Segmental retaining walls (dry stack) are gravity retaining walls that rely primarily on their mass (weight) for stability. The systems consist of concrete masonry units which are placed without the use of mortar (dry stacked), and which rely on a combination of mechanical interlock and mass to prevent overturning and sliding. The units may also be used in combination with horizontal layers of soil reinforcement which extend into the backfill to increase the effective width and weight of the gravity mass.

Limitations of the small block system (SRW) vs. "Big Block" system is the amount of tie back required for most walls higher than 4 ft.