Soil Classification – What are the Classification of Soil, How Soils are Classified?

What are the Types of Soil Classification Systems?

As mentioned earlier, many soil classification systems have been created depending on the intended use of the soil. Generally, soils are classified based on the soil grain size distribution and plasticity. Some of the popular systems of soil classifications are:

1. Textural Soil Classification
2. AASHTO Soil Classification System
3. Unified Soil Classification System (USCS)
4. Indian Standard Soil Classification System (ISSCS)

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Textural Soil Classification

The textural soil classification system is developed by the U.S Department of Agricultural System (USDA) and is widely used in the field of agriculture. This classification system is more suitable for coarse-grained soil. According to this system, boundaries between the various soil are given as follows-

Soil Fraction	Diameter (mm)
Gravel	> 2.00
Sand	2 – 0.05
Silt	0.05 – 0.002
Clay	< 0.002

A triangular classification chart has been developed using the grain size distribution for sand, silt and clay. The percentages of sand, silt, and clay size particles in the given soil sample are determined. Based on this relative percentage of sand, silt, and clay, a point is located on the triangular chart. The designation allotted to the area on which this point is located will be the classification of that soil. Besides the grain size distribution, this soil classification system does not reveal any other aspects of the soil.

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AASHTO Soil Classification System

According to the AASHTO (American Association of State Highway and Transportation Offical) soil classification system, the soil is classified into 7 inorganic groups, A -1 through A – 7, with 12 subgroups in all. It has an additional group A – 8 for peat or muck. Soils within each group are further assessed using the group index.

Group Index (GI)=0.2a+0.005ac+0.01bd

where,

a = that part of the percentage of soil particles passing the 75𝜇 greater than 35 and not exceeding 75.

b = that part of the percentage of soil particles passing the 75𝜇 greater than 15 and not exceeding 55.

c = that part of the liquid limit greater than 40 and not greater than 60.

d = that part of the plasticity index greater than 10 and not exceeding 30.

Unified Soil Classification System (USCS)

The USCS is a modified version of the soil classification system developed by A. Casagrande. In this system, coarse-grained soil is classified based on grain size distributions, whereas fine-grained soils are classified based on the plasticity of the soil. Soils are categorized into 4 major groups:

• Coarse-grained soils
• Fine-grained soils
• Organic soils
• Peat

Indian Standard Soil Classification System (ISSCS)

ISSCS is the same as USCS with one modification the fine-grained soils are subdivided into 3 subgroups of low, medium, and high compressibility, whereas, in USCS, fine-grained soils are subdivided into 2 subgroups of low and high compressibility. 4 major groups and their symbol are given below:

1. Coarse-grained soils – Gravel (G) and Sand (S)
2. Fine-grained soils – Silt (M) and Clay (C)
3. Organic soils (O)
4. Peat (Pt)

Like USCS, coarse-grained soil is categorized based on grain size distribution in this classification system, whereas fine-grained soil classification is based on plasticity. According to ISSCS, the classification based on grain size distribution is as shown below:

Boulder (mm)	Cobble (mm)	Coarse-Grained Soil					Fine-Grained Soil
Gravel		Sand			Silt (mm)	Clay (mm)
Coarse(mm)	Fine(mm)	Coarse(mm)	Medium(mm)	Fine(mm)
> 300	300 – 80	80 – 20	20 – 4.75	4.75 – 2	2 – 0.425	0.425 – 0.075	0.002 – 0.075	< 0.002

Classification of Coarse-Grained Soil (Based on ISSCS)

According to the IS soil classification system, coarse-grained soil is graded based on particle size, gradation characteristics (Cu and Cc), and percentage fineness. The soil in which 50% or more soil particles are retained on a 75-micron sieve (0.075 mm) is classified as coarse-grained soil. Coarse-grained soils are further subdivided according to 3 cases which are described below –

Case 1: %fineness < 5% by weight

The coarse-grained soils are classified as Gravel (G) if more than 50% of the coarse fraction of the soil is retained on a 4.75 mm sieve; otherwise, it is classified as Sand (S). They are further divided based on their gradation characteristics.

