Geotechnical ASTM 2017 Edition

ASTM D2487 — Unified Soil Classification System (USCS)

Standard practice for classifying mineral and organo-mineral soils for engineering purposes using grain-size distribution and plasticity characteristics.

Scope

ASTM D2487-17 covers the standard classification of soils for engineering purposes based on laboratory determination of particle-size characteristics, liquid limit, and plastic limit. It uses the Unified Soil Classification System (USCS), originally developed by Casagrande in 1942 and adopted by the US Army Corps of Engineers and US Bureau of Reclamation. The standard applies to natural mineral soils and organo-mineral soils but not to soils containing significant quantities of rock fragments larger than 3 inches (75 mm) unless specially handled.

The classification system divides soils into coarse-grained (more than 50% retained on the No. 200 sieve), fine-grained (50% or more passing the No. 200 sieve), and highly organic soils (peat). Coarse-grained soils are further subdivided into gravels and sands, each with four secondary groups based on fines content and plasticity. Fine-grained soils are classified using the plasticity chart where the liquid limit and plasticity index define the group symbol.

[FIGURE — USCS classification flowchart showing decision tree from coarse vs fine-grained through group symbols]

Purpose

The purpose of ASTM D2487 is to provide a standardized, reproducible method for assigning soils to categories that have similar engineering behavior. By classifying a soil using USCS symbols and group names, engineers can correlate the soil with expected engineering properties such as shear strength, compressibility, permeability, and compaction characteristics without performing additional tests. This classification serves as a universal language for geotechnical communication across projects, regions, and regulatory jurisdictions.

The standard is designed to be used with ASTM D6913 (sieve analysis) and ASTM D4318 (Atterberg limits), which provide the input data for classification. The system is intentionally simple enough for routine use yet comprehensive enough to capture meaningful distinctions between soil types that affect engineering performance.

Engineering Applications

The USCS is used across virtually all geotechnical engineering applications including foundation design, earthwork specification, pavement subgrade evaluation, embankment construction, slope stability analysis, and groundwater control. Specific applications include:

  • Foundation design — determining soil type for bearing capacity estimation and settlement analysis
  • Highway embankments — selecting borrow sources with suitable compaction characteristics (GW, GP, SW, SP preferred)
  • Retaining wall design — estimating lateral earth pressures based on soil classification
  • Seismic site classification — assigning Site Class A through F per ASCE 7 based on soil type and shear wave velocity
  • Drainage and filtration — identifying free-draining soils (GW, GP, SW, SP) vs impervious soils (CL, CH, ML, MH)
  • Compaction control — selecting compaction method and target density based on soil type per ASTM D698 or D1557
  • Liquefaction assessment — clean sands and silty sands (SP, SM) are most susceptible to seismic liquefaction

Design Philosophy

The USCS is based on the principle that the engineering behavior of a soil is governed primarily by its particle size distribution and the plasticity of its fine fraction. Rather than relying on a single index property, the system uses a dual-criteria approach: soils are first separated by grain size, then further subdivided by the quantity and plasticity of the fines. This two-tiered approach captures the dominant influence of coarse grains on frictional strength and permeability, while accounting for the dramatic effect that even small amounts of plastic fines can have on compressibility and moisture sensitivity.

The A-line on the plasticity chart (PI = 0.73(LL − 20)) represents an empirical boundary between inorganic clays (above the A-line) and inorganic silts (below the A-line). This boundary was established by Casagrande based on extensive testing of natural soils and reflects fundamental differences in the mineralogical composition and engineering behavior of clay minerals versus silt-sized particles.

