Segmental retaining wall
Also called: SRW, segmental block wall, interlocking block retaining wall, modular block wall, Allan Block, Versa-Lok, landscape block wall, dry-stack block wall
A wall of dry-stacked, interlocking concrete blocks — no mortar, no footing — held by their weight, a lip or pins that set a backward lean, and, when tall, layers of geogrid tied into the backfill. The standard residential wall: $35–$65 per square foot installed, and the DIY choice under 4 feet.
The blocks are cast concrete, 30–75 pounds each, with a face that’s split or smooth and a back that’s hollow or solid. They stack without mortar on a compacted gravel base with the first course buried, and each course sits back a little from the one below — the batter — set automatically by a rear lip or by fiberglass pins through the block. The wall’s stability comes from mass, from the lean, from friction between courses, and from the drained stone behind it; taller walls add geogrid between courses, a mesh that runs back into the compacted soil and turns the soil itself into part of the wall.
Manufacturers (Allan Block, Versa-Lok, Belgard, Keystone, Unilock and the big-box house brands) publish charts of how tall each block can go without grid on level ground with good soil — typically 3–4 feet — and how much grid at what spacing above that. Those charts are for the simple case; a slope above the wall, clay soil, a driveway or a structure nearby means an engineer sizes it instead, and the permit office will ask.
What makes it the DIY system: no mortar means no timing, a block can be lifted and reset, curves and corners come from tapered blocks and a chisel, and the whole thing is a set of published steps — trench, base, buried course, pipe, courses with stone behind each, caps. What makes it a contractor’s job is height, surcharge and access. The cost page prices both.