Soil classification is the systematic grouping of soils with similar engineering properties. It is the first step in every geotechnical investigation — a properly classified soil tells the engineer about its strength, compressibility, drainage, and suitability for various uses. IS 1498:1970 (the Indian Standard) is closely based on the Unified Soil Classification System (USCS), with minor differences.
Why Classify Soils?
- Predict engineering behaviour (strength, settlement, permeability)
- Select appropriate foundation type
- Determine suitability for embankment, pavement subgrade, backfill
- Communication between geotechnical engineers worldwide
- Required by IS 1892 (site investigation), IRC 37 (pavement design), and IS 8009 (foundation design)
Overview of IS 1498:1970
IS 1498:1970 classifies soils into two major divisions and 15 groups based on grain size and plasticity characteristics:
Division A: Coarse-Grained Soils (>50% retained on 75 μm sieve)
| Group Symbol | Name | Criteria |
|---|---|---|
| GW | Well-graded gravel | Cu ≥ 4 and 1 ≤ Cc ≤ 3 |
| GP | Poorly graded gravel | Not meeting GW criteria |
| GM | Silty gravel | Fines > 12%, plot below A-line |
| GC | Clayey gravel | Fines > 12%, plot above A-line |
| SW | Well-graded sand | Cu ≥ 6 and 1 ≤ Cc ≤ 3 |
| SP | Poorly graded sand | Not meeting SW criteria |
| SM | Silty sand | Fines > 12%, plot below A-line |
| SC | Clayey sand | Fines > 12%, plot above A-line |
Division B: Fine-Grained Soils (>50% passing 75 μm sieve)
| Group Symbol | Name | Plasticity |
|---|---|---|
| ML | Inorganic silt, low plasticity | LL < 35%, below A-line |
| CL | Inorganic clay, low plasticity | LL < 35%, above A-line, PI ≥ 7 |
| OL | Organic silt/clay, low plasticity | LL < 35%, LL(oven-dried)/LL(air-dried) < 0.75 |
| MI | Inorganic silt, intermediate | 35% ≤ LL < 50%, below A-line |
| CI | Inorganic clay, intermediate | 35% ≤ LL < 50%, above A-line |
| OI | Organic, intermediate plasticity | 35% ≤ LL < 50%, organic |
| MH | Inorganic silt, high plasticity | LL ≥ 50%, below A-line |
| CH | Inorganic clay, high plasticity | LL ≥ 50%, above A-line |
| OH | Organic clay/silt, high plasticity | LL ≥ 50%, organic |
| Pt | Peat | High organic content, fibrous |
The A-Line on the Plasticity Chart
The Plasticity Chart (Casagrande's chart) plots Plasticity Index (PI) vs Liquid Limit (LL). The A-line separates clays (above) from silts (below):
A-line equation: PI = 0.73 × (LL – 20)Key points on the plasticity chart:
- All points above the A-line: clay behaviour (cohesive, plastic)
- All points below the A-line: silt behaviour (low plasticity, prone to frost heave)
- U-line (upper limit): PI = 0.9(LL – 8) — no soil should plot above this; if it does, check your test results
Key Index Properties for Classification
Atterberg Limits (IS 2720 Parts 5 & 6)
| Limit | Definition | Test Method |
|---|---|---|
| Liquid Limit (LL) | Water content at which soil flows like a viscous fluid (25 blows in Casagrande device) | IS 2720 Part 5 |
| Plastic Limit (PL) | Minimum water content at which soil can be rolled into 3mm thread without crumbling | IS 2720 Part 5 |
| Plasticity Index (PI) | PI = LL – PL; range over which soil remains plastic | Calculated |
| Shrinkage Limit (SL) | Water content below which volume change ceases | IS 2720 Part 6 |
Grading Parameters (for coarse soils)
Coefficient of Uniformity: Cu = D60 / D10
Coefficient of Curvature: Cc = (D30)² / (D10 × D60)- GW: Cu ≥ 4, 1 ≤ Cc ≤ 3
- SW: Cu ≥ 6, 1 ≤ Cc ≤ 3
Activity of Clay (Skempton's Activity)
Activity (A) = PI / (% clay fraction finer than 2 μm)| Activity Value | Clay Type | Mineral |
|---|---|---|
| < 0.75 | Inactive | Kaolinite |
| 0.75–1.25 | Normal | Illite |
| > 1.25 | Active | Montmorillonite (very expansive) |
High-activity clays (black cotton soil — CH/MH group in India) have A = 1.5–7. They are notorious for swell-shrink behaviour, causing damage to light structures and pavements.
Black Cotton Soil — Special Case in Indian Geotechnics
Black cotton soil (BC soil) covers ~25% of India's land area (Deccan Plateau, Maharashtra, MP, AP, Karnataka). Classification: typically CH or MH.
- LL = 50–100%, PI = 25–60%
- Swell pressure: 10–400 kPa
- Very poor as foundation soil and pavement subgrade (CBR = 1–3%)
- Treatment: lime stabilisation, granular sub-base over thick capping layer
Practical Classification Workflow
- Perform sieve analysis (IS 2720 Part 4) — determine % passing 75 μm
- If > 50% passing 75 μm: perform Atterberg limits (IS 2720 Part 5)
- Plot PI vs LL on plasticity chart → read group symbol from chart
- If ≤ 50% passing 75 μm: calculate Cu and Cc → classify as G or S group
- For borderline cases (5–12% fines): use dual symbols (e.g., SW-SC)
- Assign field description: colour, odour, consistency (soft/stiff/hard), structure
Frequently Asked Questions
What is the difference between IS 1498 and USCS?
IS 1498:1970 closely follows USCS (ASTM D2487) but uses different LL boundaries (35% and 50% vs 50% in original USCS) and adds intermediate plasticity groups (MI, CI, OI). Both use the same A-line equation and group symbol system.
What does CL-ML mean?
CL-ML is a borderline classification for soils plotting near the A-line with PI between 4 and 7 and LL < 35%. These soils exhibit both clay and silt behaviour — intermediate cohesion and moderate permeability.
How does soil classification affect foundation design?
Classification directly informs safe bearing capacity estimates (IS 6403), expected settlement (IS 8009), and compressibility behaviour. CH soils require deeper foundations or soil improvement; SW and GW are ideal foundation soils. IS 1892 uses soil classification to guide the scope of site investigation.