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Reference Guide

Soil Classification Guide

Complete reference for the Unified Soil Classification System (USCS) and AASHTO classification with field identification tests and logging symbols

USCS Classification AASHTO Classification Field Tests Grain Size Chart Logging Symbols

USCS Classification System

The Unified Soil Classification System (USCS) per ASTM D2487 classifies soils based on grain size distribution, liquid limit and plasticity index. It uses a two-letter symbol: the first letter indicates the predominant grain size or organic content, the second describes gradation or plasticity characteristics.

Symbol Name Description Typical Uses Engineering Properties
Coarse-Grained Soils - Gravels (>50% retained on No. 4 sieve)
GW Well-Graded Gravel Gravels with wide range of particle sizes and substantial amounts of all intermediate sizes. Little or no fines. Road base, fill, drainage, foundations Excellent bearing capacity, high shear strength, very low compressibility, excellent drainage
GP Poorly-Graded Gravel Gravels predominantly one size or with missing intermediate sizes. Little or no fines. Drainage, pervious fill, coarse filter material Good bearing capacity, good shear strength, very low compressibility, excellent drainage
GM Silty Gravel Gravels with silt fines (>12% fines). Fines plot below A-line on plasticity chart. Road base, embankments, structural fill Good bearing capacity, good shear strength, low compressibility, fair to poor drainage
GC Clayey Gravel Gravels with clay fines (>12% fines). Fines plot above A-line on plasticity chart. Core material for dams, erosion protection, low-permeability fill Good bearing capacity, good shear strength, slight compressibility, poor drainage
Coarse-Grained Soils - Sands (>50% passes No. 4 sieve)
SW Well-Graded Sand Sands with wide range of particle sizes and substantial intermediate sizes. Little or no fines. Road base, fill, foundations, drainage blankets Good bearing capacity, good shear strength, very low compressibility, excellent drainage
SP Poorly-Graded Sand Sands predominantly one size (uniform) or with missing sizes (gap-graded). Little or no fines. Drainage, filter material, sand drains, backfill Fair bearing capacity, fair shear strength, very low compressibility, excellent drainage
SM Silty Sand Sands with silt fines (>12% fines). Non-plastic or low-plasticity fines below A-line. Structural fill, road subgrade, levees Fair bearing capacity, fair shear strength, slight compressibility, fair to poor drainage
SC Clayey Sand Sands with clay fines (>12% fines). Plastic fines above A-line on plasticity chart. Dam cores, liners, low-permeability fill, levees Fair bearing capacity, fair shear strength, low to medium compressibility, poor drainage
Fine-Grained Soils - Low Plasticity (LL < 50)
ML Low-Plasticity Silt Inorganic silts with slight plasticity. Very fine sands, rock flour, silty or clayey fine sands. Fill (with caution), road subgrade, levees with protection Fair bearing capacity when dry, low shear strength, medium compressibility, fair drainage. Frost susceptible.
CL Low-Plasticity Clay Inorganic clays of low to medium plasticity. Gravelly, sandy and lean clays. Dam cores, canal linings, foundations (low-rise), landfill liners Fair bearing capacity, fair shear strength, medium compressibility, very poor drainage. Moderate shrink-swell.
OL Low-Plasticity Organic Soil Organic silts and organic silty clays of low plasticity. Identified by LL reduction after oven drying. Not recommended for structural use. Landscaping only. Poor bearing capacity, low shear strength, high compressibility, poor drainage. Continues to settle.
Fine-Grained Soils - High Plasticity (LL ≥ 50)
MH High-Plasticity Silt Inorganic silts of high plasticity. Micaceous or diatomaceous fine sands and silts, elastic silts. Not suitable for most construction. Temporary fills with drainage. Poor bearing capacity, fair to poor shear strength, high compressibility, fair to poor drainage. Very frost susceptible.
CH High-Plasticity Clay Inorganic clays of high plasticity. Fat clays, expansive clays, bentonite. Landfill liners, dam cores (if controlled), slurry walls Poor to fair bearing capacity when saturated, fair shear strength, very high compressibility, practically impervious. Severe shrink-swell.
OH High-Plasticity Organic Soil Organic clays of medium to high plasticity. Identified by LL reduction after oven drying. Not suitable for construction. Remove and replace. Very poor bearing capacity, very low shear strength, very high compressibility, practically impervious.
Highly Organic Soils
Pt Peat Highly organic soils composed primarily of vegetable tissue in various stages of decomposition. Dark brown to black, spongy, distinctive organic odor. Not suitable for any structural purpose. Must be removed or bypassed. Negligible bearing capacity, negligible shear strength, extremely high compressibility, variable drainage. Long-term settlement is severe.

