Inputs
Example Data Table
| Scenario | Soil type | PI (%) | Clay (%) | Moisture (i to f) (%) | Stress (kPa) | Thickness (m) | Area (m^2) |
|---|---|---|---|---|---|---|---|
| Seasonal wetting near landscaping | Medium plastic clay | 25 | 35 | 14 to 20 | 80 | 1.5 | 25 |
| Drying due to prolonged heat | High plastic clay | 45 | 55 | 22 to 15 | 60 | 2.0 | 40 |
| Low-expansive soil zone | Silty clay | 15 | 25 | 12 to 14 | 120 | 1.2 | 30 |
Replace examples with site-specific lab results whenever available.
Formula Used
How to Use This Calculator
- Select the soil type that best matches your site observations.
- Enter PI and clay content from index testing where available.
- Provide initial and final moisture to represent seasonal change.
- Set the overburden stress and active layer thickness for the foundation zone.
- Use swell and shrink indices to align with local experience or lab swell tests.
- Click Calculate to view strain, movement, and volume results.
- Use the CSV/PDF buttons to download a record for reviews.
Tip: If you have oedometer swell percent or shrinkage data, calibrate the indices so strain aligns with test-based strain at similar stress.
Expansive soil movement and structural serviceability
Swell and shrink behavior is a leading cause of slab cracking, joint distress, and differential movement in lightly loaded buildings. The mechanism is moisture-driven volume change within an active zone beneath and around foundations. Quantifying likely heave or settlement helps engineers choose drainage, moisture control, and foundation details that protect serviceability.
Field and laboratory inputs that influence volume change
Plasticity index and clay content describe how strongly the soil responds to wetting and drying. Initial and final moisture values capture seasonal or site changes such as irrigation, leaking utilities, or drought. Overburden stress represents confining pressure that restrains deformation, while active layer thickness defines the depth that experiences meaningful moisture variation.
Understanding the calculated strain, movement, and risk class
The calculator converts moisture change into an estimated vertical strain and then multiplies by active thickness to produce movement. A safety factor can be applied to report conservative displacement for early screening. Risk classification is based on the estimated strain magnitude and can guide whether more testing, instrumentation, or mitigation is justified.
Practical mitigation options for higher swell-shrink potential
Common measures include positive surface drainage, limiting near-foundation irrigation, and installing moisture barriers or cutoff walls. Stiffened slabs, grade beams, and deeper foundations can reduce differential movement sensitivity. Where risk is elevated, confirm parameters using site-specific testing, and coordinate assumptions with local geotechnical recommendations.
Example dataset for rapid checking and documentation
The values below represent a typical seasonal wetting case in medium plastic clay. Enter them to see how the movement estimate scales with moisture change and thickness. If you later receive lab swelling data, adjust the swell and shrink indices to align the model with measured behavior.
| Soil | PI (%) | Clay (%) | Moisture (i to f) (%) | Stress (kPa) | Thickness (m) | Area (m^2) |
|---|---|---|---|---|---|---|
| Medium plastic clay | 25 | 35 | 14 to 20 | 80 | 1.5 | 25 |
FAQs
1) What does the calculator estimate?
It estimates vertical strain and movement from soil moisture change within an active layer. Outputs support early screening of heave or settlement potential for slabs and shallow foundations.
2) Why are PI and clay content included?
PI and clay content indicate how sensitive the soil structure is to wetting and drying. Higher values generally correlate with greater swell-shrink potential and larger seasonal movements.
3) How should I choose initial and final moisture?
Use values that represent expected seasonal extremes or site changes. Consider irrigation, rainfall, drainage, and leaks. If uncertain, bracket a range and compare movement results.
4) What is the role of overburden stress?
Higher confining stress restrains volume change, reducing predicted strain. Use stress representative of the active layer depth, including soil cover and any permanent foundation surcharge.
5) What do swell and shrink indices mean?
They are tunable multipliers that shape the response for swelling versus drying. Calibrate them using local experience or lab results so predicted strain matches measured behavior at similar stress.
6) Does the safety factor change the risk class?
No. The risk class is based on estimated strain before the movement safety factor. The safety factor only increases reported movement to keep early design checks conservative.
7) Can I use this for final foundation design?
Use it for preliminary evaluation and documentation. Final design should rely on site-specific geotechnical investigation, lab testing, and local standards for expansive soils and foundation performance.