2.3 Unified Soil Classification System (USCS) and Dual Symbols (ASTM D2487)

Unified Soil Classification System (USCS) and Dual Symbols (ASTM D2487)

Laboratory Verification Mandate: Under IBC Chapter 17 and ASTM D2487, the Unified Soil Classification System (USCS) is the legally binding engineering standard for classifying soils based on laboratory grain-size analysis (ASTM D6913) and Atterberg limits (ASTM D4318). As a special inspector, you must be able to read laboratory sieve reports, evaluate gradation coefficients, plot Atterberg test coordinates on the Plasticity Chart, and verify whether placed structural fill complies with contract specifications.

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1. Soil Fractions and Particle Size Boundaries (ASTM D2487)

The USCS establishes precise physical particle size boundaries that divide earth materials into five distinct fractions:

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2. Primary Classification Logic: Coarse-Grained vs. Fine-Grained

The first step in ASTM D2487 classification is determining whether the soil behaves as a granular skeletal material or a fine-grained cohesive matrix:

• Coarse-Grained Soils: More than 50% of the dry soil mass is retained on the No. 200 (0.075 mm) sieve. The engineering behavior is governed by inter-particle friction, particle interlock, and grain-size distribution.
• Fine-Grained Soils: 50% or more of the dry soil mass passes the No. 200 (0.075 mm) sieve. The engineering behavior is governed by moisture content, plasticity, and clay mineral surface chemistry.
• Highly Organic Soils (Peat - Pt): Soils composed predominantly of organic tissue (decaying roots, fibers, moss) with dark brown to black color and organic odor are classified immediately as Peat (Pt) without requiring sieve or Atterberg testing.

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3. Coarse-Grained Soil Classification Tree

If the soil is coarse-grained (> 50% retained on No. 200), classify it through the following hierarchical sequence:

Step 1: Gravel vs. Sand Distinction

Evaluate the coarse fraction (the total portion of the soil retained on the No. 200 sieve): $\text{Coarse Fraction (\%)} = 100\% - \text{\% Passing No. 200 Sieve}$

• GRAVEL (G): If more than 50% of the coarse fraction is retained on the No. 4 (4.75 mm) sieve.
• SAND (S): If 50% or more of the coarse fraction passes the No. 4 (4.75 mm) sieve.

Step 2: Percentage of Fines Passing the No. 200 Sieve

The percentage of fines dictates whether the soil is classified by gradation, plasticity, or both:

Case 1: Clean Soils (Less than 5% Passing No. 200)

Clean coarse soils contain negligible fines. Their shear strength and compaction density depend entirely on grain packing. Classification requires calculating two mathematical parameters from the cumulative grain-size distribution curve:

• Uniformity Coefficient ($C_u$): $C_u = \frac{D_{60}}{D_{10}}$

• Coefficient of Curvature ($C_c$): $C_c = \frac{(D_{30})^2}{D_{10} \times D_{60}}$ (Where $D_{10}$, $D_{30}$, and $D_{60}$ represent the particle diameters in millimeters corresponding to 10%, 30%, and 60% finer on the cumulative grain-size curve).

• Gravel Criteria (< 5% Fines):

  • Well-Graded Gravel (GW): $C_u \ge 4.0$ AND $1.0 \le C_c \le 3.0$.
  • Poorly-Graded Gravel (GP): Fails either $C_u < 4.0$ or $C_c < 1.0$ or $C_c > 3.0$.

• Sand Criteria (< 5% Fines):

  • Well-Graded Sand (SW): $C_u \ge 6.0$ AND $1.0 \le C_c \le 3.0$.
  • Poorly-Graded Sand (SP): Fails either $C_u < 6.0$ or $C_c < 1.0$ or $C_c > 3.0$.

Case 2: Coarse Soils with Substantial Fines (More than 12% Passing No. 200)

When fines exceed 12%, the fines fill the void spaces between aggregate grains, dictating permeability and shear behavior. Gradation coefficients ($C_u$ and $C_c$) are ignored, and classification is governed by Atterberg limits of the minus No. 40 fraction plotted on the Plasticity Chart:

• Gravel with Fines (> 12% Fines):
  • Silty Gravel (GM): Atterberg limits plot below the A-line or Plasticity Index ($PI$) is less than 4.
  • Clayey Gravel (GC): Atterberg limits plot on or above the A-line and $PI > 7$.
  • Dual Fines Exception (GC-GM): If the fines plot in the hatched zone of the Plasticity Chart ($4 \le PI \le 7$ and on or above the A-line), the dual symbol GC-GM is mandatory.
• Sand with Fines (> 12% Fines):
  • Silty Sand (SM): Atterberg limits plot below the A-line or $PI < 4$.
  • Clayey Sand (SC): Atterberg limits plot on or above the A-line and $PI > 7$.
  • Dual Fines Exception (SC-SM): If the fines plot in the hatched zone ($4 \le PI \le 7$ and on or above the A-line), the dual symbol SC-SM is mandatory.

