# Retaining Wall Stability Calculator

Soil and surcharge produce moments at different heights; both factors of safety must pass.

- Page: https://jianzhujisuan.com/en/retaining-wall
- Category: Foundations (https://jianzhujisuan.com/en/c/foundations)
- Chinese version: https://jianzhujisuan.com/zh/retaining-wall.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Wall height H 3 m, Backfill friction angle φ 30 °, Backfill unit weight γ 18 kN/m³, Surcharge q 10 kPa, Base width B 1.8 m, Base friction μ 0.5, Wall unit weight 24 kN/m³): **Sliding factor of safety 1.75**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Wall height H | number | 3 | m |  |
| Backfill friction angle φ | number | 30 | ° |  |
| Backfill unit weight γ | number | 18 | kN/m³ |  |
| Surcharge q | number | 10 | kPa |  |
| Base width B | number | 1.8 | m |  |
| Base friction μ | number | 0.5 | — |  |
| Wall unit weight | number | 24 | kN/m³ |  |

## Formula

- **Active coefficient**: Ka = (1 − sinφ)/(1 + sinφ)
- **Soil pressure**: Ps = 0.5·Ka·γ·H²
- **Surcharge**: Pq = Ka·q·H
- **Overturning moment**: Mo = Ps·H/3 + Pq·H/2
- **Restoring moment**: Mr = W·B/2
- **Sliding FS**: FSs = W·μ / Pa
- **Overturning FS**: FSo = Mr / Mo
- **Base pressure**: q = W/B·(1 ± 6e/B)

## Worked example

3 m wall, φ 30°, γ 18, 10 kPa surcharge, 1.8 m base, μ 0.5, wall at 24 kN/m³: Ka 0.333, 27.0 kN/m soil and 10.0 kN/m surcharge, 42.0 kNm/m overturning against 116.6 restoring, overturning FS 2.78 and sliding FS 1.75. Eccentricity is 0.324 m: all three clause 6.7.5 limits (1.3, 1.6, 0.25B = 0.45 m) are met, but it exceeds B/6 of 0.300 m, so the minimum base pressure is -6 kPa and the heel lifts. The verdict is pass, but widen the base.

## Questions

### How wide does a retaining wall need to be?

For a gravity wall the rule of thumb is a base width of 0.5 to 0.7 times the height, so a 3 m wall needs 1.5 to 2.1 m. That is only a starting point — check both factors of safety against sliding (1.3), against overturning (1.6) and the eccentricity (0.25B), all from GB 50007-2011 clause 6.7.5. The 3 m wall at 1.8 m above passes both factors at 1.75 and 2.78, yet the minimum base pressure is already negative, which means the centre of mass is too far forward; widen to about 2.1 m to clean it up.

### Is drainage behind the wall really necessary?

Always. Once the backfill saturates, water pressure is isotropic and adds a push of half gamma-w h² on top of the active pressure — for a 3 m wall that is 44 kN/m, more than the 27 kN/m of soil pressure. Most retaining wall failures are drainage failures, not strength failures. Detail a 300-500 mm granular filter or geotextile-wrapped gravel drain, weep holes at 50-100 mm diameter and 2-3 m centres falling 5% outward, with the lowest row at least 300 mm above the ground in front.

## Related tools

- [Pile Capacity Calculator](https://jianzhujisuan.com/en/pile-capacity): N value for sand, cu for clay: shaft and base worked separately, then divided by a factor of safety.
- [Bearing Capacity Calculator](https://jianzhujisuan.com/en/bearing-capacity): The three bearing factors follow from the friction angle; divide the ultimate value by a factor of safety.
- [Earth Pressure Calculator](https://jianzhujisuan.com/en/earth-pressure): All three coefficients at once; the gap between active and passive is where the money is.

---

Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.