The AASHTO system (M 145) classifies soils for highway construction based on particle size, liquid limit and plasticity index. Soils are ranked A-1 (best) through A-7 (worst) using the Group Index (GI) formula: lower GI values indicate better subgrade materials.

Group Subgroup General Description Typical Materials Subgrade Rating Group Index
Granular Materials (≤ 35% passing No. 200 sieve)
A-1 A-1-a Stone fragments, gravel and sand Well-graded gravel and sand mixtures Excellent to Good 0
A-1 A-1-b Coarse sand with minor fines Coarse sand with or without well-graded binder Excellent to Good 0
A-2 A-2-4 Silty gravel and sand Granular soils with non-plastic to slightly plastic fines Good 0
A-2 A-2-5 Silty gravel and sand Granular soils with elastic silt fines Good to Fair 0
A-2 A-2-6 Clayey gravel and sand Granular soils with plastic clay fines Good to Fair 0-4
A-2 A-2-7 Clayey gravel and sand Granular soils with high-plasticity clay fines Fair 0-4
A-3 - Fine sand Uniform fine sand, beach sand, desert sand. Non-plastic. Good 0
Silt-Clay Materials (> 35% passing No. 200 sieve)
A-4 - Silty soils Non-plastic to moderately plastic silts. LL ≤ 40, PI ≤ 10. Fair to Poor 1-8
A-5 - Elastic silty soils Diatomaceous or micaceous silts, elastic. LL > 40, PI ≤ 10. Poor 1-12
A-6 - Plastic clay Lean clays. LL ≤ 40, PI > 10. Moderate volume change. Poor 1-16
A-7 A-7-5 Elastic clay Moderately elastic clays. LL > 40, PI ≤ LL - 30. Poor to Very Poor 1-20
A-7 A-7-6 High-plasticity clay Fat clays with high volume change. LL > 40, PI > LL - 30. Very Poor 1-20
Highly Organic
A-8 - Peat and muck Highly organic soils (peat, muck). Classified by visual inspection. Not Suitable -

Field identification tests provide rapid preliminary classification without laboratory equipment. These methods are described in ASTM D2488 (Visual-Manual Procedure) and are standard practice during drilling, test pit excavation and site reconnaissance.

Visual Identification

Examine soil color, grain size, moisture and structure at the sample face or in the hand.

  • Grain size: Can individual grains be seen? Gravel is visible, sand is gritty, silt feels smooth, clay is sticky.
  • Color: Gray/brown = mineral soil. Black/dark brown = organic. Red/orange = iron oxides. Blue-gray = reducing (anaerobic) conditions.
  • Moisture: Record as dry, moist, wet or saturated. Note water table depth if encountered.
  • Structure: Note layering, lenses, root holes, fractures, mottling and any visible contamination (staining, sheen, odor).

Ribbon Test (Clay Content)

Determines clay content by rolling soil into a ribbon between thumb and forefinger.

  • Mold a walnut-sized piece of moist soil until it has a putty-like consistency.
  • Roll into a ball, then squeeze between thumb and forefinger to form a flat ribbon.
  • No ribbon forms: Silt or sand, little to no clay.
  • Ribbon 25-50 mm before breaking: Low-plasticity clay (CL).
  • Ribbon >50 mm: High-plasticity clay (CH). Feels very smooth and sticky.

Shake / Dilatancy Test

Distinguishes silt from clay by observing water migration under vibration.

  • Form a small pat of moist soil in the palm of your hand (about 15 mm diameter).
  • Shake vigorously by tapping the side of the hand against the other hand.
  • Rapid reaction: Water appears on surface quickly, disappears when squeezed. Indicates silt (ML).
  • Slow or no reaction: No water migration visible. Indicates clay (CL/CH).

Dry Strength Test

Assesses clay content by crushing a dried soil specimen between the fingers.

  • Allow a small soil specimen to air-dry completely (or use a field oven).
  • Attempt to break and crumble the dried specimen between thumb and forefinger.
  • Crumbles easily: Silt (ML/MH) or fine sand. Low dry strength.
  • Moderate resistance: Low-plasticity clay (CL). Medium dry strength.
  • Cannot be broken: High-plasticity clay (CH). Very high dry strength.

Odor Test (Organic Soils)

Detects organic content by smell, especially after heating.

  • Smell the fresh soil sample immediately after breaking or excavating.
  • Organic soils have a distinctive musty, decaying vegetation or swampy smell.
  • Heating the sample intensifies organic odor - hold near a lighter or in a sealed bag in sun.
  • Strong organic smell: Likely OL, OH or Pt classification. Confirm with lab testing (loss on ignition).
  • Chemical odor: May indicate petroleum contamination - note and handle per site health and safety plan.

