Soil classification provides a systematic way of organising soils into groups with similar engineering behaviour.
Why Classify Soils?
- Describe soils in a standardised, universally understood language
- Predict engineering behaviour without extensive testing
- Select appropriate design parameters based on classification
- Specify material requirements for construction
- Communicate effectively between field and laboratory teams
Classification Fundamentals
The Basis of Classification
All major classification systems are based on two primary characteristics:
| Characteristic | Test Method | AS Standard |
|---|---|---|
| Grain size distribution | Sieve analysis and hydrometer | AS 1289.3.6.1 / AS 1289.3.6.3 |
| Plasticity | Atterberg limits (LL, PL, PI) | AS 1289.3.1.1 / AS 1289.3.2.1 |
Particle Size Definitions (AS 1726)
| Fraction | Size Range |
|---|---|
| Boulder | > 200 mm |
| Cobble | 63–200 mm |
| Coarse gravel | 19–63 mm |
| Medium gravel | 6.7–19 mm |
| Fine gravel | 2.36–6.7 mm |
| Coarse sand | 0.600–2.36 mm |
| Medium sand | 0.212–0.600 mm |
| Fine sand | 0.075–0.212 mm |
| Silt | 0.002–0.075 mm |
| Clay | < 0.002 mm |
Unified Soil Classification System (USCS)
The USCS (ASTM D2487 / AS 1726) is the most widely used system worldwide and the primary system in Australian geotechnical practice.
Classification Flowchart
┌─────────────────────────────────────────┐
│ SIEVE ANALYSIS │
│ % passing 75 μm (No. 200 sieve) │
└──────────────────┬──────────────────────┘
│
┌──────────────────┬─────┴─────┬──────────────────┐
│ │ │ │
< 50% on < 50% on ≥ 50% on ≥ 50% on
75 μm 75 μm 75 μm 75 μm
│ │ │ │
COARSE-GRAINED COARSE-GRAINED FINE-GRAINED FINE-GRAINED
(Gravels) (Sands) (Silt & Clay) (Silt & Clay)
│ │ │ │
┌────┴────┐ ┌────┴────┐ ┌───┴───┐ ┌───┴───┐
≥ 50% < 50% ≥ 50% < 50% Above Below Above Below
gravel gravel sand sand A-Line A-Line A-Line A-Line
(G) (S) (S) (G) (Clay) (Silt) (Clay) (Silt)
│ │ │ │ │ │ │
GW,GP SW,SP SM,SC GM,GC CL,CH ML,MH OL,OH Pt
Coarse-Grained Soils (> 50% retained on 75 μm)
Gravels (> 50% of coarse fraction retained on 2.36 mm)
| Symbol | Name | Criteria |
|---|---|---|
| GW | Well-graded gravel | $C_u \geq 4$ and $1 \leq C_c \leq 3$ |
| GP | Poorly graded gravel | $C_u < 4$ or $C_c < 1$ or $C_c > 3$ |
| GM | Silty gravel | Below A-Line or PI < 4 (fines 12%+) |
| GC | Clayey gravel | Above A-Line and PI > 7 (fines 12%+) |
| GW-GM | Well-graded gravel with silt | Fines 5–12% |
| GW-GC | Well-graded gravel with clay | Fines 5–12% |
Where: $C_u = D_{60}/D_{10}$, $C_c = D_{30}^2/(D_{60} \times D_{10})$
Sands (≥ 50% of coarse fraction passes 2.36 mm)
| Symbol | Name | Criteria |
|---|---|---|
| SW | Well-graded sand | $C_u \geq 6$ and $1 \leq C_c \leq 3$ |
| SP | Poorly graded sand | $C_u < 6$ or $C_c < 1$ or $C_c > 3$ |
| SM | Silty sand | Below A-Line or PI < 4 (fines 12%+) |
| SC | Clayey sand | Above A-Line and PI > 7 (fines 12%+) |
| SW-SM | Well-graded sand with silt | Fines 5–12% |
| SW-SC | Well-graded sand with clay | Fines 5–12% |
Fines interpretation (5–12%): When fines content falls between 5% and 12%, dual symbols are used (e.g., SP-SM, SW-SC).
3.3 Fine-Grained Soils (≥ 50% passes 75 μm)
Classification is based on plasticity chart position and organic content:
| Symbol | Name | Position on Plasticity Chart |
|---|---|---|
| ML | Low-plasticity silt | LL < 50%, below A-Line |
| CL | Low-plasticity clay | LL < 50%, above A-Line |
| OL | Low-plasticity organic | LL < 50%, below A-Line zone |
| MH | High-plasticity silt | LL ≥ 50%, below A-Line |
| CH | High-plasticity clay | LL ≥ 50%, above A-Line |
| OH | High-plasticity organic | LL ≥ 50%, below A-Line zone |
| Pt | Peat / highly organic | Organic material |
Plasticity Chart
The plasticity chart is the key classification tool for fine-grained soils:
| A-Line equation: | $PI = 0.73(LL - 20)$ |
|---|
Plasticity regions:
PI
60 │
│ CH
40 │
│ U-Line: PI = 0.9(LL - 8)
35 │ ──────────────────
│ MH
20 │
│ CL
7 │ ──── A-Line: PI = 0.73(LL - 20)
4 │ ────
│ ML OL / OH
0 └──────────────────────────── LL
0 20 35 50 80 100
USCS Group Symbols Summary
| Major Division | Group Symbol | Typical Soil Name |
|---|---|---|
| Gravels | GW | Well-graded gravel |
| GP | Poorly graded gravel | |
| GM | Silty gravel | |
| GC | Clayey gravel | |
| Sands | SW | Well-graded sand |
| SP | Poorly graded sand | |
| SM | Silty sand | |
| SC | Clayey sand | |
| Silts & Clays (LL < 50) | ML | Silt |
| CL | Lean clay | |
| OL | Organic clay/silt | |
| Silts & Clays (LL ≥ 50) | MH | Elastic silt |
| CH | Fat clay | |
| OH | Organic clay/silt | |
| Highly organic | Pt | Peat |
AS 1726 — Australian Standard Classification
AS 1726:2017 "Geotechnical Site Investigations" provides the Australian framework for soil and rock description and classification.