Case 3: Borderline Fines (5% to 12% Passing No. 200)

When fines content falls within the transition zone of 5% to 12%, the soil behavior is influenced by both grain-size distribution and fines plasticity. ASTM D2487 mandates DUAL SYMBOLS combining the clean symbol and the fines symbol:

• First symbol reflects gradation criteria ($C_u$ and $C_c$): GW, GP, SW, SP.
• Second symbol reflects fines plasticity (Plasticity Chart): GM, GC, SM, SC.

The 8 standard dual symbol combinations for coarse-grained soils are:

• GW-GM: Well-graded gravel with silt ($C_u \ge 4$, $1 \le C_c \le 3$, fines plot below A-line or $PI < 4$).
• GW-GC: Well-graded gravel with clay ($C_u \ge 4$, $1 \le C_c \le 3$, fines plot on/above A-line and $PI > 7$).
• GP-GM: Poorly-graded gravel with silt (fails $C_u$ or $C_c$, fines plot below A-line or $PI < 4$).
• GP-GC: Poorly-graded gravel with clay (fails $C_u$ or $C_c$, fines plot on/above A-line and $PI > 7$).
• SW-SM: Well-graded sand with silt ($C_u \ge 6$, $1 \le C_c \le 3$, fines plot below A-line or $PI < 4$).
• SW-SC: Well-graded sand with clay ($C_u \ge 6$, $1 \le C_c \le 3$, fines plot on/above A-line and $PI > 7$).
• SP-SM: Poorly-graded sand with silt (fails $C_u$ or $C_c$, fines plot below A-line or $PI < 4$).
• SP-SC: Poorly-graded sand with clay (fails $C_u$ or $C_c$, fines plot on/above A-line and $PI > 7$).

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4. Fine-Grained Soil Classification and the Plasticity Chart

When 50% or more of the dry soil passes the No. 200 sieve, classify using the Liquid Limit ($LL$) and Plasticity Index ($PI$): $\text{Plasticity Index } (PI) = \text{Liquid Limit } (LL) - \text{Plastic Limit } (PL)$

Liquid Limit Horizon

• Low Plasticity / Compressibility (L): $LL < 50$.
• High Plasticity / Compressibility (H): $LL \ge 50$.

The Plasticity Chart Lines

• The A-Line: An empirical boundary established by Arthur Casagrande: $\text{A-Line: } PI = 0.73 \times (LL - 20)$
  • Points plotting on or above the A-Line are inorganic clays (C).
  • Points plotting below the A-Line are inorganic silts (M) or organic soils (O).
• The U-Line (Upper Limit Line): $\text{U-Line: } PI = 0.9 \times (LL - 8)$ The U-line represents the empirical upper boundary of test data for natural soils. If laboratory test results plot above the U-line, the data is invalid due to laboratory testing or calculation error, requiring immediate retesting.

The Hatched Dual-Symbol Zone: CL-ML

Between $LL < 50$, $4 \le PI \le 7$, and on or above the A-Line lies a specific transition zone shown hatched on the Plasticity Chart. Soils falling within this zone possess properties of both lean clay and inorganic silt. ASTM D2487 mandates the dual symbol CL-ML (Silty Clay).

Organic Fine-Grained Soils (OL, OH)

A soil plotting below the A-line or having low plasticity is classified as organic (OL or OH) if it contains significant organic matter. The standard laboratory verification protocol under ASTM D2487 compares liquid limits before and after oven-drying at 110°C: $\text{Liquid Limit Ratio} = \frac{LL_{\text{oven-dried}}}{LL_{\text{not dried}}} < 0.75$ If oven drying decreases the liquid limit by more than 25% (ratio < 0.75), the soil is officially classified as organic (OL if $LL < 50$, OH if $LL \ge 50$).

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5. Dual Symbols vs. Borderline Symbols (Hyphen vs. Slash)

Inspectors frequently encounter hyphenated symbols and slashed symbols on geotechnical borehole logs and laboratory test sheets. These two notations represent fundamentally different engineering circumstances:

• Dual Symbols (Codified with a Hyphen, e.g., GW-GM, CL-ML, SP-SC): Dual symbols are an official, codified classification under ASTM D2487. They are mandatory when test data falls within designated boundary ranges:
  1. Coarse-grained soils containing 5% to 12% fines (e.g., SW-SM, GP-GC).
  2. Fine-grained soils plotting in the hatched zone ($4 \le PI \le 7$ and on/above A-line: CL-ML).
  3. Coarse soils with > 12% fines where the fines plot in the hatched zone (GC-GM, SC-SM).
• Borderline Symbols (Indicated with a Slash, e.g., CL/CH, GM/SM, SP/SW): Borderline symbols are not standard USCS group symbols; they are engineering judgment notations. A slash is used when laboratory test data falls extremely close to a classification boundary (e.g., $LL = 49.5$, where the soil is on the borderline between low and high plasticity), or when field visual-manual identification under ASTM D2488 cannot definitively distinguish between two options.

