Inputs
Example data table
| Scenario | LL % | PI % | Clay % | PVR % | Swell (kPa) | Drainage | Adjusted band |
|---|---|---|---|---|---|---|---|
| Residential plot | 42 | 22 | 35 | 8 | 180 | Fair | High |
| Industrial yard | 30 | 12 | 18 | 4 | 90 | Good | Moderate |
| Cut-and-fill slope | 58 | 34 | 52 | 13 | 320 | Poor | Very High |
Recent calculation history
Formula used
The calculator builds a screening indicator by normalizing each input into a 0–1 scale and applying weights that sum to 1.00. The base score is then:
After that, a conservative safety factor is applied:
Band thresholds:
- Low: 0–24
- Moderate: 25–49
- High: 50–74
- Very High: 75–100
Weights used in the base score
| Factor | Weight | Normalization range (typical) |
|---|---|---|
| Liquid limit (LL) | 0.12 | 20–70 |
| Plasticity index (PI) | 0.12 | 0–40 |
| Clay fraction | 0.10 | 5–60 |
| Dry density | 0.06 | 1.35–2.05 (inverted) |
| Potential volume change (PVR) | 0.14 | 0–15 |
| Swell pressure | 0.12 | 0–300 |
| Active zone depth | 0.08 | 0.5–4.0 |
| Moisture variation | 0.10 | 0–12 |
| Drainage condition | 0.06 | Good/Fair/Poor |
| Surface cover | 0.04 | Paved/Mulch/Vegetation/Bare |
| Climate | 0.03 | Arid/Semi-arid/Temperate/Humid |
| Foundation type | 0.03 | Slab/Raft/Pier/Basement |
How to use this calculator
- Gather basic lab and site parameters for the near-surface soil.
- Enter LL, PI, clay fraction, density, PVR, and swell pressure values.
- Estimate active zone depth and seasonal moisture variation for the site.
- Select drainage, surface cover, climate, and foundation type conditions.
- Click Calculate Risk and review the adjusted band.
- Use the CSV or PDF buttons for reporting and record keeping.
Key soil index signals
Liquid limit and plasticity index often flag shrink–swell behavior early. Many projects treat LL above 40% and PI above 20% as a caution zone. When LL rises beyond 60% and PI exceeds 35%, the soil fabric can store more water, increasing movement risk. Use the calculator to compare lab sets from borings and see how the indicator shifts.
Volume change and pressure data
Potential volume change is a direct movement proxy. Screening ranges commonly follow: 0–3% low, 3–6% moderate, 6–10% high, and over 10% very high. Swell pressure adds distress potential under slabs and grade beams. Values near 50–100 kPa can crack brittle finishes, while 150–300 kPa can lift lightly loaded elements if moisture control is weak. Enter both PVR and swell pressure to keep the indicator responsive to lab evidence.
Active zone and moisture swing
The active zone depth represents how far seasonal wetting and drying can penetrate. For many building sites it falls between 0.8 and 2.5 m, but deep desiccation can extend beyond 3 m in dry climates with trees. Seasonal moisture variation of 3–6% is typical where drainage is controlled, while 8–12% is plausible near poor grading or leaking services. These inputs influence differential movement potential around foundations.
Site conditions that amplify risk
Drainage and surface cover change how quickly moisture equilibrates. Poor drainage can keep soils near saturation after storms, raising swell potential and softening bearing. Bare soil at the perimeter can drive faster drying, increasing shrinkage and edge drop. Climate matters because evaporation demand shapes moisture gradients; arid settings can create larger wet–dry cycles than humid regions. Use the scenario controls to test realistic combinations rather than relying on one lab number.
Interpreting the indicator for design
The score is a comparative screening tool, not a substitute for geotechnical design. Treat Low and Moderate bands as a prompt to maintain consistent moisture with grading, downspouts, and controlled irrigation. High and Very High bands indicate that movement-sensitive details may be required, such as moisture barriers, deeper support, or stiffened slabs. Export CSV or PDF outputs to document assumptions, then validate with local standards and investigations.
FAQs
1) What does the adjusted score represent?
The adjusted score scales the base indicator by your chosen safety factor and clamps it to 0–100. It helps you screen conservatively when inputs are uncertain or variability across borings is expected.
2) Can I use this for final foundation sizing?
No. It is a screening indicator for comparison and documentation. Final sizing should follow a geotechnical report, local codes, and engineered movement mitigation details suited to the structure and site.
3) Why include both PVR and swell pressure?
PVR reflects likely movement magnitude, while swell pressure reflects the stress the soil can impose. Together they better represent cracking and uplift potential than either parameter alone.
4) What if I don’t know the active zone depth?
Use a conservative estimate based on local practice, climate, and vegetation. Run several depths (for example 1.0 m, 2.0 m, 3.0 m) to bracket the risk and document sensitivity.
5) How should I choose the safety factor?
Start with 1.05–1.15 for typical uncertainty. Use higher values when data are sparse, soils vary across the site, or moisture control is difficult to guarantee during the building life.
6) Do drainage upgrades really change risk?
Yes. Grading, downspout discharge, and perimeter drains reduce wetting events and moisture gradients. Improving drainage and cover typically lowers seasonal moisture swings, which can reduce differential movement.