A-line:  PI = 0.73 × (LL − 20)
U-line (upper bound):  PI = 0.9 × (LL − 8)

Gravel (G) = material retained on No. 4 sieve (4.75 mm)
Sand (S) = material passing No. 4, retained on No. 200 (75 μm)
Fines (F) = material passing No. 200 sieve (< 75 μm)

Coefficient of uniformity:  CU = D60 / D10
Coefficient of curvature:  CC = D30² / (D60 × D10)

Important Requirements

The classification procedure follows a strict decision hierarchy. First determine the percentage passing the No. 200 (75 μm) sieve:

  • Coarse-grained soils (< 50% passing No. 200): Further divided by the percentage retained on No. 4 sieve — gravel if > 50% of coarse fraction is retained on No. 4, sand otherwise. Within each, if fines content < 5%, use gradation criteria (CU, CC). If fines 5–12%, use dual symbol. If fines > 12%, use plasticity chart.
  • Fine-grained soils (≥ 50% passing No. 200): Classify using the plasticity chart. For LL < 50, use L groups (ML, CL, OL). For LL ≥ 50, use H groups (MH, CH, OH).
  • Highly organic soils (PT): Classified by visual characteristics of peat, muck, or swamp deposits with organic content typically > 75%.
Warning:

ASTM D2487 is a classification standard, not a specification. Soils with the same group symbol can have significantly different engineering properties. Always supplement classification with index property tests (LL, PL, natural moisture content, in-situ density) for design decisions.

Key Parameters

Parameter Symbol Definition Typical Range
Effective size D10 10% passing particle diameter 0.001–10 mm
30% passing size D30 30% passing particle diameter 0.005–20 mm
60% passing size D60 60% passing particle diameter 0.01–40 mm
Uniformity coefficient CU D60/D10 1–100+
Curvature coefficient CC D30²/(D60×D10) 0.5–3.0 (well-graded)
Liquid limit LL Water content at 25 blows (Casagrande cup) 15–100+%
Plastic limit PL Water content at thread 3.2 mm diameter 10–60%
Plasticity index PI LL − PL 0–60+

USCS Group Symbols and Typical Names

Group Symbol Typical Name Criteria Engineering Use
GW Well-graded gravel CU ≥ 4, 1 ≤ CC ≤ 3 Excellent subgrade, drainage
GP Poorly-graded gravel CU < 4 or CC outside 1–3 Good drainage, low compressibility
GM Silty gravel >12% fines, PI below A-line Fair subgrade, low permeability
GC Clayey gravel >12% fines, PI at or above A-line Fair-poor subgrade, low perm.
SW Well-graded sand CU ≥ 6, 1 ≤ CC ≤ 3 Excellent subgrade, good drainage
SP Poorly-graded sand CU < 6 or CC outside 1–3 Fair subgrade, good drainage
SM Silty sand >12% fines, PI below A-line Fair subgrade, low permeability
SC Clayey sand >12% fines, PI at or above A-line Poor subgrade, low permeability
ML Low-plasticity silt LL < 50, PI below A-line Fair-poor subgrade, frost susceptible
CL Low-plasticity clay LL < 50, PI at or above A-line Fair subgrade, low compressibility
OL Low-plasticity organic LL < 50, organic, PI below A-line Poor subgrade, compressible
MH High-plasticity silt LL ≥ 50, PI below A-line Poor subgrade, compressible
CH High-plasticity clay LL ≥ 50, PI at or above A-line Poor subgrade, expansive
OH High-plasticity organic LL ≥ 50, organic, PI below A-line Very poor subgrade, highly compress.
PT Peat Highly organic, fibrous texture Unsuitable for foundation support

Plasticity Chart

The plasticity chart is a semi-logarithmic plot of plasticity index (PI) versus liquid limit (LL). The A-line separates clays (above) from silts (below). The vertical line at LL = 50 separates low plasticity (L groups) from high plasticity (H groups). The U-line (upper limit) bounds the region of natural soils; points above the U-line typically indicate measurement error or non-soil materials.

