Segmental Retaining Wall Calculator (Modular Block, Gravity)
External stability of an unreinforced segmental (modular concrete block) retaining wall, treated as a battered gravity wall: Coulomb active pressure with wall batter, wall friction, backslope and surcharge; overturning, base sliding and bearing; and the tallest number of courses your block can stack before it needs geogrid. Enter the block's depth, height, setback and infilled unit weight from the manufacturer's data.
The wall is modeled as a single leaning block Wu deep and H tall; the vertical component of the soil thrust helps resist overturning and sliding, the surcharge counts only as a driving load (and as vertical load for bearing). Bearing is checked on the Meyerhof effective width directly under the units; a gravel leveling pad spreads the load further, so this is conservative. Shear between courses, which depends on the product's tested interface strength, is not checked.
Acceptance criteria used
| Check | Minimum FoS | In this calculator |
|---|---|---|
| Base sliding | 1.5 | Yes |
| Overturning | 2.0 | Yes |
| Bearing capacity (soil footing) | 2.0 | Yes — enter qallow = qult/2.0 |
| Global stability | 1.3 | No — slope-stability analysis |
| Facing shear between units | 1.5 | No — needs the product's tested shear capacity |
Worked example
Example — five courses of a 12-in block (the calculator defaults)
Construction details that matter more than the math
- Drainage aggregate. Fill the unit cores and a zone at least 12 in behind the units with clean crushed stone, with a perforated drain at the base daylighted or tied to a storm system. A block wall with saturated backfill carries far more than the thrust computed here.
- Leveling pad. A compacted crushed-stone pad, typically about 6 in thick, under the first course.
- Embedment. Bury the bottom course; deeper embedment is needed on slopes in front of the wall and where scour or frost is a concern.
- Surcharges and tiers. A driveway, a slope above the wall, or an upper wall within about twice the lower wall's height all load the lower wall. Enter them as q or i, or treat the wall as reinforced.
References: National Concrete Masonry Association, Design Manual for Segmental Retaining Walls, 3rd ed. (method; product-specific values come from the manufacturer). Town of Clayton, NC, Segmental Block Retaining Wall Design, Manual of Specifications, Standards and Design (2010), Table 2.1. Coulomb, C.A. (1776); Meyerhof, G.G. (1953), effective-width bearing. FHWA-NHI-10-024, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Vol. I.
Related tools
- Retaining wall block calculator — blocks, gravel, geogrid rolls
- Retaining wall design criteria — safety factors and soil loads on one card
- Geogrid-reinforced block wall — when the wall is taller than the gravity limit
- Cantilever concrete retaining wall
- Soil nail wall
- Gabion wall stability
- Lateral earth pressure (Rankine)
- Terzaghi bearing capacity — derive qallow
- Presumptive bearing values