Geogrid
Also called: geo grid, geogrid reinforcement, retaining wall grid, soil reinforcement grid, reinforced soil wall, MSE wall
A polyester or polypropylene mesh laid between courses of a block retaining wall and run back 3–6 feet into the compacted backfill, so the soil mass behind the wall becomes part of it. Required above 3–4 feet, on slopes and under surcharges; cheap, and the reason block walls can go tall.
A gravity wall stands on its own weight and lean, which works up to a point — around 3–4 feet for a typical segmental block on level, well-drained ground. Above that, the soil pushing on the wall outweighs the wall. Geogrid changes the arithmetic: a sheet of stiff mesh is laid on top of a course, pinned under the next one, and stretched back into the backfill, which is then placed and compacted over it in lifts. Friction locks the grid into the soil, and the block face, the grid and a wedge of compacted soil act as one thick wall. Engineers call the result a mechanically stabilized earth wall.
What the design specifies: which courses get grid (every second or third, typically), how far back it runs (usually 60–100% of the wall’s height, so 3–6 feet on a 5-foot wall), and the grid’s strength grade. Block manufacturers publish charts for the simple cases; an engineer sizes it for a slope above, clay, a driveway or anything within a wall-height of the top. The grid must be pulled taut and the backfill compacted over it in 6–8 inch lifts, and it can’t be cut around a tree or a pipe without a detail — a gap in the grid is a weak spot in the wall.
Cost is trivial next to the block — $2–$4 per square foot of grid, a few hundred dollars on a residential wall. What it adds is excavation: the backfill zone behind the wall has to be dug out as far as the grid runs and put back compacted, which is why a 4-foot engineered wall needs a machine where a 3-foot wall needs a shovel.