Test Interpretation Summary

Combine multiple field tests for reliable classification:

  • Sandy soils: Visible grains, gritty feel, no ribbon, rapid dilatancy, low dry strength.
  • Silty soils: Smooth feel, no ribbon or weak ribbon, rapid dilatancy, low to medium dry strength.
  • Clayey soils: Sticky feel, strong ribbon, slow/no dilatancy, high dry strength.
  • Organic soils: Dark color, organic odor, fibrous texture. Always confirm with lab testing.

Soil particles are classified by size per ASTM D2487. The boundaries below are the standard divisions used in the USCS. Sieve analysis determines the coarse fraction; hydrometer analysis determines the fine fraction.

Boulders>300 mm
Cobbles75-300 mm
Gravel4.75-75 mm
Sand0.075-4.75 mm
Silt0.002-0.075 mm
Clay<0.002 mm
Boulders (>300 mm / 12 in)
Cobbles (75-300 mm / 3-12 in)
Gravel (4.75-75 mm / No. 4 - 3 in)
Sand (0.075-4.75 mm / No. 200 - No. 4)
Silt (0.002-0.075 mm)
Clay (<0.002 mm)
Fraction Size Range Sieve / Method Visible to Eye Feel Test
Boulders >300 mm (12 in) Field measurement Easily visible Cannot be held in one hand
Cobbles 75-300 mm (3-12 in) Field measurement Easily visible Can be held in one hand
Coarse Gravel 19-75 mm (3/4 in - 3 in) 3 in to 3/4 in sieve Easily visible Thumb to fist sized
Fine Gravel 4.75-19 mm (No. 4 - 3/4 in) 3/4 in to No. 4 sieve Visible Pea to thumbnail sized
Coarse Sand 2.0-4.75 mm (No. 10 - No. 4) No. 4 to No. 10 sieve Visible Like rock salt, very gritty
Medium Sand 0.425-2.0 mm (No. 40 - No. 10) No. 10 to No. 40 sieve Visible Like table salt, gritty
Fine Sand 0.075-0.425 mm (No. 200 - No. 40) No. 40 to No. 200 sieve Barely visible Like flour but still gritty
Silt 0.002-0.075 mm Hydrometer analysis Not visible Smooth, not sticky when wet
Clay <0.002 mm Hydrometer analysis Not visible Smooth, sticky and plastic when wet

Standard abbreviations and symbols used on borehole logs, test pit logs and field notes. Conventions follow ASTM D5434 (Guide for Field Logging of Subsurface Explorations).

Soil Description Abbreviations

Abbrev.Meaning
BHBorehole
TPTest Pit
MWMonitoring Well
SSSplit Spoon (sample)
STShelby Tube (undisturbed sample)
SPTStandard Penetration Test
N-valueSPT blow count (blows/300 mm)
RQDRock Quality Designation (%)
TCRTotal Core Recovery (%)
WLWater Level
GWTGroundwater Table
BGSBelow Ground Surface
TOCTop of Casing
EOBEnd of Borehole
NRNo Recovery

Moisture & Consistency

Abbrev.Meaning
DDry
MMoist
WWet
SSaturated
VLVery Loose (N < 4)
LLoose (N = 4-10)
CDCompact / Dense (N = 10-30)
VDVery Dense (N = 30-50)
VSVery Soft (qu < 25 kPa)
SoSoft (qu = 25-50 kPa)
FFirm (qu = 50-100 kPa)
StStiff (qu = 100-200 kPa)
VStVery Stiff (qu = 200-400 kPa)
HHard (qu > 400 kPa)

Contamination Indicators (Environmental Logging)

Abbrev.MeaningDescription
PIDPhotoionization DetectorField instrument reading in ppm. Screens for volatile organic compounds (VOCs).
OVAOrganic Vapor AnalyzerAlternative VOC screening instrument. Record reading in headspace or breathing zone.
PHCPetroleum HydrocarbonsVisual staining, odor or sheen indicating petroleum contamination.
LNAPLLight Non-Aqueous Phase LiquidFree product floating on groundwater (gasoline, diesel, solvents).
DNAPLDense Non-Aqueous Phase LiquidFree product sinking below groundwater (chlorinated solvents).
BTEXBenzene, Toluene, Ethylbenzene, XylenesKey volatile petroleum constituents. Measured by lab analysis.
FDField DuplicateQA/QC sample collected from same location for precision check.
TBTrip BlankLab-prepared blank carried to and from site. VOC analysis only.
EBEquipment BlankDeionized water rinsed through decontaminated sampling equipment.

This reference is for general educational purposes. Soil classification should always be performed or verified by a qualified geotechnical engineer or geoscientist. Field identifications should be confirmed with laboratory testing (sieve analysis, hydrometer, Atterberg limits) for engineering design use.

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