Classification of soil
Soil Mechanics Books

There are two soil classification systems in common use for engineering purposes. The Unified Soil Classification System is used for virtually all geotechnical engineering work except highway and road construction, where the AASHTO soil classification system is used. Both systems use the results of grain size analysis and determinations of Atterberg limits to determine soil’s classification. Soil components may be described as gravel, sand, silt, or clay. A soil comprising one or more of these components is given a descriptive name and a designation consisting of letters or letters and numbers which depend on the relative proportions of the components and the plasticity characteristics of the soil.

1. MIT System of soil classification:

Clay

• Clay is also known as Cohesive Soil, Frictionless Soil or Expansive Soil
• Composed of very fine particles (less than 0.002 mm in size)
• Flaky in shape, thus having considerable surface area
• Have high inter particle attraction and thus having sufficient cohesion
• Susceptible to swelling and shrinkage, and possess low permeability
• Commonly brown in colour

Silt

• Silt particles rang in size from 0.002 mm to 0.06 mm
• Have high capillarity and very low dry strength
• Since particle size ranges in between that of clay and sand thus possessing properties of both sand and clays i. e. it shows slight cohesion and also friction.
• The colour of silty soil is mostly brown

Sand

• Sand is also known as Frictional Soil or Cohesionless Soil
• Particle size ranging from 0.06 mm to 2 mm.
• It may be rounded to angular in shape
• Grey in colour
• No plasticity
• Possess high strength in confined state and has considerable frictional resistance
• Angular particles have high frictional resistance than rounded ones
• It has high permeability and low capillarity

Gravel

• Particle size ranges from 2 mm to 60 mm
• Gravel form good foundation material
• Shows high frictional resistance
• Angular particles have high frictional resistance than rounded ones
• The gravels produced by crushing of rocks are angular in shape while those taken from riverbeds are sub-rounded to rounded

Cobbles or Boulders

• Particles larger than gravels are commonly known as Cobbles or Boulders
• Cobbles range in size from 60 mm to 200 mm
• The material larger than 200 mm are designated as Boulders

2. AASHTO classifications of soils:

The AASHTO system classifies soils into seven primary groups, named A-1 through A-7, based on their relative expected quality for road embankments, sub-grades, sub-bases, and bases. Some of the groups are in turn divided into subgroups, such as A-1-a and A-1-b. Furthermore, a Group Index may be calculated to quantify a soil’s expected performance within a group. To determine a soil’s classification in the AASHTO system, one first determines the relative proportions of gravel, coarse sand, fine sand, and silt-clay.

AASHTO Classification Chart

Note: Plasticity index of A-7-5 subgroup is equal to or less than the LL - 30. Plasticity index of A-7-6 subgroup is greater than LL - 30

In the AASHTO system:

• gravel is material smaller than 75 mm (3 in.) but retained on a No. 10 sieve;
• coarse sand is material passing a No 10 sieve but retained on a No. 40 sieve; and fine sand is material passing a No. 40 sieve but retained on a No. 200 sieve.
• Material passing the No. 200 sieve is silt-clay and is classified based on Atterberg limits.
• It should be noted that the division between gravel and sand is made at a smaller size (No. 10 sieve) in the AASHTO system than in the unified system (No. 4 sieve).

Secondly, if any fines are present, Atterberg limits are determined and the plasticity index is calculated. A soil is a granular material if less than 35% of the soil by weight passes the No. 200 sieve (#200). Granular materials are classified into groups A-1 through A-3. Soils having more than 35% passing the No. 200 sieve are silt-clay and fall in groups A-4 through A-7. Having the proportions of the components and the plasticity data, one enters one of the two alternatives AASHTO classification tables and checks from left to right until a classification is found for which the soil meets the criteria. It should be noted that, in this scheme, group A-3 is checked before A-2. Soils classified as A-1 are typically well-graded mixtures of gravel, coarse sand, and fine sand. Soils in subgroup A-1-a contain more gravel whereas those in A-1-b contain more sand.

