Foundation Input Form
Use this advanced soil bearing calculator for shallow foundations. The page stays in a single-column flow, while the calculator fields use a responsive 3-column, 2-column, then 1-column layout.
Plotly Graph
This chart compares the pressure values from your most recent calculation.
Example Data Table
These sample rows are illustrative and help you understand how the calculator inputs may be organized during early foundation checks.
| Case | Shape | B / Dia (m) | L (m) | Df (m) | γ (kN/m³) | c (kPa) | φ (°) | Load (kN) | FOS |
|---|---|---|---|---|---|---|---|---|---|
| Warehouse footing | Strip | 2.00 | 6.00 | 1.50 | 18.00 | 20.00 | 28.00 | 900 | 3.00 |
| Column pad | Square | 2.40 | 2.40 | 1.80 | 19.00 | 15.00 | 30.00 | 1200 | 3.00 |
| Tank support | Circular | 3.20 | 3.20 | 1.20 | 17.50 | 10.00 | 26.00 | 1400 | 2.75 |
Formula Used
This calculator uses a Terzaghi-style shallow foundation approach with footing shape adjustments. The selected failure mode controls whether full soil strength or reduced local shear strength is used.
qallow,gross = qna + q
Nc = (Nq - 1) / tanφ
Nγ = 1.5(Nq - 1)tanφ
Shape factors used:
- Strip footing: sc = 1.0, sq = 1.0, sγ = 1.0
- Square footing: sc = 1.3, sq = 1.0, sγ = 0.8
- Circular footing: sc = 1.3, sq = 1.0, sγ = 0.6
A simple groundwater reduction is applied to the γ-term when the water table lies at or near the footing base. Local shear reduces both cohesion and friction contribution before factor generation.
How to Use This Calculator
- Choose the footing shape and failure mode.
- Enter the service load and footing dimensions.
- Fill in soil unit weight, cohesion, and friction angle.
- Add surcharge and water table depth if relevant.
- Provide a factor of safety for allowable pressure.
- Enter elastic modulus and Poisson ratio for settlement.
- Press Calculate Soil Bearing to view results above the form.
- Use the CSV or PDF buttons to export the result summary.
FAQs
1) What does this soil bearing calculator estimate?
It estimates gross ultimate, net ultimate, net allowable, and gross allowable bearing pressures for shallow foundations. It also checks service pressure, utilization, reserve pressure, required footing size, and an elastic settlement estimate.
2) Which footing shapes are supported?
It supports strip, square, and circular shallow footings. Each option uses different shape factors, footing area equations, and settlement influence values. This makes the output more practical for early design comparisons.
3) Why is factor of safety important?
Factor of safety reduces ultimate soil capacity to a safer allowable value. It helps account for uncertainty in soil properties, load estimates, groundwater changes, and construction variation during foundation design.
4) How does groundwater affect bearing capacity?
Groundwater reduces effective stress and lowers the contribution of soil unit weight in the bearing equation. When the water table is close to the footing base, allowable capacity usually decreases.
5) What is the difference between gross and net pressure?
Gross pressure includes the overburden pressure already acting at the foundation level. Net pressure removes that overburden effect, making it useful when comparing foundation load increase against the original soil state.
6) When should I use local shear instead of general shear?
Use local shear for looser or more compressible soils where failure is less distinct. The calculator reduces strength inputs for this mode, giving a more conservative estimate than general shear assumptions.
7) Is the settlement result a final design settlement?
No. It is a quick elastic estimate for screening. Final settlement design may require layered soil data, consolidation analysis, field testing, stress distribution checks, and project-specific code requirements.
8) Can this calculator replace a geotechnical report?
No. It is best for concept studies, checks, and education. Final foundation design should still rely on geotechnical investigation, laboratory testing, site conditions, and qualified engineering judgment.