Gravel:Coarse fraction retained on 4.75 mm > 50%

1. GW ⇒ Well-graded gravel
   C_u ≥ 4 and 1 ≤ C_c ≤ 3
2. GP ⇒ Poorly graded gravel
   C_u < 4, or 1 > C_c, or C_c > 3

Sand:Coarse fraction retained on 4.75 mm < 50%

1. SW ⇒ Well-graded sand
   C_u ≥ 6 and 1 ≤ C_c ≤ 3
2. SP ⇒ Poorly graded sand
   C_u < 6, or 1 > C_c, or C_c > 3

Case 2: 5% < %fineness < 12% by weight

In this case, dual symbols will be used as a borderline case. For example, GW – GM. The first part of the dual symbol represents gradation characteristics (well-graded/poorly-graded), and the second part of the dual symbol represents the type of fines (silt or clay)

Gravel:Coarse fraction retained on 4.75 mm > 50%

1. GW-GC ⇒ Well-graded gravel containing clay as fine
   C_u ≥ 4 and 1 < C_c < 3 and clay fraction > silt fraction
2. GP-GC ⇒ Poorly graded gravel containing clay as fine
   C_u < 4 or 1 > C_c or C_c > 3 and clay fraction> silt fraction
3. GW-GM ⇒ Well-graded gravel containing silt as fine
   Cu ≥ 4 and 1 < Cc < 3 and clay fraction < silt fraction
4. GP-GM ⇒ Poorly graded gravel containing silt as fine
   C_u < 4, or 1 > C_c, or C_c > 3 and clay fraction < silt fraction

Sand:Coarse fraction retained on 4.75 mm < 50%

1. SW-SC ⇒ Well-graded sand containing clay as fine
   C_u ≥ 6 and 1 < C_c < 3 and clay fraction > silt fraction
2. SP-SC ⇒ Poorly graded sand containing clay as fine
   C_u < 6 or 1 > C_c or C_c > 3 and clay fraction> silt fraction
3. SW-SM ⇒ Well-graded sand containing silt as fine
   C_u ≥ 6 and 1 < C_c< 3 and clay fraction < silt fraction
4. SP-SM ⇒ Poorly graded sand containing silt as fine
   C_u < 6, or 1 > C_c, or C_c> 3 and clay fraction < silt fraction

Case 3: %fineness > 12% by weight

In this case, coarse-grained soil is classified based on its grain size and plasticity chart. The subgroups for this case are as follows:

Gravel:Coarse fraction retained on 4.75 mm > 50%

1. GC ⇒ Clayey gravel
   I_p > 7% (clay fraction > silt fraction)
2. GM ⇒ Silty gravel
   I_p < 4% (silt fraction > clay fraction)

Sand:Coarse fraction retained on 4.75 mm < 50%

1. SC ⇒ Clayey sand
   I_p > 7% (clay fraction > silt fraction)
2. SM ⇒ Silty sand
   I_p < 4% (silt fraction > clay fraction)

One thing to remember in this case is if the plasticity index is between 4% -7%, then dual symbols will be used. (GC-GM or SC-SM)

Classification of Fine-Grained Soil (Based on ISSCS)

According to the IS, soil classification system, fine-grained soils are those in which more than 50% of soil grains pass through a 75-micron sieve (0.075). Fine-grained soils are categorized using the plasticity chart and compressibility (w_L). Liquid limit (w_L) and plastic limit (w_P) are determined for soil, and the plasticity index is calculated (I_P = w_L– w_P). Based on this I_P and w_L, the soil is on the plasticity chart. The plasticity chart is shown in the diagram below:

The A-line is a boundary that represents the relationship between I_P and w_L. The equation of A-line is given as

I_P = 0.73(w_L– 20)

If the soil lies above A-line (IP of soil > IP of A-line), then it is clay(C), and if the soil lies below A-line (IP of soil < IP of A-line), then it is either silt (M) or organic soil (O). Another boundary line, the U-line, represents the upper limit beyond which no soil should exist. If the soil goes above this boundary, the test to determine wL and wP is performed again. The equation for U-line is given as

I_P = 0.9(w_L – 8)

Based on the soil classification, the fine-grained soils are further classified based on their compressibility according to the following cases:

Case 1: w_L< 35%

The soil has low compressibility and is classified as low plastic soil. The low plastic soil can be denoted as CL (low plastic inorganic clay), ML (low plastic silt) or OL (low plastic organic clay)

Case 2: 35% < w_L< 50%

The soil has intermediate compressibility and is classified as medium plastic soil. The medium plastic soil can be denoted as CI (medium plastic inorganic clay), MI (medium plastic silt) and OI (medium plastic organic clay)

Case 3: w_L> 50%

The soil has high compressibility and is classified as highly plastic soil. The highly plastic soil can be denoted as CH (high plastic inorganic clay), MH (high plastic silt) and OH (high plastic organic clay).

Organic and inorganic soils are plotted in the same area on the plasticity chart. Organic soils are distinguished from inorganic soils by odour and colour or liquid limit test on oven dry samples. If the w_L of the oven dry sample is less than 3/4th of the in-situ soil sample, then the soil is organic, otherwise inorganic. Highly organic clay, like peat, is classified as Pt.

For more information about soil classification, you can refer to the following video on Byju Exam Prep’s official youtube channel.

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