Soils in group A-3 are typically fine sands that may contain small amounts of non-plastic silt. Group A-2 contains a wide variety of “borderline” granular materials that do not meet the criteria for groups A-1 or A-3. Soils in group A-4 are silty soils, whereas those in group A-5 are high-plasticity elastic silt. Soils in group A-6 are typically lean clays, and those in group A-7 are typically highly plastic clays. Within groups containing fines, one may calculate a group index to further evaluate relative quality and supporting value of a material as sub-grade. The group index is calculated according to the following empirical formula:

Group index F 35 – ( )0.2 0.005 LL 40 – ( ) + [ ] + 0.01 F 15 – ( )PI 10 – ( )

3. Unified soil classification system (USCS):

The Unified Soil Classification System is based on the airfield classification system developed by Casa Grande during World War II. With some modification it was jointly adopted by several U.S. government agencies in 1952. Additional refinements were made and it is currently standardized as ASTM D 2487-93. It is used in the U.S. and much of the world for geotechnical work other than roads and highways. In the unified system soils are designated by a two-letter symbol: the first identifies the primary component of the soil, and the second describes its grain size or plasticity characteristics. For example, poorly graded sand is designated SP and low plasticity clay is CL. Five first-letter symbols are used:

G for gravel
S for sand
M for silt
C for clay
O for organic soil

Clean sands and gravels (having less than 5% passing the No. 200 sieve) are given a second letter P if poorly graded or W if well graded. Sands and gravels with more than 12% by weight passing the No. 200 sieve are given a second letter M if the fines are silty or C if fines are clayey. Sands and gravels having between 5 and 12% are given dual classifications such as SP-SM. Silts, clays, and organic soils are given the second letter H or L to designate high or low plasticity. The specific rules for classification are summarized as follows and described in detail in ASTM D 2487.

1. Organic soils are distinguished by a dark-brown to black color, an organic odor, and visible fibrous matter.
2. For soils that are not notably organic the first step in classification is to consider the percentage passing the No. 200 sieve.
3. If less than 50% of the soil passes the No. 200 sieve, the soil is coarse grained, and the first letter will be G or S;
4. if more than 50% passes the No. 200 sieve, the soil is fine grained and the first letter will be M or C.

For coarse-grained soils, the proportions of sand and gravel in the coarse fraction (not the total sample) determine the first letter of the classification symbol. The coarse fraction is that portion of the total sample retained on a No. 200 sieve. If more than half of the coarse fraction is gravel (retained on the No. 4 sieve), the soil is gravel and the first letter symbol is G. If more than half of the coarse fraction is sand, the soil is sand and the first letter symbol is S. For sands and gravels the second letter of the classification is based on gradation for clean sands and gravels and plasticity of the fines for sands and gravels with fines.

For clean sands (less than 5% passing the No. 200 sieve), the classification is well-graded sand (SW) if C ≥ 6 and 1 £ Cc £ 3. Both of these criteria must be met for the soil to be SW, otherwise the classification is poorly graded sand (SP). Clean gravels (less than 5% passing the No. 200 sieve) are classified as well-graded gravel (GW) if Cu ≥ 4 and 1 £ Cc £ 3. If both criteria are not met, the soil is poorly graded gravel (GP). For sands and gravels where more than 12% of the total sample passes the No. 200 sieve, the soil is a clayey sand (SC), clayey gravel (GC), silty sand (SM), or silty gravel (GM).

The second letter is assigned based on whether the fines classify as clay (C) or silt (M) as described for fine-grained soils below. For sands and gravels having between 5 and 12% of the total sample passing the No. 200 sieve, both the gradation and plasticity characteristics must be evaluated and the soil is given a dual classification such as SP-SM, SP-SC, GW-GC, etc. The first symbol is always based on gradation, whereas the second is always based on plasticity. For fine-grained soils and organic soils, classification in the unified system is based on Atterberg limits determined by the fraction passing the No. 40 sieve. The liquid limit and plasticity index are determined and plotted on the plasticity chart. The vertical line at LL = 50 separates high-plasticity soils from low-plasticity soils. The A-line separates clay from silt. The equation of the A-line is

PI = 0.73 (LL – 20)

The U-line is not used in classification but is an upper boundary of expected results for natural soils. Values plotting above the U-line should be checked for errors. Inorganic soils with liquid limits below 50 that plot above the A-line and have PI values greater than 7 are lean clays and are designated CL; those with liquid limits above 50 that plot above the A-line are fat clays and are designated CH. Inorganic soils with liquid limits below 50 that plot below the A-line are silt and are designated ML; those with liquid limits above 50 that plot below the A-line are elastic silts and are designated MH.

The plasticity chart has a shaded area; soils that plot in this area (above the A-line with PI values between 4 and 7) are silty clay and are given the dual symbol CL-ML. If the soil under consideration is the fines component of a dually classified sand or gravel, the soil is classified as SM-SC or GM-GC. Soils with sufficient organic contents to influence properties that have liquid limits below 50 are classified as OL; those with liquid limits above 50 are classified as OH. Soils that are predominantly organic, with visible vegetable tissue, are termed peat and given the designation Pt.

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