Cohesion caution
Long-term drained walls may require c = 0; verify the soil model.
Engineering
Retaining Wall Capacity Calculator
Estimate Rankine active force and toe moment from wall height, soil unit weight, friction angle, and cohesion.
RETAINING WALL CAPACITY
Turn soil parameters into a transparent active-force estimate
This simplified Rankine screen exposes the coefficient, active force, toe moment, and allowable-moment utilization instead of hiding assumptions in one capacity number.
Decision boundary:For preliminary geotechnical screening of Rankine active thrust and toe moment; groundwater, surcharge, wall friction, and final code design are excluded.
Active coefficient-
Tension-crack depth (m)-
Active force (kN/m)-
Toe moment (kN*m/m)-
Moment utilization-
Allowance / demand-
Height / base ratio-
Moment status-
DECISION MAP
Retaining Wall Capacity Calculator evidence path
Use the estimate to identify which soil or geometry assumption needs verification before a full geotechnical design.
| Quantity | Formula basis | Current value | Unit | Decision role |
|---|
LIVE CALCULATION PROCESS
Formula, symbols, substitution, and final check
Ka = (1-sin phi)/(1+sin phi); z0 = 2c/(gamma sqrt(Ka)); Pa = 0.5 Ka gamma(H-z0)^2; M = Pa(H-z0)/3
This simplified Rankine screen assumes level drained backfill, no wall friction, and a triangular positive-pressure zone below the calculated tension crack. Cohesion is not carried as a uniform negative pressure, and the page rejects cases where the simplified crack depth reaches the full wall height.
| Symbol | Meaning | Unit | Current input |
|---|---|---|---|
| H | Retained wall height | m | 4.2 |
| gamma | Bulk soil unit weight | kN/m3 | 18.5 |
| phi | Effective soil friction angle | degrees | 30 |
| c | Effective cohesion used in the screen | kPa | 5 |
| B | Recorded wall base width | m | 2.8 |
| Mallow | Entered allowable toe moment | kN*m/m | 210 |
HOW TO USE
Turn soil parameters into a transparent active-force estimate
- Enter wall height, base width, soil unit weight, friction angle, cohesion, and the moment allowance.
- Confirm that the soil parameters share the same drained interpretation and unit basis.
- Check the calculated tension-crack depth before using the positive pressure height.
- Review Ka, active force, toe moment, geometry ratio, and allowance utilization separately.
- Pass groundwater, surcharge, wall-friction, and soil-test evidence to the qualified geotechnical review.
SUBJECT FUNDAMENTALS
Terms behind Retaining Wall Capacity Calculator
- Rankine coefficient
- Active earth-pressure coefficient
- Unit weight
- Soil weight per volume
- Friction angle
- Effective shear-friction parameter
- Cohesion
- Effective cohesive intercept
- Toe moment
- Active force moment about toe
CALCULATION METHOD
Why this formula fits the topic
Ka = (1-sin phi)/(1+sin phi); z0 = 2c/(gamma sqrt(Ka)); Pa = 0.5 Ka gamma(H-z0)^2; M = Pa(H-z0)/3
This simplified Rankine screen assumes level drained backfill, no wall friction, and a triangular positive-pressure zone below the calculated tension crack. Cohesion is not carried as a uniform negative pressure, and the page rejects cases where the simplified crack depth reaches the full wall height.
DEEPER ANALYSIS
Where professional judgement still matters
Water pressure
Hydrostatic pressure is not included and can dominate demand.
Geometry
Backfill slope, surcharge, and wall friction alter the simple Rankine result.
WORKED DECISION CASES
Two realistic uses
Cohesive screening case
For H = 4.2 m, gamma = 18.5 kN/m3, phi = 30 degrees, and c = 5 kPa, Ka is 0.333 and the tension crack is about 0.936 m. The positive active force is about 32.84 kN/m and acts one third of the 3.264 m pressure height above the base.
Cohesion excluded for long-term review
Using the same height, unit weight, and friction angle with c = 0 makes z0 = 0. Active force rises to about 54.39 kN/m and toe moment to about 76.15 kN*m/m, exposing how the cohesion assumption changes the screen.
Technical glossary
- Active pressure
- Lateral soil pressure acting toward the wall movement direction.
- Rankine Ka
- The active coefficient from the friction-angle expression.
- Unit weight
- Soil weight per unit volume used in the pressure triangle.
- Friction angle
- The effective internal friction angle used by the model.
- Cohesion
- The effective shear intercept entered for this screening case.
- Toe moment
- The active resultant moment about the wall toe.
EVIDENCE AND DATA LINEAGE
What to retain with the result
Retain soil test values, groundwater level, wall geometry, and the chosen earth-pressure theory.
LIMITS AND EXCLUSIONS
What this model does not establish
- No surcharge, water, seismic, wall friction, bearing, or global stability analysis.
- The cohesion term can be non-conservative outside its validated soil model.
- Use a qualified geotechnical engineer and governing code.
RELIABLE SOURCES
References for method and context
FREQUENTLY ASKED QUESTIONS
Retaining Wall Capacity Calculator questions
Does this include surcharge?
No; enter a separate load model for surcharge.
Is cohesion always valid?
No. Long-term drained design often treats effective cohesion conservatively.
Why is base width an input?
It supplies the reported height-to-base ratio for geometry context; it does not alter the Rankine force equation.
Can phi be 90 degrees?
No; the model rejects values at or above 89 degrees.
Does it include water?
No. Add hydrostatic pressure in the governing design.
Is this a code capacity?
No; it is a transparent preliminary estimate.
IMPORTANT NOTE
Before relying on the exported result
Preliminary Rankine estimate only; not a final geotechnical design.
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