Why Classification Matters
Before any geotechnical analysis — bearing capacity, settlement, slope stability, or compaction — the engineer must characterize the soil being worked with. The Unified Soil Classification System (USCS), codified in ASTM D2487, provides a standard language for describing and grouping soils by their engineering behavior. A soil's USCS group symbol is the first piece of information another engineer needs to begin reasoning about it.
Particle-Size Boundaries
The USCS divides soil into two broad families based on the No. 200 sieve (0.075 mm):
- Coarse-grained soils retain more than 50% on the No. 200 sieve. They are further split at the No. 4 sieve (4.75 mm): material retained is gravel (G); material passing is sand (S).
- Fine-grained soils pass more than 50% through the No. 200 sieve. They are classified as silt (M) or clay (C) based on plasticity, not particle size alone.
Within gravel and sand, gradation quality drives the second letter: well-graded (W) or poorly graded (P), with silty (M) or clayey (C) modifiers when fines exceed 12%. The coefficient of uniformity Cu = D60/D10 and coefficient of curvature Cc = (D30)²/(D10 × D60) from grain-size analysis (ASTM D422) are used to assign W vs P: GW requires Cu ≥ 4 and 1 ≤ Cc ≤ 3; SW requires Cu ≥ 6 and 1 ≤ Cc ≤ 3.
Atterberg Limits and the Plasticity Chart
Fine-grained soils are classified primarily by Atterberg limits, which describe how water content affects soil consistency:
- Liquid Limit (LL) — water content at which soil transitions from plastic to liquid behavior (ASTM D4318, Casagrande cup or fall-cone method).
- Plastic Limit (PL) — water content at which soil transitions from plastic to semi-solid; determined by rolling threads to 3.2 mm diameter.
- Plasticity Index (PI) — the range of water content over which soil behaves plastically: PI = LL − PL.
The Casagrande Plasticity Chart plots PI vs LL and is the heart of fine-grained classification. The A-line (PI = 0.73 × (LL − 20)) separates clays (above) from silts (below). The U-line (PI = 0.9 × (LL − 8)) is the upper boundary of natural soils. The LL = 50 vertical line separates low-plasticity (L) from high-plasticity (H) soils.
Group Symbols and Their Meanings
| Symbol | Name | Key Characteristics |
|---|---|---|
| GW | Well-graded gravel | Cu ≥ 4, Cc 1–3, < 5% fines; excellent drainage, high strength |
| GP | Poorly graded gravel | Fails Cu or Cc criteria; uniform gradation; free-draining |
| GM | Silty gravel | > 12% non-plastic fines; reduced drainage, frost-susceptible |
| GC | Clayey gravel | > 12% plastic fines; susceptible to softening when wet |
| SW | Well-graded sand | Cu ≥ 6, Cc 1–3; good foundation material when dense |
| SP | Poorly graded sand | Uniform or gap-graded; liquefaction-susceptible when loose and saturated |
| SM | Silty sand | Fines reduce permeability; frost-susceptible |
| SC | Clayey sand | Plastic fines; cohesion adds strength but swelling is possible |
| ML | Low-plasticity silt | LL < 50, below A-line; highly frost-susceptible, collapsible when loose |
| MH | High-plasticity silt | LL ≥ 50, below A-line; elastic, compressible, volume-change prone |
| CL | Low-plasticity clay | LL < 50, above A-line, PI > 7; moderate shrink-swell; common residual soil |
| CH | High-plasticity clay | LL ≥ 50, above A-line; expansive, very compressible; problematic for foundations |
| OL/OH | Organic silt/clay | Organic matter indicated by odor, color, LL drop after oven-drying > 75% |
| Pt | Peat | Fibrous, very high water content; not suitable for structural support |
Field Identification Techniques
When lab equipment is unavailable, ASTM D2488 provides a visual-manual procedure. Key field tests include:
- Dry strength test: a soil pat dried and then crushed — high dry strength indicates clay (CH or CL); little or none indicates silt (ML).
- Dilatancy (shake) test: a saturated pat shaken in the palm — rapid sheen indicates silty behavior (ML, SM); no reaction indicates clay.
- Thread test: rolling soil to 3.2 mm threads at PL — clays form tough threads; silts crumble easily.
- Ribboning: squeezing soil between thumb and forefinger — long, smooth ribbon indicates high plasticity (CH).
Engineering Significance by Group
USCS symbols translate directly to performance expectations. GW and SW soils are preferred structural fill and subgrade materials because of high strength, good compactability, and free drainage. SP soils, while free-draining, are liquefaction-susceptible when loose and saturated — a critical PE exam distinction. CH soils present the greatest foundation challenges: high compressibility, low shear strength when saturated, and volume change with moisture fluctuation. ML soils are notoriously frost-susceptible; ice lensing occurs because their pore size is large enough to draw capillary water but fine enough to allow suction gradients.
Organic soils (OL, OH, Pt) should never be used as structural fill and typically require removal and replacement or deep foundation bypass. Always verify organics with the ASTM D2974 loss-on-ignition test when field indicators are ambiguous.
Dual Symbols and Borderline Cases
When a soil plots near classification boundaries, ASTM D2487 allows dual symbols (e.g., GW-GM, CL-ML). These borderline soils require engineering judgment and conservative assumptions. If the Plasticity Chart plots a soil between the A-line and the U-line with a PI between 4 and 7, the CL-ML dual symbol applies — a soil that behaves inconsistently depending on moisture state.
For PE exam problems, always start with the No. 200 sieve split (coarse vs fine), then apply the appropriate classification tree. Carry Cu and Cc for coarse soils; LL, PL, and PI for fine soils. The Plasticity Chart must be memorized with the A-line equation.
For further reading, see Geotechnical Studio or browse related articles at SPT Correlations and Compaction Specifications.