Formula used
The calculator applies method-specific bearing factors, shape factors, depth factors, and simplified inclination corrections. It adjusts surcharge and unit weight for groundwater. Final foundation design still requires geotechnical and structural review.
How to use this calculator
- Select SI or US units before entering project values.
- Choose a bearing-capacity method, foundation shape, and design basis.
- Enter footing dimensions, soil strength, unit weights, and groundwater depth.
- Add service or factored loads, moments, horizontal force, uplift, and direct eccentricity.
- Enable advanced mode for settlement, field correlations, stability, and custom factors.
- Calculate and compare bearing pressure, settlement, contact, and stability results.
- Export the completed summary to CSV, PDF, print, or clipboard.
Example construction data
| Input | Example value | Purpose |
|---|---|---|
| Footing size | 2.0 m × 3.0 m | Defines gross and effective bearing area. |
| Embedment depth | 1.2 m | Establishes soil surcharge at foundation level. |
| Cohesion and friction | 15 kPa and 30° | Controls bearing factors and strength terms. |
| Soil unit weight | 18 kN/m³ | Controls overburden and the Nγ contribution. |
| Service vertical load | 950 kN | Creates average and eccentric contact pressure. |
| Groundwater depth | 3.0 m | Determines whether buoyancy corrections apply. |
| Elastic modulus | 25 MPa | Supports immediate settlement screening. |
Understanding soil bearing capacity results
Ultimate bearing capacity describes the pressure associated with a modeled soil failure condition. Allowable capacity reduces that value using a selected safety approach. Applied contact pressure should remain below the governing resistance.
Foundation performance also depends on settlement, groundwater, load eccentricity, and nearby slopes. A footing may pass shear capacity while exceeding movement limits. Compare both bearing and settlement before choosing dimensions.
Eccentric moments reduce the effective footing width and length. Large eccentricity can produce zero or tensile contact pressure. Partial-contact behavior then needs a more detailed soil-structure analysis.
Field-test correlations can provide helpful comparison values when used carefully. Their coefficients depend on soil type, energy corrections, scale, and local practice. Project-specific geotechnical recommendations should govern final values.
The sliding, overturning, and uplift checks are preliminary screening calculations. Structural footing shear, flexure, punching, anchorage, and global stability remain separate design tasks. Review every warning before issuing construction documents.
This tool supports alternatives, checks, and transparent calculation records. It does not identify weak layers automatically from boring logs. Qualified professionals must confirm assumptions using site investigation evidence.
Frequently asked questions
What is the difference between ultimate and allowable bearing capacity?
Ultimate capacity represents a modeled soil failure pressure. Allowable capacity applies safety or resistance factors and may also be limited by settlement or a geotechnical report.
Which bearing-capacity method should I select?
Use the method required by your project criteria and geotechnical basis. Compare multiple methods when no single method is mandated.
How does groundwater affect the result?
Groundwater reduces effective unit weight below the water table. That reduction can lower surcharge and the unit-weight contribution to bearing capacity.
Why does eccentricity reduce capacity?
Eccentric loading shifts the resultant and reduces the effective contact area. Smaller effective dimensions increase demand and may reduce calculated resistance.
Can I use an SPT correlation as the final design value?
Use correlations only when their assumptions, corrections, and local calibration are appropriate. A geotechnical engineer should confirm their project applicability.
Does passing bearing capacity guarantee acceptable settlement?
No. Shear capacity and settlement are different limit states. A footing can remain stable while moving more than the structure can tolerate.
What does a negative minimum contact pressure mean?
It indicates that the full-contact rectangular pressure equation predicts tension. Soil cannot sustain tension, so partial-contact analysis may be necessary.
Can this calculator design footing reinforcement?
No. It screens geotechnical bearing, settlement, and stability. Structural flexure, one-way shear, punching shear, and detailing require separate calculations.
Can I use the result without a soil investigation?
Presumptive or assumed properties may support early planning only. Final design should use site-specific information appropriate to the structure and ground conditions.
Why are custom correction factors available?
They allow experienced users to apply a selected standard, research method, or project-specific interpretation. Inputs should be documented in the exported report.