Expansive Subgrade Soil Classifier & Stabilization Sizer
Classify AASHTO soil groups ($A-1$ to $A-7-6$), calculate Group Index ($GI$), evaluate subgrade volumetric swelling potential, and determine recommended undercut depths, non-expansive capping layers, and lime/cement stabilization dosages referencing ERA 2013 design criteria.
Report Metadata
Laboratory Soil Test Parameters
ERA Subgrade Undercut & Stabilization Recommendations
Black Cotton Soil indicators detected: Recommended treatment includes 600mm undercut, woven geotextile separator, and 500mm non-expansive capping layer (CBR ≥ 15%, PI < 12%) or in-situ 4% hydrated lime stabilization.
The configured criterion indicates this treatment based on the entered parameters. Verify against project-specific geotechnical investigation and governing specifications.
- Execute excavation during dry weather to prevent heavy machinery bogging.
- Do NOT allow water to pond in excavated subgrade trenches.
- Compact capping layer to 95% Modified AASHTO T-180 dry density.
EXPANSIVE SUBGRADE SOIL & STABILIZATION REPORT
Reference Basis: Selected geotechnical criteria (Based on selected ERA 2013 criteria)
| Project Name: | [Insert Project Title] | Date / Time: | 9/23/2026 |
| Station / Borrow Pit: | [Insert Section Details] | Design Standard: | ERA Geotechnical Manual 2013 |
| Evaluated By: | User-entered project information | AASHTO Group: | A-7-6 (34) |
1. Laboratory Soil Classification & Volumetric Swell
| Liquid Limit (LL) | Plastic Limit (PL) | Plasticity Index (PI) | Passing #200 (%) | Free Swell (%) | Swelling Rating |
|---|---|---|---|---|---|
| 65% | 24% | 41% | 78% | 110% | Very High (Expansive Black Cotton Soil) |
2. Recommended Subgrade Improvement & Stabilization Schedule
| Undercut Depth | Capping Bed (CBR ≥ 15%) | Lime Dosage (%) | Cement Dosage (%) | Geotextile Fabric |
|---|---|---|---|---|
| 600 mm | 500 mm | 4% | 3% | RECOMMENDED |
ENGINEERING VERIFICATION: Review and verify the result against project-specific data, governing specifications and professional engineering judgment before final use.
Geotechnical Guidance (ERA 2013 Manual)
1. Black Cotton Soil Characteristics
Black cotton soils contain active montmorillonite clay minerals. They exhibit extreme volume change (high swelling when wet, deep shrinkage cracking when dry). Never place subbase directly on untreated A-7-6 subgrade.
2. Lime Stabilization Chemistry
Hydrated lime Ca(OH)₂ reacts with active clay minerals via cation exchange and pozzolanic reactions, permanently lowering PI and raising CBR from 2% to > 15%.
3. Moisture Ingress Prevention
Extend non-expansive capping layers across the full embankment width into the side slopes to prevent edge moisture infiltration.
Expansive Soil & Stabilization Sizer Documentation
Technical specifications, mathematical formulas, and civil guidelines
1. Functional Purpose & Methodology
Expansive soils—predominantly smectite-rich clay formations known commonly as Black Cotton Soils—undergo massive volumetric swelling upon moisture intake and deep shrinkage cracking upon desiccation. Placing highway pavements directly over untreated expansive subgrades leads to severe longitudinal edge cracking, differential pavement heave, and premature structural failure. The Expansive Soil & Stabilization Sizer classifies subgrades per AASHTO M-145, computes Group Index (GI), and specifies mitigation measures referencing the ERA Geotechnical Design Manual (2013) and ERA Pavement Design Manual (2013).
The tool evaluates laboratory index parameters (Liquid Limit, Plasticity Index, percent passing No. 200 sieve, and free swell index) to determine required undercut excavation depths, non-expansive capping layer thicknesses (CBR ≥ 15%), and chemical stabilization dosages (hydrated lime or Portland cement).
2. Practical Step-by-Step Instructions
- Input Laboratory Soil Tests: Enter Liquid Limit (LL%), Plastic Limit (PL%), Percentage passing standard #200 sieve (F200%), and 4-Day Soaked Free Swell Index (%).
- Review AASHTO Soil Group: The tool automatically calculates Plasticity Index (PI = LL - PL) and Group Index (GI), assigning AASHTO groups from A-1-a to A-7-6.
- Inspect Swelling Hazard Rating: Review the categorized swelling severity: Low, Medium, High, or Very High (Expansive Black Cotton).
- Evaluate Engineering Treatments: Inspect recommended subgrade undercut excavation depth, non-expansive granular capping layer thickness, impermeable geomembrane/geotextile barrier placement, and chemical stabilization dosages.
- Export Geotechnical Submittal Sheet: Click Print Clean Report to export a black-bordered geotechnical engineering calculation sheet for client submittal.
3. Mathematical Formulations & Variable Definitions
Soil classification and Group Index equations follow AASHTO M-145 standards:
- AASHTO Group Index (GI) Formula:GI = (F200 - 35) × [ 0.2 + 0.005 × (LL - 40) ] + 0.01 × (F200 - 15) × (PI - 10)Where F200 = percentage passing 0.075mm sieve (expressed as whole number), LL = Liquid Limit (%), and PI = Plasticity Index (%).
- Group Index Rounding Rule: GI is rounded to the nearest non-negative integer. For A-2-6 and A-2-7 subgroups, only the second term is evaluated: GI = 0.01 × (F200 - 15) × (PI - 10).
- Plasticity Index (PI):PI = LL - PL
- A-7-5 vs. A-7-6 Subgroup Criteria:If PI ≤ (LL - 30) → A-7-5 | If PI > (LL - 30) → A-7-6
4. Standard Treatment Guidelines (ERA Geotechnical Design Manual 2013)
Standard treatment envelopes per ERA 2013 Chapter 6 (Expansive Clays):
| Swell Potential | Soil Index Criteria | Recommended Undercut | Capping Layer (CBR ≥ 15%) | Stabilization Option |
|---|---|---|---|---|
| Low | PI < 20, Free Swell < 50% | None (0 mm) | 150 mm (if CBR < 5%) | Standard compaction at OMC |
| Medium | PI 20-35, Free Swell 50-80% | 300 mm | 300 mm | 2% - 3% Hydrated Lime |
| High | PI 35-50, Free Swell 80-120% | 600 mm | 500 mm | 3% - 5% Lime + Geotextile |
| Very High (Black Cotton) | PI > 50, Free Swell > 120% | 1,000 mm (1.0 m) | 600 mm + sealed shoulders | 4% - 6% Lime or Lime-Flyash |
5. Output Interpretation & Construction Controls
Specialized geotechnical construction controls on expansive subgrades:
- Compaction Moisture: Expansive soils must be compacted at 1% to 3% wet of Optimum Moisture Content (OMC) at 90% to 93% MDD to minimize swell potential.
- Paved Shoulders: Shoulder widths must be fully sealed with asphalt or double surface treatment to prevent rainwater infiltration into the subgrade edge.
- Side Drain Depth: Longitudinal side ditches must be placed at least 3.0m away from the toe of fill and lined with concrete or impermeable geomembrane.
6. Related Technical Guides & Workflow Integration
To model the complete structural cross-section over expansive subgrades, use the ERA Pavement Design & Overlay Calculator, and consult our guide on ERA 2013 Highway Design Standards.