Soil classification

This chapter covers the following:

• AASHTO soil classification
• USCS soil classification

AASHTO soil classification system

The AASHTO Soil Classification System was developed by the American Association of State Highway and Transportation Officials. It classifies soils based on their suitability as subgrade material for highway construction. You’ll most often see it used to evaluate soil performance for pavement subgrades and embankments.

Basic criteria used

• Grain size distribution
• Liquid limit (LL)
• Plasticity index (PI)

Particle size limits (as per AASHTO)

Fraction	Size (mm)	Sieve No.
Gravel	$>2.0$	No. 10
Sand	$0.075-2.0$	No. 200 - No. 10
Silt & Clay (Fines)	$<0.075$	No. 200

Soil groups and classification

Granular Materials (≤ 35% passing No. 200 sieve):

• Groups: A-1, A-2, A-3

Silt-Clay Materials (> 35% passing No. 200 sieve):

• Groups: A-4, A-5, A-6, A-7

Group index (GI)

The group index (GI) is a number used to further rate subgrade quality within the AASHTO groups. It is calculated as:

$$GI=(F-35)\left[0.2+0.005(LL-40)\right]+0.01(F-15)(PI-10)$$

Where:

• $F$: Percent passing No. 200 sieve
• $LL$: Liquid limit
• $PI$: Plasticity index

If GI < 0, it is taken as zero. A higher GI generally indicates poorer subgrade performance.

Summary table of AASHTO classification

Group	Description	% fines	LL	PI	Subgrade quality
A-1-a	Gravel + Sand	≤ 35%	≤ 40	≤ 6	Excellent
A-1-b	Sand + Gravel	≤ 35%	≤ 40	≤ 6	Good
A-2	Silty/Clayey Sand	≤ 35%	≤ 40	≤ 10	Fair
A-3	Fine Sand	≤ 35%	≤ 40	NP	Poor drainage
A-4	Silty Soil	> 35%	≤ 40	≤ 10	Marginal
A-5	Highly Silty Soil	> 35%	> 40	≤ 10	Very poor
A-6	Clayey Soil	> 35%	≤ 40	> 10	Low strength
A-7-5	Silty Clay	> 35%	> 40	≤ PI line	Very poor
A-7-6	Plastic Clay	> 35%	> 40	> PI line	Worst

Example

Given:

• $F=42\%$
• $LL=45$
• $PI=18$

Step 1: Group type

Since $F>35\%$, the soil is a Silt-Clay material.

Because $LL>40$ and $PI>10$, it falls in the A-7 group.

Next, compare $PI$ to the $A-$line:

• $PI$ above $A-$line $\Rightarrow$ A-7-6

Step 2: Calculate GI

Substitute the given values into the GI equation:

$$GI=(42-35)\left[0.2+0.005(45-40)\right]+0.01(42-15)(18-10)$$

Compute step by step:

$$GI=7(0.2+0.025)+0.01(27)(8)=7(0.225)+2.16=1.575+2.16=3.735\approx 4$$

Final classification:

A-7-6 (GI = 4) $\to$ Poor subgrade quality

Unified soil classification system (USCS)

The Unified Soil Classification System (USCS) classifies soils based on particle size distribution and plasticity characteristics. In geotechnical engineering, it’s commonly used to help predict soil behavior.

Objectives

• Classify soil for engineering purposes
• Predict behavior (shear strength, permeability, compaction)
• Standardize communication among engineers and geologists

Major soil categories

1. Coarse-grained soils: More than 50% retained on No. 200 sieve
   • Gravels ($G$)
   • Sands ($S$)
2. Fine-grained soils: More than 50% passes No. 200 sieve
   • Silts ($M$)
   • Clays ($C$)
3. Highly organic soils (Pt): Peat or organic-rich soils

Grain size boundaries

Soil type	Size range
Gravel	$>4.75\text{mm}$
Sand	$0.075\text{mm}-4.75\text{mm}$
Silt/Clay	$<0.075\text{mm}$

Coarse-grained soil classification

Step 1: Gravel or Sand?

• Gravel (G): If more than 50% of the coarse fraction is $>4.75\text{mm}$
• Sand (S): If more than 50% of the coarse fraction is $<4.75\text{mm}$

Step 2: Gradation criteria

• Coefficient of uniformity (Cu):

$$C_u=\frac{D_{60}}{D_{10}}$$

• Coefficient of curvature (Cc):

$$C_c=\frac{(D_{30}{)}^2}{D_{10}\cdot D_{60}}$$

Well-graded conditions:

• Gravel: $C_u>4,1<C_c<3$
• Sand: $C_u>6,1<C_c<3$

Fine-grained soil classification

Atterberg limits:

• Liquid limit (LL)
• Plastic limit (PL)
• Plasticity index (PI):

$$PI=LL-PL$$

Plasticity chart:

• Below A-line: Silts (ML, MH)
• Above A-line: Clays (CL, CH)

Symbol	Description
CL	Inorganic clay (low plasticity)
CH	Inorganic clay (high plasticity)
ML	Inorganic silt (low plasticity)
MH	Inorganic silt (high plasticity)
OL	Organic silt/clay (low plasticity)
OH	Organic silt/clay (high plasticity)

Organic soils

• Identified by color, odor, and lower specific gravity
• Designation: Pt (peat)

USCS symbols overview

Soil type	Well-graded	Poorly-graded	With fines
Gravel	GW	GP	GM (silty), GC (clayey)
Sand	SW	SP	SM, SC
Fines	-	-	ML, CL, MH, CH

Examples:

• SW: Well-graded sand
• CL: Inorganic clay (low plasticity)
• SP-SM: Poorly graded sand with silt

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Example

Given:

• $55\%$ passing No. 200 sieve $\to$ Fine-grained
• $LL=42\%$, $PL=24\%$
• $PI=42-24=18$

Interpretation:

• $PI=18$, $LL=42$ $\to$ above A-line $\to$ Clay
• $LL<50$ $\to$ Low plasticity

USCS symbol: CL

Summary table

Group	Description
GW	Well-graded gravel
GP	Poorly-graded gravel
GM	Silty gravel
GC	Clayey gravel
SW	Well-graded sand
SP	Poorly-graded sand
SM	Silty sand
SC	Clayey sand
ML	Inorganic silt (low LL)
CL	Inorganic clay (low LL)
MH	Inorganic silt (high LL)
CH	Inorganic clay (high LL)
OL	Organic silt/clay
OH	Organic silt/clay (high LL)
Pt	Peat

Please refer to the FE Handbook for detailed classification charts.