hydrostatic pressure against the back of the wall. On a 6-foot wall, that pressure can double or triple what dry soil would push against.

Most residential walls aren't engineered to hold saturated backfill. The design assumes water will drain out through the backfill and exit through weep holes or a perforated drain pipe. Skip the drainage layer, and you've built a dam. Dams need a lot more wall than most homeowners are ready to pay for.

Recycled concrete aggregate does the same job as virgin crushed stone in this application. It's angular, it interlocks, it drains fast, and it's a fraction of the price. That's the case for using it.

The Two RCA Grades You Care About

For retaining wall backfill, two grades of RCA do all the work:

3/4 inch clean RCA. No fines, all clean stone. This is the drain layer. Water moves through it fast, and it doesn't clog with silt.
Crusher run RCA. A mix of stone sizes down to fines, similar to a 411 or 610 gradation. Compacts tight and holds shape. This is the bulk fill.

Some jobs use just one grade. Bigger or wetter walls use both. The trick is knowing which combination fits your wall.

For a fuller breakdown of RCA sizing, our post on aggregate sizes and grades covers the technical side.

Sizing by Wall Height

Wall height drives everything. Taller walls hold back more soil, which means more water pressure, which means more drainage capacity. Here's the working guide most contractors use:

| Wall Height | Drainage Layer Width | Drain Pipe | Bulk Fill Behind Drain | |-------------|---------------------|-----------|----------------------| | Under 3 ft | 6 to 12 inches of 3/4 clean | Optional | Native soil or crusher run | | 3 to 4 ft | 12 inches of 3/4 clean | 4-inch perf pipe | Crusher run | | 4 to 6 ft | 12 to 18 inches of 3/4 clean | 4-inch perf pipe, gravel bedded | Crusher run to grade | | 6 to 8 ft | 18 to 24 inches of 3/4 clean | 4-inch perf pipe, wrapped in filter fabric | Crusher run to grade | | Over 8 ft | Engineered design required | Engineered | Engineered |

Anything over 4 feet in Tennessee typically needs a permit and often an engineer's stamp. Don't guess on the engineered ones.

How to Layer the Backfill

Order matters. Here's the sequence from wall to soil:

The wall block or timbers. Level base, proper batter, whatever your wall system calls for.
A row of drainage stone right against the wall. 3/4 inch clean RCA, minimum 6 inches wide for a small wall, up to 24 inches for a tall one.
A perforated drain pipe at the base. Set on top of a 4-inch bed of clean stone, sloped 1 to 2 percent to daylight or a drain outlet. Wrap the pipe in filter fabric on walls taller than 4 feet.
Filter fabric between the drainage layer and the bulk fill. Keeps silt from washing into the drain stone and clogging it.
Bulk fill behind the drainage layer. Crusher run RCA works great here. Compact in 6-inch lifts.
Cap with 6 inches of topsoil or native fill. This sheds surface water off the top of the wall rather than letting it soak straight in.

Skip any of those layers and you're gambling. The filter fabric between the drain and the bulk fill is the one most DIY installs miss, and it's the one that turns a working drainage system into a clogged useless one within a couple years.

Real Cost Example: 30 Foot Long, 6 Foot Tall Wall

Let's price out a common wall size. Thirty feet long, six feet tall, terraced backfill against a sloped yard. Here's the aggregate math:

Drainage layer. 18 inches wide, 6 feet tall, 30 feet long. That's 22.5 cubic feet per linear foot, or 675 cubic feet total. Convert to tons: about 27 tons of 3/4 inch clean RCA.

Drain pipe bedding. 4-inch bed under the pipe, 12 inches wide, 30 feet long. Roughly 10 cubic feet, or half a ton of clean stone. Rolled into the drainage layer estimate.

Bulk fill. Depends on how much soil you excavated and how much you're backfilling. For this size wall, budget another 15 to 25 tons of crusher run RCA.

Total aggregate: roughly 42 to 52 tons.

At virgin stone pricing (call it $35 to $50 per ton delivered), you're looking at $1,470 to $2,600 in stone.

At RCA pricing (our starting range is $15 to $35 per ton before delivery), the same job runs $630 to $1,820. That's a savings of $800 to $1,000 on the aggregate alone.

Numbers move around based on your specific job. Run yours through our aggregate calculator once you've got wall dimensions locked in.

RCA for Gabion Basket Walls

Gabion walls are a special case. Instead of block or timber, wire baskets full of rock do the work. RCA in the 2-inch to 6-inch range is ideal for gabion fill: angular, interlocking, cheaper than virgin quarried stone, and it drains well.

For gabion faces where appearance matters, some folks use RCA in the interior and a decorative stone on the visible face. Our RCA vs river rock vs 57 stone post covers appearance differences.

Where People Get It Wrong

A few mistakes we see over and over:

Using crusher run against the wall instead of clean stone. Crusher run has fines. Fines wash into your drain pipe and clog it. Use clean stone as the drainage layer.
Skipping the filter fabric. Silt migrates into your drain stone from the bulk fill side within a couple years. Filter fabric stops it. It costs almost nothing.
Forgetting to slope the drain pipe. A perforated pipe that runs level is a pipe full of stagnant water. Slope it 1 to 2 percent to daylight.
Compacting the drainage layer. Don't. Compaction closes up the voids that let water move. Compact the bulk fill instead.
Undersizing the drainage width. A 4-inch drainage layer behind a 6-foot wall is not enough. Go wider than you think you need.

What SMSG Will Carry for Retaining Wall Jobs

Our materials page lists what we stock. For retaining walls, that's:

3/4 inch clean RCA for drainage layers
Crusher run RCA for bulk fill
Larger sized RCA on request for gabion fill

Delivery is available across Blount, Knox, Loudon, and Sevier County. Pickup at 245 Brookdale Rd, Maryville. For current pricing, check the pricing page.

Ready to Spec Your Wall

Every retaining wall lives or dies by its backfill. RCA gives you the drainage performance of virgin stone at a lower price, which means bigger drainage layers on the same budget.

Call (865) 999-0857 to talk through your wall project, or visit our contact page for the map to 245 Brookdale Rd, Maryville. Hours: Monday through Friday 7am to 5pm, Saturday 8am to noon.

Recycled Concrete Aggregate for Retaining Walls: Which Grade Do You Actually Need?