Soil Particle size| 3 Classification types

soil particle size

Soil particle size – Grain size analysis of soils, classifications are done to find their engineering properties. construction materials for home construction.

These properties required for evaluation of suitability of the soil, for use a foundation or construction materials for home construction.

Soil particle size Classification

soil particle size
Soil particle size-Sand in construction
This classification of soil is based on the grain size of particles.
The engineering behaviour of soil with small particles will differ from the soil with large particles.
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In many types and varieties, natural soils occur
The soils are grouped into 2 categories for engineering purposes.
1.Coarse-grained Soils
2.Fine-grained Soils

1. Coarse-grained soils:

soil particle size-coarse gravel
soil particle size-coarse gravel
Gravels and Sand are examples of coarse-grained soils.
Relative density and soil particle size are the main index properties of coarse-grained soils.
The soil Particle size of Gravel
a.Fine gravel : 4.75mm to 20mm
b.Coarse gravel: 20mm to 80mm
c.Cobble: 80mm to 300mm
d.Boulder: > 300mm.
Soil Particle sizes of Sand:
a.Fine sand: 0.075 mm to 0.425mm
b.Medium sand : 0.425mm to 2mm
c.Coarse sand : 2mm to 4.75mm.

2. Fine-grained soils:

soil particle size-clay
soil particle size-clay
Silt and Clay are examples of fine-grained soils.
Consistency and Atterbergs’s limits are the main index properties of fine-grained soils are

Soil Particle size of Silt
a.Silt – 0.002 to 0.075mm
The soil particle size of Clay
a. Clay – </=0.002mm

3 Classification types of soil

The following are the 3 classification types of soil.

  1. ASSHTO  Classification
  2. UCS classification
  3. ICS classification

Need for Classification

For understanding the nature, the items group.
Classification attempts to choose those criteria which are relevant to the purpose of view.
The soils that are grouped in order of performance for one type of physical conditions will not necessarily have the same order of performance under different physical conditions

Index Properties

The soils are classified and identified based on Index properties.
The properties of soils which are not of primary interest to the geotechnical engineer, but which are indicative of the engineering properties are called Index properties. 

Given below are the Engineering properties:


Permeability of soil is the property of soil mass which permits the flow of water through its interconnected voids.

2.Shear strength

Shear strength is the resistance to shearing stresses just before failure.


Consolidation is defined as the compression of soil under steady pressure, such as the weight imposed by a structure or earth filling.

Categories of Index properties

The Index properties are divided into two categories:
1. Properties of individual soil particle size.
2.Aggregate properties. (properties of soil mass)

1. Properties of individual soil particle size:

Properties of individual  particle size can be determined for a disturbed sample & they depend upon the individual grains and are independent of the manner of soil formation.

2. Aggregate properties

Aggregate properties of soil depend on soil formation, soil history and soil structure and these properties can be determined from undisturbed samples or preferably from in situ tests.

Soil classification tests

Simple tests which are required to determine the Index properties are known as classification tests.
Soil classification tests are required for evaluation of suitability of the soil, for use a foundation or construction materials for home construction.
These classification tests are proved to be a very useful tool for the soil engineer as it gives guidelines of properties like permeability, compressibility and shear strength.

 IS particle size classification

American Association of State Highways and Transport Officials (AASHTO) classifications

This classification is useful for classifying soils for highways.
The particle size and plasticity characteristics are required to classify this soil.
Soils classified like A1, A2, A3…….A7.
A1 group soils are Stone fragments, gravel and sand
A2 group soils are Silty or clayey gravel sand
A3 group soils are fine sand
A4, A5 group soils are silty soils
A6, A7 group soils are clayey soils.
A1, A2, A3 are excellent to good for subgrade.
A4,A5,A6,A6 are Fair to poor for subgrade.
Fine grade soils are the soils if 35% of soil passes through sieve no 200(0.075mm).
Sieve no 10(2mm size) is used to divide the soil into gravel and sand.
In this system, there is no place for organic soils

Unified Soil Classification system (UCS)

Casagrande in 1948 developed this system.
This system is used in all types of engineering problems involving soils.
The criteria in this classification :
a. Grain size distribution
b.Plasticity characteristics
c. Compressibility.
In this system, the soils are classified into 15 groups
If more than 50% soil passes through 0.075mm its is classified as fine-grained.
Sieve  475mm is used for dividing the soil into gravel and sand. 

