Methodology & code references

Every number this calculator produces comes from a published code table or a standard geotechnical method. Nothing is invented. Here is exactly what we use, so an engineer or a plan reviewer can check us.

1. Lateral earth pressure: IBC Table 1610.1

The push of the retained soil is taken as a design lateral soil load (an equivalent-fluid pressure, in psf per foot of depth) from IBC Table 1610.1. The total horizontal force on a wall of height H is the area of the pressure triangle,P = ½ · EFP · H², acting at H/3 above the base. We use these active values (and the at-rest column when the wall is restrained):

For a sloped backfill we increase the pressure by the ratio of Rankine active coefficients Ka(β)/Ka(0). A uniform surcharge q adds a rectangular pressure Ka·q over the full height (Ka taken as EFP/γ for consistency with the code value). Where a row combines two USCS classes (ML, CL) we use the more conservative value (60 pcf, the CL figure; ML alone is 45). IBC lists high-plasticity clays (CH/MH) as unsuitable backfill with no tabulated value; if you select one, we apply a conservative 100 pcf and warn you to replace it.

2. Presumptive bearing: IBC Table 1806.2

The allowable pressure under the base is the presumptive load-bearing value from IBC Table 1806.2:

• Crystalline bedrock: 12,000 psf
• Sedimentary and foliated rock: 4,000 psf
• Sandy gravel and/or gravel (GW, GP): 3,000 psf
• Sand, silty sand, clayey sand, silty/clayey gravel (SW, SP, SM, SC, GM, GC): 2,000 psf
• Clay, sandy clay, silty clay, clayey silt, silt and sandy silt (CL, ML, MH, CH): 1,500 psf

3. Stability: the three classical checks

Modelling the wall as a rectangular mass of width B and height H with unit weight γ_wall (conservative for a segmental wall, which also gains soil weight on its setback heel that we ignore on the safe side):

• Overturning about the toe: FS = W·(B/2) / (driving moment) ≥ 2.0.
• Sliding: FS = W·μ / (horizontal force) ≥ 1.5, μ = base-friction coefficient.
• Bearing: from the resultant eccentricity e, the peak toe pressure must stay below the allowable value, and ideally e ≤ B/6 (resultant in the middle third, no heel uplift).

The solver scans base widths from 0.3·H upward and returns the smallest that passes all three. If none passes, it recommends a reinforced (geogrid) SRW or an engineered cantilever and estimates the geogrid layers and length (NCMA practice: length ≥ 0.6·H, min 4 ft; a layer roughly every two courses).

4. Footing depth below the frost line

The base must bear below the frost line so frost heave can't lift it (IRC R403.1.4 / IBC §1809.5). We recommend a minimum depth below grade of the larger of: your state's typical frost depth, one buried course (≈ 10% of wall height), and a 12-inch practical minimum to reach stable bearing soil. Frost depth varies within a state, so the per-state figure is a representative permit value. Confirm yours with the local building department.

5. Friction angle & unit weight

Representative effective friction angles and moist unit weights are typical design values from standard references (NAVFAC DM-7.1; Coduto, Foundation Design). They drive the Rankine slope/surcharge adjustment and the base friction. Your geotechnical report always governs over these defaults.

Important limits

This tool produces a planning estimate, not a stamped design. A wall retaining more than 4 ft, or any wall with a surcharge, requires an engineered design (IRC R404.1 / IBC §1807.2) and a local permit. Confirm soil parameters with a geotechnical investigation and the final design with a licensed engineer.