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Gabion Wall Stability Calculator

Gravity-wall external stability for stepped gabion walls: Rankine active pressure with uniform surcharge, factors of safety against overturning and sliding, resultant eccentricity with the middle-third check, and peak bearing pressure. Wall cross-section modeled as a trapezoid (base width to top width, back face vertical against the fill).

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Geometry: back face vertical against the fill, front face stepped from B at the base to T at the top (trapezoid). Level backfill, no water pressure — gabions are free-draining, but the backfill must drain too (provide a granular drainage zone or geotextile-wrapped chimney). Repeat the check at each course interface using the geometry above that level. Sloping backfill, seismic, or water loading require the extended analysis.

$$ W = \gamma_g H \frac{B+T}{2}, \qquad \bar{x} = B - \frac{B^2 + BT + T^2}{3(B+T)}, \qquad e = \frac{B}{2} - \frac{M_r - M_o}{\Sigma V} $$
x̄ centroid distance from the toe (back face at x = B) · Mr = W·x̄ · Mo = Pa,soil·H/3 + Pa,surcharge·H/2 · ΣV = W (level fill, no key).

Acceptance criteria

Customary static factors of safety for gravity walls
CheckMinimumBasis
Overturning about toe2.0ΣM resisting / ΣM driving
Sliding on base1.5μ·ΣV / ΣH, μ = tan φfoundation
Eccentricitye ≤ B/6Resultant in middle third, full base contact
Bearingqmax ≤ qallowTrapezoidal distribution, (ΣV/B)(1 + 6e/B)

These are the classical service-load criteria (Das; Terzaghi & Peck); LRFD projects use factored loads and resistance factors per AASHTO instead — same mechanics, different bookkeeping.

Worked example

Example — 9-ft stepped gabion wall, stream bank

Given: H = 9 ft (three 3-ft courses), B = 6 ft, T = 3 ft, γg = 110 pcf, backfill γ = 120 pcf, φ = 30°, no surcharge.
Ka = tan²(45 − 15) = 0.333 · Pa = ½ × 0.333 × 120 × 9² = 1,620 lb/ft at H/3 = 3 ft
W = 110 × 9 × (6+3)/2 = 4,455 lb/ft · x̄ = 6 − (36+18+9)/(3×9) = 3.67 ft from toe
FoSOT = (4455 × 3.67)/(1620 × 3) = 16,350/4,860 = 3.4 ✓ · FoSSL = (0.577 × 4455)/1620 = 1.59 ✓
e = 3.0 − (16,350 − 4,860)/4,455 = 3.0 − 2.58 = 0.42 ft < B/6 = 1.0 ft ✓
qmax = (4455/6)(1 + 6×0.42/6) = 743 × 1.42 = 1,055 psf — all external checks pass; verify each course interface and basket internals per manufacturer.

References: Das, B.M., Principles of Foundation Engineering, retaining-wall chapters. Terzaghi, K. & Peck, R.B. (1967). Soil Mechanics in Engineering Practice. FHWA-NHI-10-024/025, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes (gravity-wall fundamentals). Maccaferri, Gabion Walls Design Guide (fill unit weights, internal checks).

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