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D2487-17 – Standard Test Method Technical Guide
⚙️ Standard Scope and Primary Classification Basis
ASTM D2487-17 defines a practice for the classification of mineral and organo-mineral soils for engineering purposes. It relies on laboratory determination of particle-size characteristics, liquid limit, and plasticity index. The system is the ASTM version of the Unified Soil Classification System (USCS), originally developed from the Airfield Classification System by A. Casagrande in the early 1940s and adopted by U.S. Government Agencies in 1952. This standard is limited to naturally occurring soils passing the 3-inch (75-mm) sieve. Professional judgment is essential, as this standard is intended for qualitative application only.
⚠️ Important Caveat (Scope 1.4 & Note 3): This standard is for qualitative application. For detailed designs of important structures, it must be supplemented by quantitative laboratory tests or performance data under expected field conditions. This standard cannot replace education or professional experience.
🗂️ Group Symbols, Dual Symbols, and Borderline Classifications
The system relies on a combination of group symbols and group names. A single group symbol is usually assigned, but specific conditions require a dual symbol. According to Note 1 of the standard, a dual symbol is required when a soil contains 5 to 12 percent fines (e.g., GP-GM, SP-SM) or when the liquid limit and plasticity index plot in the crosshatched area of the plasticity chart (e.g., CL-ML). Borderline conditions can be indicated with a slash (e.g., CL/CH, GM/SM) to warn of expansive potential, particularly when the liquid limit of clayey soils is close to 50.
| 🟦 Major Division | 📏 Group Symbol | 📐 Typical Group Name | ⚡ Fines / PI Criteria |
|---|---|---|---|
| Coarse-Grained (Gravel) | GW / GP / GM / GC | Well-Graded / Silty / Clayey Gravel | Fines <5% or >12%; Plastic / Non-Plastic fines |
| Coarse-Grained (Sand) | SW / SP / SM / SC | Well-Graded / Silty / Clayey Sand | Fines <5% or >12%; Plastic / Non-Plastic fines |
| Fine-Grained (Silt & Clay) | ML / CL / MH / CH | Silt / Clay (Low/High Plasticity) | 50%+ fines; LL <50 / >50; “A-Line” position |
Dual Symbol Examples from the Standard
When the fines content is at the boundary or the plasticity characteristics are transitional, the standard dictates specific dual nomenclature to capture the soil’s engineering behavior accurately.
| 🔍 Scenario | ⚡ Dual Symbol | 🎯 Interpretation |
|---|---|---|
| 5-12% Fines (Non-plastic) | GP-GM / SP-SM | Borderline well or poorly graded with silty fines |
| 5-12% Fines (Plastic) | GP-GC / SP-SC | Borderline well or poorly graded with clayey fines |
| Crosshatched Area (LL < 50) | CL-ML | Intermediate silty clay with unique plasticity |
| LL near 50 boundary | CL/CH or CH/CL | Alert to expansive potential in borderline clays |
💡 Expert Tip: When a soil classifies with a borderline symbol (e.g., CL/CH or GM/SM), the first symbol should be the one based strictly on this standard. Pay particular attention to soils near the LL=50 boundary, as these can have expansive characteristics which are critical for foundation design.
📐 Key Measured Properties and the Plasticity Chart
The classification system is heavily dependent on the relationship between Liquid Limit (LL) and Plasticity Index (PI). These are plotted on the Casagrande Plasticity Chart, which features the “A-Line” (defined by the equation PI = 0.73 (LL − 20)). Soils plotting above the A-Line are classified as clays (e.g., CL, CH), while soils below the A-Line are classified as silts (e.g., ML, MH). The vertical boundary at LL = 50 separates low plasticity (L) from high plasticity (H) soils. A crosshatched area is defined on the chart where the dual symbol CL-ML is mandatory. The standard also references specific criteria for organic soils (OL, OH) and highly organic soils (Pt).
❓ Frequently Asked Questions
🔍 What is the difference between a dual symbol (dash) and a borderline symbol (slash)?
A dual symbol (e.g., GP-GM) is required by the standard for specific technical conditions (5-12% fines or crosshatched plasticity). A borderline symbol (e.g., GM/SM or CL/CH) is assigned at the discretion of the user to indicate the soil’s properties are near the boundary of another classification group, particularly when the test results are close to a boundary. The first symbol is always the one derived from the standard.
💡 When is a dual symbol specifically required by the standard?
A dual symbol is required in exactly two cases: (1) when the soil contains between 5% and 12% fines (passing the No. 200 sieve), and (2) when the plot of the liquid limit and plasticity index values falls into the crosshatched area of the plasticity chart, typically indicating borderline silty clay behavior (CL-ML).
⚡ Can D2487 be used for materials like crushed rock or shale?
While the scope is officially limited to naturally occurring soils (Section 1.3), Note 2 states that the group names and symbols may be used as a descriptive system applied to materials such as shale, claystone, shells, and crushed rock. See Appendix X2 of the standard for further guidance on this descriptive application.
📌 What does the “A-Line” represent on the Plasticity Chart?
The A-Line is defined by the equation PI = 0.73 (LL − 20). It is the empirical boundary separating primarily inorganic clays (soils plotting above the line) from primarily inorganic silts (soils plotting below the line). The position of a soil relative to the A-Line is a fundamental criterion for assigning the proper group symbol in the USCS.