Last reviewed 16 Sept 2026 · 6 min read
Why classify soils
Classification places a soil in a group of similar engineering behaviour using simple index properties (grain size and plasticity), so engineers can communicate, anticipate problems and choose tests and construction methods before detailed testing.
Particle-size and textural classifications
- Particle-size classification names soil by size ranges (IS: gravel 4.75–80 mm, sand 0.075–4.75 mm, silt 0.002–0.075 mm, clay < 0.002 mm).
- Textural classification (triangular chart of % sand, silt, clay, e.g. US Bureau of Public Roads/USDA) names soils such as sandy loam, silty clay — common in agriculture and highway work but ignores plasticity.
Indian Standard Soil Classification System (IS 1498:1970)
Based on the Unified system, with three plasticity divisions for fine-grained soils.
Major divisions
| Division | Criterion | Primary letters |
|---|---|---|
| Coarse-grained soils | More than 50% (by mass) retained on the 75 µm sieve | G (gravel), S (sand) |
| Fine-grained soils | More than 50% passing 75 µm | M (silt), C (clay), O (organic) |
| Highly organic soils | Fibrous, dark, odorous | Pt (peat) |
Gravel vs sand: if more than half of the coarse fraction is retained on the 4.75 mm sieve → gravel; otherwise sand.
Secondary letters
| Letter | Meaning |
|---|---|
| W | Well graded |
| P | Poorly graded |
| M | With non-plastic/low-plastic fines (silty) |
| C | With plastic fines (clayey) |
| L | Low plasticity/compressibility ( < 35%) |
| I | Intermediate ( 35–50%) |
| H | High ( > 50%) |
Coarse-grained soils — groups
| Fines (passing 75 µm) | Classification |
|---|---|
| Less than 5% | GW / GP, SW / SP by and (well graded: gravel , sand , and ) |
| More than 12% | GM / GC, SM / SC by where the fines plot on the plasticity chart (below A-line or → M; above A-line and → C) |
| 5% to 12% | Borderline — dual symbols, e.g. GW-GM, SP-SC |
| between 4 and 7 above A-line | Dual symbol such as SM-SC |
Fine-grained soils — the plasticity chart
Plot plasticity index () against liquid limit ().
- Above the A-line → clays (C); below → silts (M) or organic soils (O).
- Vertical lines at = 35% and 50% separate L, I, H.
Groups: CL, CI, CH (inorganic clays); ML, MI, MH (inorganic silts); OL, OI, OH (organic silts and clays, identified by odour, colour and a sharp drop in LL after oven drying); Pt (peat).
The hatched zone just above the A-line with between 4 and 7 at low LL is designated CL-ML.
Scale: horizontal axis 0–100% liquid limit; the A-line meets the axis at = 20%.
Field identification of fine-grained soils
| Test | Procedure | Clay | Silt |
|---|---|---|---|
| Dilatancy (shaking) | Shake a wet pat; watch water appear/disappear | None to slow | Quick reaction |
| Toughness | Roll a thread near the plastic limit | Tough, stiff thread | Weak, crumbly |
| Dry strength | Crush a dried lump between fingers | High | Low (powders easily) |
| Feel / washing | Rub in water | Smooth, sticky, hard to wash off | Gritty, washes off |
Organic soils: dark colour, musty odour, spongy feel.
Other classification systems
Unified Soil Classification System (USCS)
Same letter symbols as IS 1498, but fine-grained soils have only two plasticity divisions: L ( < 50%) and H ( ≥ 50%).
AASHTO (HRB) system — for highway subgrades
Soils are grouped A-1 to A-7 (A-1 best as subgrade; A-4 to A-7 silt–clay materials; A-8 peat) using sieve analysis (2 mm, 425 µm, 75 µm) and Atterberg limits. Within groups, the group index ranks quality:
- = % passing 75 µm in excess of 35 (0 to 40)
- = % passing 75 µm in excess of 15 (0 to 40)
- = liquid limit in excess of 40 (0 to 20)
- = plasticity index in excess of 10 (0 to 20)
GI ranges from 0 (good subgrade) to 20 (very poor); higher GI → thicker pavement.
Worked examples
A soil has 3% passing 75 µm; 70% of the coarse fraction passes 4.75 mm; = 0.15 mm, = 0.40 mm, = 1.10 mm. Classify.
Solution. Coarse-grained (97% retained on 75 µm). More than half the coarse fraction passes 4.75 mm → sand. Fines < 5% → W or P.
(> 6); (< 1) → not well graded → SP (poorly graded sand).
A soil has 78% passing 75 µm, = 45%, = 22%. Classify.
Solution. Fine-grained. . A-line at = 45: → 23 > 18.25 → above A-line → clay. between 35 and 50 → CI (clay of intermediate plasticity).
A subgrade soil: 60% passing 75 µm, = 48%, = 18%. Find GI.
Solution. ; (max); ; .
Frequently tested points
- IS 1498: coarse-grained if > 50% retained on 75 µm; gravel vs sand at 4.75 mm on the coarse fraction.
- Fines < 5%: W/P; > 12%: M/C; 5–12%: dual symbols.
- A-line ; above = clay, below = silt/organic.
- IS divisions L < 35%, I 35–50%, H > 50%; USCS uses only L/H at 50%.
- Dilatancy quick for silts; dry strength high for clays.
- ; AASHTO A-1 best, A-7 poorest (A-8 peat).
- Using the whole sample (instead of the coarse fraction) to decide gravel vs sand.
- Classifying fine soils by LL alone without checking the A-line.
- Forgetting the limits (0–40, 0–20) on , , , in the group index.
- Classification groups soils by grain size and plasticity.
- IS 1498 uses G, S, M, C, O, Pt with W, P, M, C, L, I, H qualifiers.
- The plasticity chart and A-line separate clays from silts.
- Field tests identify fine soils quickly; AASHTO and group index serve highway work.