Soil Description Format (AS 1726)
The standardised description format follows this sequence:
| Order | Attribute | Example |
|---|---|---|
| 1 | Particle size | CLAYEY SAND |
| 2 | Plasticity | of low plasticity |
| 3 | Colour | brown |
| 4 | Moisture condition | moist |
| 5 | Structure | faintly laminated |
| 6 | Fabric | with sub-angular quartz |
| 7 | Strength/consistency | medium dense |
| 8 | Additional | with rootlets |
| 9 | Stratum name | Residual soil |
| 10 | Geological formation | Hawkesbury Sandstone |
Example: "Clayey SAND of low plasticity, brown, moist, dense, containing sub-angular sand grains — Residual soil from Hawkesbury Sandstone."
Consistency of Fine-Grained Soils (AS 1726)
| Classification | Undrained Shear Strength ($s_u$) | Field Guide |
|---|---|---|
| Very soft (vs) | < 12 kPa | Squeezes between fingers; ball changes shape |
| Soft (so) | 12–25 kPa | Easily indented by thumb; moulded by moderate finger pressure |
| Firm (fi) | 25–50 kPa | Indented by strong thumb pressure |
| Stiff (st) | 50–100 kPa | Cannot be indented by thumb; can be indented by thumbnail |
| Very stiff (vst) | 100–200 kPa | Indented only with difficulty by thumbnail |
| Hard (ha) | > 200 kPa | Cannot be indented by thumbnail |
Density of Coarse-Grained Soils (AS 1726)
| Classification | SPT N-value | Relative Density |
|---|---|---|
| Very loose (vl) | 0–4 | < 15% |
| Loose (lo) | 4–10 | 15–35% |
| Medium dense (md) | 10–30 | 35–65% |
| Dense (de) | 30–50 | 65–85% |
| Very dense (vd) | > 50 | > 85% |
Field Identification Methods
When laboratory testing is not available, field methods provide preliminary classification:
| Test | How It's Done | Interpretation |
|---|---|---|
| Dilatancy test | Shake a wet soil pat in the palm | Rapid water appearance = silt; slow = clay |
| Dry strength test | Crush a dry sample between fingers | High strength = clay; crumbles easily = silt |
| Toughness test | Roll soil to 3 mm thread near PL | Stiff/firm = clay; weak = silt |
| Shine test | Cut soil with knife | Dull surface = silt; shiny = clay |
| Hand texturing | Rub soil between fingers | Gritty = sand; smooth = silt; sticky = clay |
Visual-Manual Procedure
Step 1: Determine coarse/fine fraction
- Examine through 75 μm sieve or wash sample
- If > 50% retained — coarse-grained
Step 2: For coarse-grained soils
- Estimate gravel vs sand fraction
- Check grading (well-sorted vs poorly-sorted)
- Check fines type (silt vs clay via dilatancy)
Step 3: For fine-grained soils
- Perform hand thread test for approximate PI
- Check dry strength and dilatancy
- Determine approximate LL by rapid method
Step 4: Assign provisional USCS group
- Record as dual symbol if borderline (e.g., SM/SC)
Classification Worked Examples
Example 1: Well-Graded Sand
| Property | Value |
|---|---|
| % passing 75 μm | 2% |
| $D_{10}$ | 0.20 mm |
| $D_{30}$ | 0.65 mm |
| $D_{60}$ | 1.80 mm |
Classification: SW (Well-graded sand)
Example 2: High-Plasticity Clay
| Property | Value |
|---|---|
| LL | 62% |
| PL | 28% |
| PI | 34% |
A-Line check: $0.73(62 - 20) = 30.7$
PI (34) > 30.7, so above A-Line
LL (62) ≥ 50% → High plasticity
Classification: CH (Fat clay)
Engineering Practice
| Application | Classification Relevance |
|---|---|
| Foundation design | Allowable bearing capacity, settlement estimates |
| Earthworks | Compaction characteristics, borrow source selection |
| Pavement design | Subgrade CBR estimation, material suitability |
| Retaining walls | Earth pressure parameters, drainage requirements |
| Slope stability | Shear strength parameters, drainage behaviour |
| Tunnelling | Excavation method, support requirements |
| Ground improvement | Method selection (vibro, deep mixing, drainage) |