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6. Comprehensive USCS Laboratory Classification Reference

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7. Worked Laboratory Soil Classification Example

Laboratory Test Data

A laboratory report for a potential structural fill source provides the following results:

• Total Dry Mass of Sample: 1,200.0 g
• Mass Retained on 3-in. (75 mm) Sieve: 0.0 g (0% cobbles/boulders)
• Mass Retained on No. 4 (4.75 mm) Sieve: 216.0 g
• Mass Retained on No. 200 (0.075 mm) Sieve: 876.0 g
• Mass Passing No. 200 Sieve: 108.0 g
• Grain-Size Curve Values: $D_{10} = 0.082\text{ mm}$, $D_{30} = 0.28\text{ mm}$, $D_{60} = 0.92\text{ mm}$
• Atterberg Limits (Minus No. 40 Fraction): Liquid Limit ($LL$) = 34%, Plastic Limit ($PL$) = 20%

Step-by-Step Derivation

Step 1: Calculate Percentage Fractions

$\text{\% Passing No. 200 (Fines)} = \left(\frac{108.0\text{ g}}{1,200.0\text{ g}}\right) \times 100\% = 9.0\%$ $\text{\% Retained on No. 200 (Coarse Fraction)} = 100\% - 9.0\% = 91.0\%$

• Check: Since 91.0% > 50%, the soil is COARSE-GRAINED.

Step 2: Determine Gravel vs. Sand

$\text{\% Gravel (Retained on No. 4)} = \left(\frac{216.0\text{ g}}{1,200.0\text{ g}}\right) \times 100\% = 18.0\%$ $\text{\% Sand (Passing No. 4 and Retained on No. 200)} = 91.0\% - 18.0\% = 73.0\%$ Evaluate the coarse fraction (91.0% total):

• Gravel portion of coarse fraction: $\frac{18.0%}{91.0%} \times 100% = 19.8%$
• Sand portion of coarse fraction: $\frac{73.0%}{91.0%} \times 100% = 80.2%$
• Check: Since sand (80.2%) $\ge$ 50% of the coarse fraction, the base soil is SAND (S).

Step 3: Evaluate Fines Percentage and Flag Dual Symbol

The percentage of fines passing the No. 200 sieve is 9.0%.

• Rule Check: 9.0% falls strictly between 5% and 12%. Therefore, under ASTM D2487, a DUAL SYMBOL IS MANDATORY. The first symbol must describe gradation; the second must describe fines plasticity.

Step 4: Determine Gradation Symbol (SW vs. SP)

Compute $C_u$ and $C_c$: $C_u = \frac{D_{60}}{D_{10}} = \frac{0.92}{0.082} = 11.22$ $C_c = \frac{(D_{30})^2}{D_{10} \times D_{60}} = \frac{(0.28)^2}{0.082 \times 0.92} = \frac{0.0784}{0.07544} = 1.039$

• Criteria for SW: $C_u \ge 6.0$ ($11.22 \ge 6.0 \rightarrow \text{Pass}$) AND $1.0 \le C_c \le 3.0$ ($1.0 \le 1.039 \le 3.0 \rightarrow \text{Pass}$).
• Result: Both criteria are satisfied. The gradation symbol is SW (Well-Graded Sand).

Step 5: Determine Fines Plasticity Symbol (SM vs. SC)

Calculate Plasticity Index ($PI$): $PI = LL - PL = 34 - 20 = 14$ Calculate A-Line coordinate at $LL = 34$: $PI_{\text{A-Line}} = 0.73 \times (LL - 20) = 0.73 \times (34 - 20) = 0.73 \times 14 = 10.22$

• Plot Comparison: The soil's actual $PI = 14$. Since $14 > 10.22$ (plots above the A-line) and $PI > 7$, the fines classify as Clay (SC).

Step 6: Final Classification

Combining the well-graded sand gradation symbol with the clayey fines symbol yields:

• USCS Group Symbol: SW-SC
• Official USCS Group Name: Well-graded sand with clay and gravel (because gravel is $\ge 15%$, "with gravel" is appended to the group name per ASTM D2487).