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

Within A-7, the subgroup depends on how $PI$ compares to $LL-30$: A-7-5 applies when $PI\le LL-30$, and A-7-6 applies when $PI>LL-30$.

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.

Exam tip: The GI formula, the $C_u$/$C_c$ gradation equations, and the plasticity chart (A-line) used later in this chapter are all printed in the FE Reference Handbook’s soil classification section. The tested skill is locating and applying the right formula under time pressure, not memorizing it. When you calculate GI, carry the intermediate terms unrounded and round only the final result, to the nearest whole number (never below zero).

Summary table of AASHTO classification

Group	Description	% fines (passing No. 200)	LL	PI	General subgrade rating
A-1-a	Stone fragments, gravel, and sand	≤ 15%	-	≤ 6	Excellent to good
A-1-b	Stone fragments, gravel, and sand	≤ 25%	-	≤ 6	Excellent to good
A-2-4	Silty or clayey gravel and sand	≤ 35%	≤ 40	≤ 10	Excellent to good
A-2-5	Silty or clayey gravel and sand	≤ 35%	> 40	≤ 10	Excellent to good
A-2-6	Silty or clayey gravel and sand	≤ 35%	≤ 40	> 10	Excellent to good
A-2-7	Silty or clayey gravel and sand	≤ 35%	> 40	> 10	Excellent to good
A-3	Fine sand	≤ 10%	-	NP	Excellent to good
A-4	Silty soils	> 35%	≤ 40	≤ 10	Fair to poor
A-5	Silty soils	> 35%	> 40	≤ 10	Fair to poor
A-6	Clayey soils	> 35%	≤ 40	> 10	Fair to poor
A-7-5	Clayey soils	> 35%	> 40	> 10, $\le LL-30$	Fair to poor
A-7-6	Clayey soils	> 35%	> 40	> 10, $>LL-30$	Fair to poor

The granular groups also have coarser-sieve limits: A-1-a needs ≤ 50% passing the No. 10 and ≤ 30% passing the No. 40 sieve, A-1-b ≤ 50% passing the No. 40, and A-3 ≥ 51% passing the No. 40. LL and PI are measured on the fraction passing the No. 40 sieve.

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.

Within the A-7 group, the subgroup depends on how $PI$ compares to $LL-30$:

• $PI\le LL-30\Rightarrow$ A-7-5
• $PI>LL-30\Rightarrow$ A-7-6

Here, $LL-30=45-30=15$, and $PI=18>15$, so the soil falls in 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) $\to$ a silt-clay soil rated fair to poor as a subgrade; within a group, the higher the GI, the less suitable the soil

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: 50% or more passes No. 200 sieve
   • Silts ($M$)
   • Clays ($C$)
3. Highly organic soils (Pt): Peat or organic-rich soils

Watch out: AASHTO and USCS draw the coarse/fine line at different points. AASHTO calls a soil “granular” when 35% or less passes the No. 200 sieve, while USCS calls a soil “coarse-grained” only when more than 50% is retained on the No. 200 sieve. A soil with, say, 40% passing No. 200 counts as fine-grained under AASHTO’s more conservative cutoff but still coarse-grained under USCS - the two systems’ thresholds aren’t interchangeable.

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: $C_u\ge 4,1\le C_c\le 3$
• Sand: $C_u\ge 6,1\le C_c\le 3$

Example: Checking gradation

A sand sample has $D_{10}=0.1\text{mm}$, $D_{30}=0.3\text{mm}$, and $D_{60}=1.0\text{mm}$. Is it well-graded?

$$C_u=\frac{D_{60}}{D_{10}}=\frac{1.0}{0.1}=10$$

$$C_c=\frac{(D_{30}{)}^2}{D_{10}\cdot D_{60}}=\frac{(0.3{)}^2}{0.1\times 1.0}=\frac{0.09}{0.1}=0.9$$

$C_u=10$ satisfies $C_u\ge 6$, but $C_c=0.9$ falls outside the required $1\le C_c\le 3$ range. Both conditions must hold for a well-graded sand, so this soil is poorly-graded (SP) even though it passes the $C_u$ check.

Answer: Poorly-graded sand (SP)

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)

Within each of these, the liquid limit sets low versus high plasticity: LL < 50 gives low plasticity (ML, CL), and LL ≥ 50 gives high plasticity (MH, 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: OL or OH for organic silts and clays (low or high liquid limit), and Pt for peat and other highly organic soils

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

Coarse-grained soils with 5-12% fines take dual symbols, such as SP-SM for a poorly-graded sand with silt.

Please refer to the FE Handbook for detailed classification charts.