Calculator inputs
The page stays in a single main column. Inside the calculator, inputs use three columns on large screens, two on medium, and one on mobile.
Example data table
This sample set illustrates a typical rising pressure-settlement response. Use it as a guide only, not as a design recommendation.
| Point | Pressure (kPa) | Settlement (mm) | Secant Modulus (MN/m³) |
|---|---|---|---|
| 1 | 10 | 0.35 | 28.5714 |
| 2 | 20 | 0.78 | 25.6410 |
| 3 | 40 | 1.60 | 25.0000 |
| 4 | 60 | 2.46 | 24.3902 |
| 5 | 80 | 3.35 | 23.8806 |
Formula used
k = q / s
kreg = Σ(si × qi) / Σ(si2)
kcorrected = kbasis × (Bplate / Bfoundation)n × Cshape × Cdepth × Cmoisture × Coverburden
kdesign = kcorrected / SF
qallow = kdesign × sallow
sestimate = qdesign / kdesign
Karea = kdesign × A
Pressure is converted to kPa and settlement to meters during calculation. This keeps the core modulus in kN/m³ before any display-unit conversion.
How to use this calculator
- Choose your preferred pressure, settlement, area, and modulus units.
- Enter one or more pressure-settlement pairs from a plate load or similar test.
- Select the evaluation basis: last secant, average secant, or origin regression.
- Enter plate width, footing width, exponent, and project adjustment factors.
- Add safety factor, allowable settlement, design pressure, and contact area.
- Press Calculate Modulus to see the result summary above the form.
- Review the graph, detailed tables, and downloadable CSV or PDF report.
- Compare design modulus and predicted settlement before using values in models.
FAQs
1. What is subgrade reaction modulus?
It is a Winkler-style soil support parameter linking contact pressure to settlement. Higher values indicate stiffer support under the loaded area.
2. Why are there several modulus results?
Different engineers may use a last secant value, an average secant value, or a regression-based value. This calculator shows all three, then applies your chosen basis for design adjustments.
3. Why does footing width reduce the modulus?
A larger foundation spreads stress differently than a smaller test plate. Many screening methods lower the reported plate modulus as the loaded width increases.
4. What does the size exponent do?
It controls how strongly plate results are scaled to the foundation width. A higher exponent gives a larger reduction when the foundation is much wider.
5. Should I use this for final foundation design?
Use it for checking, comparing, or predesign work. Final foundation values should come from project-specific geotechnical recommendations and the governing design code.
6. What is the area spring rate output?
It multiplies design modulus by contact area to estimate an equivalent vertical spring rate. This can help when building simplified structural or finite-element support models.
7. Why does moisture factor matter?
Soil stiffness may change with moisture condition, saturation, and seasonal variation. The factor lets you quickly reduce or increase screening values during sensitivity studies.
8. Which basis should I choose?
For smooth datasets, origin regression is often stable. For conservative screening, engineers sometimes choose the last secant value near service load levels.