What this calculator does
A retaining wall has to resist three things at once: being pushed over, being pushed along its base, and the pressure of the soil beneath its toe. This calculator checks all three and reports the factor of safety against each, so you know which one to fix first.
How to use it
- Enter the retained height, the base width and the weight of the wall stem per metre of length.
- Enter the soil unit weight, friction angle and any surcharge from traffic or adjacent structures.
- Add the allowable bearing pressure and any cohesion to complete the checks.
- Factors of safety below about 1.5 for overturning and sliding, or below 2 for bearing, mean the wall needs a wider base, a heel or better drainage.
Governing formula
Ka = (1 − sin φ)/(1 + sin φ) ; Pa = ½γH²Ka + qKaH Where:
-
φ= Soil friction angle [°] -
γ= Soil unit weight [kN/m³] -
H= Retained height [m]
Derived Equations:
FS_o = W·(B/2) / Σ(Pa·y) FS_s = (W·tan(2φ/3) + cB) / Pa σ_max = W/B · (1 + 6e/B) Worked example — 3 m wall, 2.2 m base, φ = 30°
The figures below were produced by running this page's own engine with the example inputs (Retained height (m), Base width (m), Wall weight per metre of length (kN/m), Soil unit weight (kN/m³), Soil friction angle (°), Surcharge load (kPa), Soil cohesion at the base (kPa), Allowable bearing pressure (kPa)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Active earth pressure coefficient | 0.33 Ka |
|---|---|
| Active thrust | 37 kN/m |
| Factor of safety — overturning | 0.48 – |
| Factor of safety — sliding | 0.18 – |
| Factor of safety — bearing | 2.48 – |
| Maximum bearing pressure | 60.47 kPa |
| Assessment | Not stable: FS(o) 0.48, FS(slide) 0.18, FS(bearing) 2.48. Widen the base or drain the backfill. |
- Ka = 1/3 at 30°, so the triangular soil thrust alone is 0.5 × 18 × 9 × 1/3 = 27 kN/m.
- The 10 kPa surcharge adds a further 10 kN/m acting at mid-height.
- The governing check is usually overturning for a narrow base — widen the base or add a heel until all three factors are comfortable.
Assumptions & limitations
Engineering calculation. Rankine active earth pressure with three limit states — overturning, sliding and bearing — assuming a vertical wall with dry horizontal backfill and a base friction of two thirds of φ.
- Rankine theory assumes a vertical wall and horizontal, dry, clean backfill; inclined walls or cohesionless backfill with a slope need Coulomb values.
- No passive resistance in front of the toe is credited — a heel key or toe drain changes the sliding result significantly.
- Does not check global stability, structural design of the wall itself, or seismic loading.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS EN 1997-1 — geotechnical design, retaining structures.
- GECP 7 / CIRIA C580 — design of retaining walls.
- Das, Principles of Geotechnical Engineering — Rankine earth pressure theory.