[FIGURE — Plasticity chart with A-line (PI = 0.73 × (LL − 20)), U-line, and zones for CL, ML, CH, MH, OL, OH]

Practical Engineering Notes

Note:

When fines content is between 5% and 12%, ASTM D2487 requires a dual symbol (e.g., SP-SM, GW-GC). The first symbol represents the gradation-based classification assuming no fines, and the second represents the plasticity-based classification of the fines fraction. This dual symbol conveys that the soil has transitional behavior.

Field Tip:

For preliminary field classification (ASTM D2488 visual-manual method), use the feel test: gritty = silt, smooth/sticky = clay. Roll a moist thread — if it can be rolled to 3 mm without crumbling, the soil has plastic fines. Dilatancy (shaking test) helps distinguish silt from clay — silts exhibit rapid water expulsion and shiny surface.

Typical Workflow

  1. Perform sieve analysis per ASTM D6913 to determine grain-size distribution and percent passing No. 200.
  2. If ≥ 50% passes No. 200, perform Atterberg limits per ASTM D4318 (LL by Casagrande cup or fall cone, PL by thread method).
  3. Compute CU and CC for coarse-grained soils with ≤ 12% fines.
  4. Plot PI vs LL on plasticity chart to classify fine-grained soils.
  5. Assign group symbol per Table 1 of ASTM D2487.
  6. Assign group name per Table 2 (e.g., "poorly graded sand with silt" for SP-SM).
  7. Record classification in the geotechnical report with supporting index properties.

Common Mistakes

  • Using D30 for CU calculations — CU uses D60 and D10 only; CC uses D30.
  • Classifying by group symbol alone — always include the group name per ASTM D2487 Section 7 for complete identification.
  • Ignoring organic content — organic soils (OL, OH, PT) have distinct compressibility and require special handling.
  • Incorrect A-line application — the A-line equation uses natural LL values, not adjusted ones.
  • Misclassifying borderline cases — soils plotting within ±2% PI of the A-line require dual symbol classification.

Best Practices

  • Always report both group symbol and group name (e.g., "CL — Lean Clay with Sand").
  • Supplement classification with natural moisture content and in-situ density for design.
  • Use the visual-manual method (ASTM D2488) for field classification during drilling, but confirm with laboratory classification.
  • For gap-graded soils or soils with unusual gradation, note this in the classification report.
  • Reconcile USCS with AASHTO classification (ASTM D3282) for highway projects — they disagree about 20% of the time.

Limitations

  • Does not apply to soils with more than 30% retained on 3-inch (75 mm) sieve unless special procedures are used (supplemented by ASTM D5511/D5512 for large particles).
  • Classification alone does not provide design parameters — correlation with engineering properties requires local experience or site-specific testing.
  • May not distinguish between soils with similar index properties but different mineralogy or stress history that affect behavior.
  • The system is less reliable for residual soils, tropical soils, and cemented soils where particle bonding or mineralogy dominates behavior.

Related CivilFlow Calculators

Related Formulas

See the Geotechnical Formulas section for particle-size distribution, Atterberg limits, compaction, and permeability equations.

Related Handbook Chapters

Refer to the Engineering Handbook for geotechnical design guidance including bearing capacity, settlement analysis, and earth pressure theory.

Related Blog Articles

Related Learn Pages

Explore the Geotechnical Engineering learn page for foundational concepts in soil mechanics and foundation engineering.

Related Glossary Terms

Visit the Glossary for definitions of USCS group symbols, plasticity index, Atterberg limits, and other geotechnical terms.

References

  • ASTM D2487-17, "Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)," ASTM International, 2017.
  • ASTM D4318-17, "Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils," ASTM International, 2017.
  • ASTM D6913-17, "Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis," ASTM International, 2017.
  • Casagrande, A., "Classification and Identification of Soils," Transactions of ASCE, Vol. 113, 1948, pp. 901–930.
  • Holtz, R.D., Kovacs, W.D., and Sheahan, T.C., "An Introduction to Geotechnical Engineering," 2nd Ed., Pearson, 2011.