IS soil classification or Indian standard classification (ICS)

This system is similar to the UCS system with one basic difference in the classification of fine-grained soils.
The fine-grained soil in UCS is divided into 2 categories Low and high compressibility.
But in ICS system they are classified into three categories low, medium and high compressibility.
The code recommended for this system is IS 1498-1970.
According to code, the soils are divided into three divisions.
a.Coarse-grained soils (8 groups)
b.Fine-grained soils (9 groups)
c.Highly organic soils (1 group)

a. Coarse-grained Soils

Coarse-grained soil is the soils if more than 50% of soil retained on 0.075mm sieve.
They are two types:
I.Gravel (4 groups)
II. Sand (4 groups)

I. Gravel(G)

If more than 50% of coarse fraction retained on 4.75mm IS sieve then the coarse-grained soil is classified as gravel
 They are  4 types of Gravels:
  • Clayey gravel (GC)
  • Poorly graded soil (GP)
  • Well graded gravel (GW)
  • Silty gravel (GM)

II. Sand

Coarse-grained soils classified as sand if more than 50% of coarse fraction passes on 4.75mm IS sieve.
Sands are of 4 types. They are.
  • Well grade sand (SW)
  • Poorly graded sand (SP)
  • Silty sand (SM)
  • Clayey sand (SC)

Coefficient of Uniformity (Cu)

The uniformity of soil is expressed by a term known as Uniformity of coefficient, Cu.
where, D60 – Particle size is such that 60% of the soil is finer than this size.
            D10 – Particle size is such that 10% of soil is finer than this size.
                      & D10 is also called an effective size.

Given below  Cu values for coarse-grained soil

1. Well graded gravel-GW, Cu>4
2. Poorly graded gravel-GP, Cu posses no values.
3. Well graded sand-SW, Cu>6
4. Poorly grade sand-SP, Cu posses no values.

Coefficient of Curvature (Cc)

The shapes of the particle size distribution is determined by the coefficient of curvature & that coefficient also termed as the coefficient of graduation.
        D30 -Particle size is such that 30% of the soil is finer than this size.

Given below  Cc values for coarse-grained soil

1. Cc posses no valves for poorly grade sand-SP,
2. Cc is between 1 and 3 for Well graded gravel-GW,
2. Cc  posses no valves for Poorly graded gravel-GP,
3. Cc is between 1 and 3 for Well graded sand-SW,

b. Fine-grained soils

Soil is classified as fine-grained soil if more than 50% of soil passes through 0.075mm sieve.
The following are the types of fine-grained soils.
  • Inorganic silts (M)
  • Inorganic clays (C)
  • Organic silts & clay (O)
 Their types are further divided on the basis of the characteristics of compressibility :
1. Low compressible inorganic silt (ML)
2. Intermediate compressible inorganic silt (MI)
3. High compressible inorganic silt (MH)
4. Low compressible inorganic clay (CL)
5. Intermediate compressible inorganic clay (CI)
6. High compressible inorganic clay (CH)
7.High compressible organic clay and silt (OH)
8.Low compressible organic clay silt (OL)
9.Intermediate compressible organic clay and silt (OI)

Liquid Limit

The stages of water content at which the soil changes from the liquid state to the plastic state is called Liquid limit.
compressibility characteristics of the soil are expressed based on Liquid limit.
The clay practically behaves like a liquid but possesses a small shear strength, at the liquid limit,
On the amount of clay mineral, The liquid limit of soil depends.
The particle will be smaller, If the surface charge is stronger and
If the amount of adsorbed water is greater, the liquid limit will be higher.
If the liquid limit is
a.  < than 35% it is called Low compressible
b. Between 35 % to 50 %, it is intermediate compressible
c.  > than 50% ,it is high compressible.

Plasticity chart

Based on plasticity chart fined grained soils are classified which was first explained by Mr.Casagrande.
Plasticity chart equation can be addressed by Ip = (WL-20) x 0.73

For fine-grained soils plasticity chart analysis

1. Inorganic clays are plotted above A-line.
2. Inorganic silts are plotted below A-line.
3. Clays, Organic silts  are plotted under A-line
4. Black cotton soils lie along with a band partly higher and partly under A-line.

Group Index of Soil (GI)

The soil Group index  dependent on :
a. % of soil passing 0.075mm sieve.
b. Liquid limit
c. Plastic limit.
Group index can be calculated by GI=0.2a + 0.005ac + 0.01bd
where ,    a=(% soil passing 0.075mm sieve)-35% range (0 to 40)
                b=(% soil passing 0.075mm sieve)-35% range (0 to 40)
                c=Wl-40%(range 0 to 20),Wl is Liquid limit.
                d=Ip-20% (Range 0 to 20),Ip is Plasticity Index.
if Group index valve is negative (-ve), it is recorded as 0 and Group index valves range from 0 to 20
if Group index =0, the soil is best for pavement and GI=20, the soil is poor for pavement.

Sedimentation Analysis

Silt and clay soil particles size cannot be separated by sieving., therefore sedimentation tests are preferred for fine-grained soils.
The common sedimentation test is:
a.Hydrometer test
b.Pipette test
Sedimentation analysis can be performed for determining soil particles less than 0.075mm size.
The sedimentation analysis is dependent on the Stroke’s law.
 spherical particles terminal velocity is calculated by practising stoke’s law.
For particle diameters varying from 0.2 mm to 0.0002 mm, the Stroke’s law is proven.
particle size distribution in soil
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About Ashwini Kumar

Ashwini Kumar is a graduate in civil engineering from NIT Warangal. He has 8+ years of experience in the construction field. He is the author, founder of the Easy Home Builds blog

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