Soil Classification

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

Particle Size Analysis

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: The percentage of soil passing a specific sieve.
Coefficient of Uniformity ( $C_u$ ): $C_u = D_{60} / D_{10}$
Coefficient of Curvature ( $C_c$ ): $C_c = (D_{30})^2 / (D_{10} \times D_{60})$

Atterberg Limits

Used for fine-grained soils (silts and clays) to describe consistency at varying water contents.

Liquid Limit (LL): Boundary between liquid and plastic states.
Plastic Limit (PL): Boundary between plastic and semi-solid states.
Plasticity Index (PI): The range of water content where soil behaves plastically. $PI = LL - PL$

Unified Soil Classification System (USCS)

Classifies soils into four major groups:

Coarse-grained: More than 50% retained on No. 200 sieve.
Fine-grained: 50% or more passes No. 200 sieve.
Organic soils: High organic content.
Peat: Highly organic.

USCS Symbols

First Letter (Soil Type):

G: Gravel
S: Sand
M: Silt
C: Clay
O: Organic
Pt: Peat

Second Letter (Gradation or Plasticity):

W: Well-graded
P: Poorly graded
H: High plasticity ( $LL > 50$ )
L: Low plasticity ( $LL < 50$ )

AASHTO Classification System

Used primarily for highway subgrade classification. Soils are classified into groups A-1 through A-7.

Group Index (GI)

Used to evaluate the quality of a soil as a subgrade material. $GI = (F - 35)[0.2 + 0.005(LL - 40)] + 0.01(F - 15)(PI - 10)$

F: Percent passing No. 200 sieve.
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).

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