Segmental Retaining Wall: What It Is, How It Works and When It Needs Geogrid
A plain-English guide to SRWs: how dry-stacked block holds back a hill, gravity vs. geogrid walls, the block specs that matter, and when an engineer is required.
A segmental retaining wall is a wall of dry-stacked concrete units, laid without mortar, that holds back soil mostly with its own weight. Most block walls you see in backyards and along commercial parking lots are segmental retaining walls, or SRWs, built from systems like Allan Block, Versa-Lok and Belgard.
They’re popular for good reasons: they flex instead of cracking, they don’t need a footing below the frost line, and they go up fast. Here’s how they work, the two main types, and the details that separate a wall that lasts from one that bulges.
Quick answer
A segmental retaining wall (SRW) is a gravity wall of dry-stacked concrete units that relies on its mass, plus friction and interlock between courses, to hold back soil. Short walls, typically up to about 3–4 ft depending on the block, stand on their own. Taller walls add layers of geogrid that tie the block to the soil behind it. Every SRW sits on a compacted leveling pad at least 6 in. thick, with at least 12 in. of clean, free-draining stone behind the units.
How a segmental retaining wall works
The Concrete Masonry & Hardscapes Association (CMHA) describes SRWs as gravity walls that rely primarily on their mass. Three details make a stack of loose block act like one structure:
- Setback. Each course steps back slightly from the one below, so the finished wall leans into the hill.
- Interlock. Lips, pins or keys lock each course to the one below, so units can’t slide forward.
- Stone fill. Hollow cores get filled with crushed stone, and the stone behind the wall adds weight, drains water and helps lock the units together.
Because nothing is mortared, an SRW can move slightly as the ground freezes and thaws without cracking. That’s why it sits on a compacted stone pad instead of a footing below the frost line like a poured concrete wall.
Gravity vs. reinforced segmental walls
| Gravity (conventional) SRW | Reinforced SRW | |
|---|---|---|
| How it holds | The block’s weight, setback and interlock | Block plus layers of geogrid that create a reinforced soil mass behind the wall |
| Typical height | Up to about 3–4 ft, per the block maker’s chart | Taller walls, designed by an engineer |
| Excavation | Base trench plus about 12 in. of stone behind the block | Back far enough for the geogrid: at least 0.6 times the wall height and never under 4 ft (CMHA) |
| Typical use | Garden walls, terraces, low walls with nothing heavy above | Tall walls, weak soils, driveways or slopes above |
Geogrid is a flat polymer grid laid between courses and run back into compacted fill. Once it’s in, the block and the reinforced soil act together as one big, heavy wall. Our retaining wall design guide covers grid lengths and spacing on a worked example.
The parts of an SRW
- Leveling pad: compacted crushed stone or unreinforced concrete, at least 6 in. thick.
- Buried first course: keeps the bottom of the wall from kicking out.
- Units and caps: the units are dry-stacked; only the caps are glued down, with concrete adhesive.
- Drainage stone: at least 12 in. of free-draining stone behind the units.
- Drain pipe: a perforated pipe at the base on taller walls and wet sites, vented to daylight. See retaining wall drainage.
- Geogrid and compacted backfill: on reinforced walls, placed in layers and compacted as the wall goes up.
What makes a good SRW block
SRW units are made to ASTM C1372. According to CMHA’s summary, the standard calls for an average compressive strength of at least 3,000 psi across three units, with no single unit under 2,500 psi, and dimensions within ±1/8 in.
In freeze-thaw country, ask for freeze-thaw test results under ASTM C1262. Unless the block has a proven field record, all five test specimens must lose less than 1% of their weight after 100 cycles, or four of five must lose less than 1.5% after 150 cycles.
Beyond the standard, look at unit weight and depth (heavier, deeper units go taller without grid), the connection type, and whether the maker publishes gravity-wall charts and installation guides. Our comparisons of the top segmental retaining wall systems and Allan Block vs. Versa-Lok cover the brands.
SRW vs. poured concrete vs. big block
| Segmental block (SRW) | Poured concrete | Big block (precast modular) | |
|---|---|---|---|
| Units | Dry-cast, typically under 150 lb, hand-placed | Formed and poured in place | Wet-cast, from a few hundred to several thousand lb, machine-placed |
| Footing | Compacted stone pad | Reinforced footing below the frost line | Compacted stone pad |
| Height without reinforcement | About 3–4 ft | Set by the engineer’s design | Can exceed 20 ft as a gravity wall |
| Handles movement | Flexes | Rigid; cracks if it moves | Flexes |
When does an SRW need an engineer?
Under the International Residential Code, section R404.4, walls that retain more than 48 in. of unbalanced fill, or walls over 24 in. tall that resist loads in addition to soil, like a driveway or a fence, must be designed to accepted engineering practice. Many towns add their own permit and setback rules, so call your building department before you dig.
Below those thresholds, a short gravity wall on good soil is a reasonable DIY project with the block maker’s installation guide in hand.
FAQ
What is a segmental retaining wall?
A wall of dry-stacked concrete units, laid without mortar, that holds soil with its weight, setback and interlock. Taller versions add geogrid to tie the block to the soil behind it.
Do segmental retaining walls need mortar?
No. SRW units are dry-stacked. Only the caps are glued down, with a concrete adhesive.
How tall can a segmental retaining wall be?
Without reinforcement, usually about 3–4 ft depending on the block, so check the maker’s gravity-wall chart. With geogrid and an engineered design, much taller walls are built on commercial and highway projects.
Does an SRW need a footing below the frost line?
No. SRWs sit on a compacted leveling pad at least 6 in. thick, with the first course buried. The flexible, dry-stacked wall tolerates small frost movement.
What’s the difference between an SRW and a big-block wall?
SRW units are dry-cast and usually under 150 lb, so they’re placed by hand and need geogrid at modest heights. Wet-cast big blocks weigh hundreds to thousands of pounds, are set by machine and can stand much taller as gravity walls.
How long does an SRW last?
Roughly 50–100 years when it’s built on a solid base with drainage and the right reinforcement. Most early failures come from water or the base, not the block.
Sources: CMHA, Guide to Segmental Retaining Walls; CMHA, sampling and testing SRW units; CMHA, segmental retaining wall design (geogrid length); Redi-Rock, SRW vs. precast modular block; ICC, 2021 International Residential Code, chapter 4 (R404.4).
Related: how long retaining walls last, by wall type.
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