About Retaining Wall Design Calculator
The retaining wall design calculator evaluates the stability and reinforcement of gravity and cantilever walls using Rankine or Coulomb active earth pressure. It is used by geotechnical and structural engineers to check overturning, sliding, and bearing factors of safety, base pressures, and the ACI 318 reinforcement of the stem, toe, and heel.
Define the wall geometry, the backfill and foundation soils, the surcharge, and the water condition. The tool returns the active and passive forces, the overturning, sliding, and bearing factors of safety, the base pressure distribution and eccentricity, and the required reinforcement and material quantities, all in real time.
How It Works
- Compute the active earth pressure coefficient Ka (Rankine or Coulomb) and passive Kp from the backfill friction angle and geometry.
- Resolve the driving forces from earth pressure, surcharge, and water, and the resisting forces from wall and soil weight plus passive resistance.
- Take moments about the toe to find the overturning and sliding factors of safety, the resultant eccentricity, and the base pressures qmax and qmin.
- Compare qmax with the allowable bearing capacity for the bearing factor of safety, then design the stem, toe, and heel reinforcement per ACI 318.
Worked Example
For a backfill with phi = 30 degrees and horizontal surface, the Rankine active coefficient is Ka = tan^2(45 - phi/2) = tan^2(30) = 0.333. With a vertical resultant V = 240 kN/m on a base width B = 3.0 m at eccentricity e = 0.2 m (within the middle third), qmax = V/B*(1 + 6e/B) = 80*(1 + 0.4) = 112.0 kPa and qmin = 80*(1 - 0.4) = 48.0 kPa.
Formulas
- Rankine active coefficient (horizontal backfill)
Ka = tan^2(45 - phi/2)- Passive coefficient
Kp = tan^2(45 + phi/2)- Active force resultant
Pa = 0.5 * Ka * gamma * H^2- Factors of safety
FoS_OT = MR/MO ; FoS_SL = (V*tan(delta_b) + c_b*B + Pp) / FH- Base pressure distribution
qmax = V/B*(1 + 6*e/B) ; qmin = V/B*(1 - 6*e/B)
Standards & References
- Eurocode 7 (EN 1997)
- ACI 318-19
- AS 4678
Frequently Asked Questions
When should I use Rankine versus Coulomb earth pressure?
Rankine theory assumes a smooth (frictionless) wall and is simplest for a vertical back with a horizontal or uniformly sloping backfill. Coulomb theory accounts for wall-soil friction delta and an inclined wall face, giving a lower active thrust. The tool computes both, so you can select the appropriate one.
What factors of safety are checked?
The tool checks overturning (FoS = resisting moment / overturning moment, typical target 2.0), sliding (resisting friction plus base cohesion and passive resistance divided by the horizontal thrust, target 1.5), and bearing (allowable bearing capacity divided by qmax, target around 3.0).
What is the middle-third rule?
If the resultant of all forces stays within the middle third of the base (eccentricity e <= B/6), the entire base remains in compression and qmin is positive. Outside the middle third the heel lifts and the bearing pressure redistributes, which the tool flags.
Does the calculator design the reinforcement?
Yes. For cantilever walls it computes the bending moment and shear at the stem, toe, and heel and solves the required flexural steel per ACI 318 (strength reduction factor 0.9, minimum ratio 0.0018), checks the concrete shear capacity, and reports concrete and rebar quantities.