Soil Classification - Theory & Concepts

Soil Classification

Learning Objectives

Understand the purpose of classifying soils in geotechnical engineering.
Analyze particle size distribution using Sieve and Hydrometer methods.
Calculate and interpret Atterberg Limits for fine-grained soils.
Classify soils using the Unified Soil Classification System (USCS) and AASHTO methods.

Soil classification systems allow engineers to group soils with similar engineering properties based on their physical characteristics. The two most common systems are the Unified Soil Classification System (USCS) and the AASHTO Classification System.

Particle Size Analysis

The first step in classification is determining the distribution of particle sizes.

Gradation

The distribution of particle sizes within a soil sample. Soils can be well-graded (wide range of sizes) or poorly graded (uniform size or gap-graded).

Sieve Analysis

Used for coarse-grained soils (gravel and sand). The soil is shaken through a stack of sieves with decreasing mesh sizes.

Percent Finer ( $F$ ): The percentage of soil passing a specific sieve.
Coefficient of Uniformity ( $C_u$ ): Measures the range of particle sizes.
Coefficient of Curvature ( $C_c$ ): Measures the shape of the gradation curve.

Gradation Types:

Well-graded (W): Wide range of sizes.
Poorly graded (P): Uniform size (SP) or gap-graded (GP).

Coefficient of Uniformity

Measures the range of particle sizes in a soil sample; larger values indicate a wider gradation.

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

Variables

Symbol	Description	Unit
$C_u$	Coefficient of uniformity	-
$D_{60}$	Diameter corresponding to 60% finer by weight	-
$D_{10}$	Diameter corresponding to 10% finer by weight (effective size)	-

Uniformity Criteria

Well-graded Gravel (GW): $C_u \ge 4$
Well-graded Sand (SW): $C_u \ge 6$

Coefficient of Curvature

Measures the shape of the particle-size gradation curve; used alongside C_u to classify well-graded soils.

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

Variables

Symbol	Description	Unit
$C_c$	Coefficient of curvature (or gradation)	-
$D_{30}$	Diameter corresponding to 30% finer by weight	-
$D_{60}$	Diameter corresponding to 60% finer by weight	-
$D_{10}$	Diameter corresponding to 10% finer by weight (effective size)	-

Curvature Criteria

Well-graded soils (GW or SW) must have $1 \le C_c \le 3$ .

Hydrometer Analysis (Stokes' Law)

Used for fine-grained soils (silts and clays) passing the No. 200 sieve. It relies on Stokes' Law, which states that the settling velocity ( $v$ ) of a spherical particle in a fluid is proportional to the square of its diameter ( $D$ ).

By measuring the specific gravity of the soil-water suspension over time using a hydrometer, engineers can calculate the settling velocities and thus determine the distribution of microscopic particle sizes (e.g., separating silt from clay fractions at the 0.002 mm boundary).

Stokes' Law (Settling Velocity)

Predicts the settling velocity of fine soil particles in suspension; the basis for hydrometer analysis of silts and clays.

v = \frac{\gamma_s - \gamma_w}{18 \mu} D^2

Variables

Symbol	Description	Unit
$v$	Settling velocity of the particle	-
$\gamma_s$	Unit weight of soil solids	-
$\gamma_w$	Unit weight of water	-
$\mu$	Dynamic viscosity of the fluid	-
$D$	Diameter of the spherical particle	-

Interactive Sieve Analysis

Interactive Simulation

Explore how particle distribution affects the gradation curve using the simulation below.

Aggregate Gradation Curve

Loading chart...

Interpretation:

A well-graded soil has a good representation of particle sizes over a wide range. This leads to high density and stability as smaller particles fill the voids between larger ones. Best for structural fill and base courses.

Atterberg Limits

Atterberg limits are used for fine-grained soils (silts and clays) to describe their consistency at varying water contents.

Atterberg Limits

The water contents at which soil behavior changes between solid, semi-solid, plastic, and liquid states.

Liquid Limit (LL)

The water content at which soil transitions from a plastic to a liquid state.

Plasticity Index (PI)

The range of water content where soil behaves plastically.

Consistency Limits

Liquid Limit (LL): The water content at which soil transitions from a plastic to a liquid state. Determined by the Casagrande cup method (25 blows).
Plastic Limit (PL): The water content at which soil transitions from a semi-solid to a plastic state. Determined by rolling a 3mm thread.
Plasticity Index (PI): The range of water content where soil behaves plastically.
Liquidity Index (LI): Used to predict the stress history and in-situ state.

Plasticity Index

The range of water contents over which a fine-grained soil behaves plastically; a key index for classifying clays and silts.

