contractor · 2026-05-01
Calculate block count, base gravel, drainage stone, and total cost for a segmental retaining wall (SRW) install.
| Wall length (ft) | 30 |
| Wall height (inches) | 36 |
| Block length (inches) | 17.5 |
| Block height (inches) | 6 |
| Block $ each | $6 |
| Cap block $ per linear foot | $12 |
| Base gravel $ per yard (delivered) | $55 |
| Drainage stone $ per yard (delivered) | $60 |
| Filter fabric (lump sum) | $120 |
| Labor $ per linear foot | $35 |
| Blocks needed | 126 |
| Base gravel cubic yards | 0.8 |
| Drainage stone cubic yards | 3.3 |
| Block + cap cost | $1,053 |
| Base + drain stone cost | $366 |
| Labor cost | $1,050 |
A segmental retaining wall (SRW) is half visible block face and half hidden drainage system. Skip the drainage and the wall fails — sometimes year 2, often year 5.
The fabric is non-negotiable. Without it, fines wash into the drain stone, clog it, build hydrostatic pressure, and push the wall over.
Most jurisdictions require an engineered design above:
Below those limits, manufacturer's published prescriptive details (Allan Block, Versa-Lok, Belgard) are accepted.
Skipped base, no batter (walls should lean back into the slope ~1 in per foot of height), no drain stone, no fabric, and stacking vertical instead of running-bond. Get the base right and the rest follows.
Soil pressure increases with the square of wall height. A 6-ft wall doesn't have 50% more force on it than a 4-ft wall — it has roughly 125% more. The block patterns, geogrid spacing, and base widths above 4 ft require a stamped design specific to your soil. Insurance/code/lawyer reasons all align here.
Under 3 ft, on stable soil, with a flat top: usually yes per most manufacturer prescriptive details. Anything taller, sloped above, or carrying a surcharge needs grid every 2 courses to bind the block to the backfill mass. Don't guess — read the manufacturer's spec for your block.
Properly built: 50+ years. The blocks are precast concrete, the failure modes are drainage and base. Walls fail because the base settled (didn't compact), water pressure built up (no drain stone or no weep), or fines clogged the system (no filter fabric). Get those three right and the wall outlasts you.
1 inch of setback per 12 inches of height is standard for most SRW systems. The block lip on the back face automatically creates this — don't try to defeat it. The lean is what lets the wall resist soil pressure passively.