Inputs
Enter Retaining Wall and Backfill Data
Use consistent US customary units. The calculator reports force per linear foot of wall.
Worked Example
Example Data Table
| Input | Example Value | Purpose |
|---|---|---|
| Wall height | 12 ft | Defines the retained soil prism. |
| Unit weight | 120 pcf | Estimates soil self-weight. |
| Friction angle | 30 degrees | Develops static and seismic coefficients. |
| Cohesion | 75 psf | Reduces calculated active lateral pressure. |
| Uniform surcharge | 250 psf | Adds lateral force through the coefficient. |
| Horizontal coefficient | 0.15 | Represents pseudo-static horizontal inertia. |
| Estimated seismic thrust | 3,806 lb/ft | Approximate result using the listed assumptions. |
Calculation Basis
Formula Used
This calculator applies a simplified Mononobe-Okabe active-pressure coefficient for a vertical, smooth wall and level backfill. Cohesion is then used as a screening adjustment.
Negative calculated thrust is set to zero. This avoids treating soil as capable of carrying sustained tension. The seismic increment equals the calculated seismic thrust minus the static active thrust.
Practical Steps
How to Use This Calculator
- Enter the retained wall height in feet.
- Enter the selected backfill unit weight in pounds per cubic foot.
- Provide the effective friction angle and verified cohesion value.
- Enter the uniform surcharge behind the retaining wall.
- Enter the project horizontal seismic coefficient.
- Select Calculate Load to display thrust, increment, location, and moment.
- Check messages about tension cracks or high coefficients.
- Export the displayed calculation as CSV or PDF when needed.
- Complete separate stability, drainage, structural, and code checks before construction.
Engineering Context
Cohesion and Seismic Lateral Loading
Cohesive soil can show apparent strength because particles bond together. This bond reduces active lateral pressure in an earth pressure model. The reduction is strongest near the top of the retained soil. A tension zone can form when the calculated pressure becomes negative. Real soil cannot carry sustained tensile stress. The calculator limits net thrust to zero or above.
Earthquake shaking changes the direction of the apparent gravity field. The horizontal seismic coefficient represents that effect in a pseudo-static check. The tool first finds a static active coefficient. It then finds a seismic active coefficient using a simplified Mononobe-Okabe expression. The wall is assumed vertical and smooth. Backfill is assumed level. Wall friction, groundwater pressure, and vertical seismic acceleration are excluded.
The result is expressed per linear foot of wall. It combines soil weight, uniform surcharge, and a cohesion adjustment. The seismic increment compares the calculated seismic thrust with the static thrust. The resultant height is a simplified composite location. Static force is placed at one-third of wall height. The increment is placed at approximately sixty percent of wall height. These positions are useful for preliminary overturning checks. They are not a substitute for a complete pressure distribution.
Use drained or undrained parameters only when the project method permits them. Obtain values from a qualified geotechnical investigation. Do not use apparent cohesion from unsaturated samples without engineering review. Cohesion can reduce over time. It can also change after excavation, wetting, cycling, cracking, or remolding. Seismic loading may cause strength loss in sensitive soils. Liquefaction, lateral spreading, and slope movement require separate evaluation.
Check the inputs before relying on the result. Confirm wall height, backfill unit weight, surcharge location, and the selected horizontal coefficient. Ensure the friction angle exceeds the inertia angle. Otherwise the simplified seismic coefficient is outside its valid range. Review drainage conditions carefully. Water pressures are not included. Add hydrostatic and seepage effects separately where they apply.
A retaining wall design also needs sliding, overturning, bearing, structural, settlement, global stability, and seismic displacement checks. Nearby foundations and utilities can change the load path. Construction sequencing matters. Site-specific codes may require different load combinations or more rigorous dynamic analysis. Treat this calculation as a transparent screening step. Final design decisions require a licensed engineer with the complete site information.
Common Questions
Frequently Asked Questions
1. What does this calculator estimate?
It estimates cohesion-adjusted seismic active earth thrust per linear foot of a vertical retaining wall. It also reports a static comparison, a pseudo-static increment, a simplified resultant location, and an estimated base moment.
2. Is this a final retaining wall design?
No. It is a preliminary screening calculation. Final wall design requires project-specific geotechnical, structural, drainage, foundation, seismic, and code checks performed by qualified professionals.
3. What cohesion value should I enter?
Enter a verified design cohesion value selected for the project condition. Do not assume apparent cohesion remains available after wetting, cracking, repeated loading, excavation, or long-term weather exposure.
4. Why is the horizontal seismic coefficient needed?
The horizontal seismic coefficient represents pseudo-static earthquake inertia. It creates an inertia angle and increases the active pressure coefficient in the simplified seismic calculation.
5. Why can the calculated thrust be zero?
Cohesion may make the mathematical lateral pressure negative near the wall top. Soil is not assumed to carry sustained tension, so the calculator limits net thrust to zero rather than reporting a negative force.
6. Does the calculator include water pressure?
No. Hydrostatic pressure, seepage, crack water, and drainage effects are excluded. Add them separately using the applicable water levels, drainage details, and project design method.
7. Can this tool be used for sloping backfill?
No. The listed equation assumes level backfill and a vertical, smooth wall. Sloping grades, wall batter, and wall friction need a suitable method with matching geometry and assumptions.
8. Why must the friction angle exceed the inertia angle?
The simplified seismic coefficient requires a physically valid failure wedge. When the inertia angle reaches or exceeds the friction angle, the expression becomes unsuitable for this screening calculation.
9. Which units does the calculator use?
Use feet for height, pounds per cubic foot for unit weight, pounds per square foot for cohesion and surcharge, and a unitless horizontal coefficient. Results are pounds per linear foot.
10. What is a uniform surcharge?
A uniform surcharge is a broadly distributed vertical pressure behind the wall. Typical sources include paved areas, stored materials, or building loads when they can be represented as uniform.
11. How should I use the reported result?
Use it as one input to preliminary wall stability checks. Compare it with the project load combinations, then verify sliding, overturning, bearing, structural capacity, drainage, and global stability.