PI = LL - PL

Variables

Symbol	Description	Unit
$PI$	Plasticity index	-
$LL$	Liquid limit	-
$PL$	Plastic limit	-

Liquidity Index

Indicates the in-situ consistency of a fine-grained soil relative to its Atterberg limits; used to predict undrained shear strength.

LI = \frac{w_{in-situ} - PL}{PI}

Variables

Symbol	Description	Unit
$LI$	Liquidity index	-
$w_{in-situ}$	In-situ water content	-
$PL$	Plastic limit	-
$PI$	Plasticity index	-

A-Line

An empirical line on the plasticity chart ( $PI = 0.73(LL - 20)$ ) that separates clay (above) from silt (below).

Unified Soil Classification System (USCS)

The USCS classifies soils into four major groups based on the percentage passing the No. 200 sieve (0.075 mm).

USCS Group Symbols

Primary Letter (Soil Type):

G: Gravel (> 50% of coarse fraction is retained on No. 4 sieve)
S: Sand (> 50% of coarse fraction passes No. 4 sieve)
M: Silt (Passes No. 200, below A-line)
C: Clay (Passes No. 200, above A-line)
O: Organic (High organic content)
Pt: Peat (Highly organic)

Secondary Letter (Gradation or Plasticity):

W: Well-graded (Clean gravels/sands with $<5\\%$ fines)
P: Poorly graded (Clean gravels/sands with $<5\\%$ fines)
M: Silty (Gravels/sands with $>12\\%$ fines)
C: Clayey (Gravels/sands with $>12\\%$ fines)
H: High plasticity ( $LL > 50$ )
L: Low plasticity ( $LL < 50$ )

USCS Dual Symbols

Soils with 5% to 12% fines require dual symbols (e.g., SW-SM, GW-GC) to indicate both gradation and the nature of the fines.

Basic USCS Classification Procedure

STEP-BY-STEP

Determine if the soil is coarse-grained ( $>50\%$ retained on No. 200 sieve) or fine-grained ( $>50\%$ passes No. 200 sieve).
If coarse-grained, determine if it is Gravel ( $>50\%$ of coarse fraction retained on No. 4 sieve) or Sand ( $>50\%$ of coarse fraction passes No. 4 sieve).
Determine the percentage of fines. If $<5\%$ , use $C_u$ and $C_c$ to assign W or P. If $>12\%$ , use Atterberg limits to assign M or C. If $5-12\%$ , use a dual symbol.
If fine-grained, plot LL and PI on the Plasticity Chart to classify as Silt (M) or Clay (C) and determine plasticity (L or H).

Interactive Plasticity Chart

Interactive Simulation

Use the chart below to determine the USCS classification for fine-grained soils based on Liquid Limit (LL) and Plasticity Index (PI).

USCS Plasticity Chart

Liquid Limit (LL): 45

Plasticity Index (PI): 20

Result

Lean Clay

The A-Line separates clays (above) from silts (below). The vertical line at LL=50 separates low plasticity (L) from high plasticity (H).

AASHTO Classification System

The AASHTO classification system is used primarily for highway subgrade classification. Soils are classified into seven major groups: A-1 to A-7.

Group Index (GI)

Used to evaluate the quality of a soil as a subgrade material. The higher the GI, the poorer the soil.

Group Index

AASHTO index that quantifies the suitability of a soil as a highway subgrade material; higher values indicate poorer quality.

GI = (F_{200} - 35)[0.2 + 0.005(LL - 40)] + 0.01(F_{200} - 15)(PI - 10)

Variables

Symbol	Description	Unit
$GI$	Group Index	-
$F_{200}$	Percent passing No. 200 sieve (whole number)	-
$LL$	Liquid limit	-
$PI$	Plasticity index	-

Group Index Calculation Rules

If $GI < 0$ , set $GI = 0$ .
Round to the nearest whole number.
For Groups A-2-6 and A-2-7, use only the second term (PI term).

Key Takeaways

USCS is the preferred system for geotechnical engineering, while AASHTO is used for highway engineering.
Sieve Analysis separates coarse particles; Hydrometer Analysis utilizes Stokes' Law to determine the size distribution of fine particles (silts/clays) based on settling velocity.
The Plasticity Chart uses the A-line ( $PI = 0.73(LL-20)$ ) to separate clays (above) from silts (below).
Atterberg Limits (LL, PL, PI) are critical for classifying fine-grained soils and predicting their engineering behavior (swelling, shrinkage, strength).
Group Index (GI) in AASHTO quantifies the suitability of soil as a subgrade material; higher GI = poorer quality.

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