Pavement

Table of Contents

Appendix A Unified Soil Classification System

Major DivisionsGroup SymbolsTypical NamesField Identification Procedures
(Excluding particles, larger than 75-0 mm and basing fractions on estimated mass)
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Coarse-grained soils
More than half of material is larger than 75 μm sieve size
The 75 μm sieve size is about the smallest particle visible to the naked eye Gravels
More than half of coarse fraction is > 4.75 mm sieve size
Clean gravels
(little or no fines)
GW Well-graded gravels, gravel s and mixtures, little or no fines. Wide range in grain sizes and
substantial amounts of all intermediate particle sizes.
GP Poorly-graded gravels, gravel-sand mixtures, little or no fines. Predominantly one size or a range of sizes with some intermediate sizes missing.
Gravels with fines
(appreciable amount of fines)
GM Silty gravels, gravel. Sand-silt mixtures. Non-plastic fines or fines with low plasticity (for identification procedures, see ML below).
GC Clayey gravels, gravel sand-clay mixtures. Plastic fines (for identification procedures see CL below)
Sands
More than half of coarse fraction is <4.75 mm sieve size
Clean sands
(little or no fines)
SW Well graded sands, gravely sands, little or no fines. Wide range in grain size and substantial amounts of all intermediate particle sizes.
SP Poorly-graded sands, gravely sands, little or no fines. Predominantly one size or a range of sizes with some intermediate sizes missing.
Sands with fines
(appreciable amount of fines)
SM Silty sands, sand-silt mixtures. Non-plastic fines or fines with low plasticity (for identification procedures see ML below).
SC Clayey sands, sand-clay mixtures. Plastic fines (for identification procedures see CL below).
Fine-grained soils
More than half of material is smaller than 75μm sieve size
     Identification procedures on fractions smaller than 425 µm sieve size
Dry Strength (crushing characteristics) Dilatancy (reaction to shaking) Toughness (consistency near PL
Silts and Clays
Liquid Limit
< 50
ML Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity. None to slight Quick to slow None
CL Inorganic clays of low to medium plasticity, gravely clays, sandy clays, silty clays, lean clays. Medium to high None to very slow Medium
OL Organic silts and organic silty clays of low plasticity. Slight to medium Slow Slight
Silts and Clays
Liquid Limit
> 50
MH Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts. Slight to medium Slow to none Slight to medium
CH Inorganic clays of high plasticity, fat clays. High to very high None High
OH Organic clays of medium to high plasticity, organic silts. Medium to high None to very slow Slight to medium
Highly Organic Soils Pt Peat and other highly organic soils. Readily identified by colour, odour, spongy feel and frequently by fibrous texture.
  1. Boundary Classification: Soils possessing characteristics of two groups are designated by combinations of group symbols. For example GW-GC, well graded gravel sand mixture with clay binder.
Information Required for Describing SoilsLaboratory Classification Criteria
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For undisturbed soils add information on stratification, degree of compactness, cementation, moisture conditions and drainage characteristics.

Give typical name; indicate approximate percentages of sand and gravel, max. size, angularity, surface condition, and hardness of the coarse grains, local or geologic name and other pertinent descriptive information, and symbol in parentheses.

EXAMPLE:

Silty sand, gravelly, about 20% hard, angular gravel particles 13.2mm maximum size, rounded and sub-angular sand grains coarse to fine, about 15% non-plastic fines with low dry strength well compacted and moist in place; alluvial sand (SM).

Use grain size curve in identifying the fractions as given under field identification Determine percentage of gravel and sand from grain size curve. Depending on percentage of fines (fraction smaller than 75 µm sieve size) coarse grained soils are classified as follows
Not meeting all grading requirements for GW
Atterberg limits
below the A
Line or PI < 4
Above the A line
With PI between 4 and 7 are borderline cases requiring use of dual symbols.
Atterberg limits
below the A
Line with PI < 7
Not meeting all grading requirements for SW
Atterberg limits
below the A
line or PI < 4
Above the A line
With PI between 4 and 7 are borderline cases requiring use of dual symbols.
Atterberg limits
below the A
line with PI < 7

Give typical name, indicate degree and character of plasticity, amount and maximum size of coarse grains, colour in wet condition, odour, if any local or geologic name, and other pertinent descriptive information; and symbol in parentheses.

For undisturbed soils add information, on structure, stratification, consistency in undisturbed and remoulded states, moisture and drainage conditions.

EXAMPLE:

Clayey silt, brown, slightly plastic, small percentage of fine sand, numerous vertical root holes, firm and dry in place, loess. (ML).

PLASTICITY CHART for laboratory classification of the fine-grained soils

  1. All sieve sizes of this chart are AS standard.
  2. Cu = Coefficient uniformity. Cc = Coefficient of curvature. D60 = Grain diameter at 60% passing.
    D30 = Grain diameter of 30% passing. D10 = Grain diameter at 10% passing.