HOW TO IDENTIFY & CLASSIFY SOIL ON SITE? [STEP-BY-STEP GUIDE]

The principle terms used by civil engineers to describe soil are:

  • Gravel (particle size larger than 4.75 mm)
  • sand (particle size within 4.75mm to 0.075mm)
  • Silt and clay (particle size less than 0.075 mm)

Most natural soils contain a mixture of two or more of these components and may contain a mixture of organic matter in a partially or completely decomposed state. The mixture is named for the component that has the greatest influence on its behavior, and the other components are indicated by adjectives.

For example, silty clay has predominantly clay properties, but contains considerable amounts of silt.


While identifying and classifying soil in the field, we need to use the following steps in logical sequence.

  1. identify as coarse-grained soil or fine-grained soil
  2. Coarse Grain Soil Classification
  3. fine-grained soil classification

step 1 [Identifying as Coarse Grained Soil or Fine Grained Soil]

In the first step we try to identify the soil: Coarse Grain Soil (CGS) or Fine-grained soil (FGS).

Coarse-grained soils are those soils whose individual particles are visible to the naked eye. Sandy soils and gravelly soils come in this group.

Fine-grained soils are those soils whose individual particles are not visible to the naked eye. The silty soil and clay soil come in this group.


To know more about coarse grained soil and fine grained soil please read my post What are the primary differences between coarse grained soil and fine grained soil

In order to identify the soil as CGS or FGS the following procedure is followed in the field:

  • Take a representative sample of soil and spread it on a flat surface or the palm of the hand.
  • Remove all particles whose size is larger than 75 mm.
  • If the soil contains more than 50% visible particles then the soil is called coarse-grained soil.
  • If the soil contains less than 50% visible particles then the soil is called fine grained soil.

Once the soil is identified as coarse-grained soil or fine-grained soil, we further divide it based on the percentage of different sizes present in the soil mass. Coarse-grained soils can be subdivided into gravel or sandy soils.


step 2 [Classifying Coarse Grained Soil]

  • If the percentage of gravel is more than that of sand then the coarse grained soil is called gravel soil.
  • If the percentage of fine (ie silt and clay) present in gravel soil is less than 5%, it is recognized as clear gravel, Clean gravel can be classified as either Well Graded Gravel (GW) or Poor Grade Gravel (GP),
  • Guinea worm– if all particle sizes are well represented
  • GP – If there is an excess or absence of intermediate particle size.
  • If the percentage of fine (ie silt and clay) present in gravel soil is more than 12%, it is recognized as dirty gravel, Dirty gravel can be classified as either Silty Gravel (GM) or clay gravel (GC),
  • GM– If there is little or no plasticity in the fine
  • GC– If the fine is of low to medium to high plasticity.
  • Gravel with a 5 to 12% fine is given a range classification, which is usually done in the laboratory.
  • If the percentage of sand is more than that of gravel then the coarse grained soil is called sandy soil.
  • If the percentage of fines (ie silt and clay) present in sandy soil is less than 5%, it is recognized as clean sand, Clean sand can be classified as either Well Graded Sand (SW) or Poor Grade Sand (SP),
  • SW– if all particle sizes are well represented
  • SP – If there is an excess or absence of intermediate particle size.
  • If the percentage of fine (ie silt and clay) present in sandy soil is more than 12%, then it is recognized as dirty sand, Dirty sand can be classified as either silty sand (SM) or Clay Sand (SC),
  • SM– If there is little or no plasticity in the fine
  • scheduled caste– If the fine is of low to medium to high plasticity.
  • Sands with 5 to 12% fines are given a range classification, which is usually done in the laboratory.

step 3 [Classifying Fine Grained Soil]

There are mainly two types of fine grained soils i.e. silt and clay. The distinction between silt and clay cannot be based on particle size because the important physical properties of the two materials are only indirectly related to the size of the particles. Furthermore, since both are microscopic, physical properties other than particle size should be used as criteria for field identification. There are 4 field tests to describe or classify silt and soil that we can easily do on the field. these

  • latency
  • dry strength
  • toughness
  • Spread

Let us discuss each of them one by one.

1. dry strength test

Dry power provides a basis for distinction. A small clay brick is molded and allowed to air dry. It is then broken and a small piece of about 1.0 cm size is pressed between the thumb and forefinger. The effort required to break the piece provides a basis for describing the power as too low, low, medium, high or too high. A piece of clay can be broken only with great effort, while a piece of silt is easily crushed.

2. latency or shaking test

Since silt is much more permeable than clay, a dilution test can also be used to differentiate between the two materials. In this test a small amount of clay is mixed with water to a very soft consistency in the palm of the hand. Then the back of the hand is lightly patted. If the clay is silky, the water quickly rises to its surface and gives it a shiny or shiny appearance. Then if the clay patch is deformed, in some cases by squeezing and in others by stretching, water flows back into it and leaves the surface dull. Generally, the higher the proportion of soil in the sample, the slower the reaction of the test. The response is described as fast, slow, or none.

3. Toughness / Plasticity Test

The property of plasticity is characteristic of soil and can be used as the basis for a simple field test. At certain moisture content a soil that contains a substantial amount of clay can be deformed and re-formed by hand without disintegration. Thus, if a sample of moist soil can be manipulated between the palms of the hands and fingers and twisted into a long thread, it undoubtedly contains a significant amount of clay. As moisture is lost during continuous manipulation, the soil reaches a state of non-plasticity and tends to crumble. Just before reaching the crumpled stage, a highly plastic clay can be twisted into a long thread with a diameter of about 3 mm, which has enough strength to support its own weight. On the other hand, silt can rarely be spun into a thread whose diameter is as small as 3 mm without any serious cracking, and there is a complete reduction in tensile strength unless a small amount of clay is present. The record of a simple plasticity test should indicate not only whether a plastic thread can be formed, but also the rigidity of the thread as it nears the collapsing stage. This condition is described as weak and friable, moderate or hard.

4. dispersion test

The fourth procedure, known as the dispersion test, is also useful for distinguishing between silt and clay, and for estimating the relative amounts of sand, silt, and clay in a material. A small amount of clay is dispersed with water in a glass cylinder or test tube and then allowed to solidify. The coarser particles fall out first and the finer particles remain in suspension the longest. Generally sand freezes in 30 to 60 seconds. The silt-shaped material settles in 15 to 60 minutes, while the clay-shaped material remains in suspension for at least several hours and usually several days until the clay particles combine into clusters or floccules. .

After doing these tests we can use the table given below to classify the soil as silt or clay.

specific name dry strength latency reaction hardness of plastic thread settling time deflection test
sandy silt less than none Fast weak to friable 30 seconds to 60 minutes
silt at the very least Fast weak to friable 15 to 60 minutes
clay silt low to medium fast slow medium 15 minutes to several hours
Sandy clay from bottom to top slow for someone medium 30 seconds to several hours
silty clay medium to high slow for someone medium 15 minutes to several hours
Soil very high from high None Harsh several hours to days
organic silt low to medium Slow weak to friable 15 minutes to several hours
organic soil medium to high None Harsh several hours to days


Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.