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Field Identification of Soil | Gravel V/S Sand, Silt & Clay| Particle Size Classification

In soil engineering, soil is defined as an unconsolidated material, composed of solid particles, produced by rock disintegration. In this article, we are going to discuss about identification and classification of soil, Field Identification of Soil, Descriptive Soil Classification, and Particle size Classification.

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What is Soil Mechanics?

Dr. Karl Tezaghi says soil mechanics is how to use laws of mechanics and hydraulics to solve engineering problems with sediments and other unconsolidated soil particles, regardless of whether or not rocks have an admixture of organic constituents.

Field Identification of Soil
Fig. Field Identification of Soil

For engineering purposes, soil is a loose or moderately cohesive natural aggregate of mineral grains, inorganic or organic.

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Soil is a disintegrated rock, according to a geologist. Agriculture experts define soil as the loose mantle at the surface of the earth that favors plant growth.

Identification And Classification Of Soil

The classification of soil is the division of soil into different groups so that the soil in a specific group has similar behavior.  It is a type of labeling for different levels of soil. It is a good idea to organize or group the different types of soil into broad groups with similar characteristics because of the wide variety of soil that covers the earth. The behavior of groups is easier to study than the behavior of individual soils.

A soil classification system must meet the following fundamental requirements in order for it to be beneficial to the geotechnical engineers:

I) It should have a limited number of groups. 

iii) It should be based on the engineering features that are most relevant to the classification purpose.

iii) It should be straightforward and use words that are easy to understand..

It should be remembered that soil classification is no substitute for precise analysis based on engineering properties. Advanced tests on undisturbed samples should be done to determine the engineering properties for the final design of a massive structure.

Field Identification of Soil

Mainly, coarse-grained and fine-grained soils are distinguished according to whether the individual soil grains are visible with the naked eye.  Thus, grain size itself may be sufficient to differentiate gravel from sand, but silt and clay cannot be distinguished by this method.  Understanding how to differentiate gravel from sand, silt, and clay from soil makes it easier to identify soil in the field. The steps are given briefly below. 

1) Gravel from sand

For this identification Visual inspection is used.  Gravel is defined as soil particles larger than 4.75 mm and smaller than 80 mm; sand is defined as soil particles from 4.75 mm to 0.075 mm. If possible, field identification of sand and gravel should also include mineralogical composition. 

2) Silt VS sand

Fine sand and silt cannot be easily distinguished by visual inspection. Silt may appear slightly darker in color. However the dispersion test can distinguish the two. Pouring a spoonful of sample into a jar of water is part of this test. If the particles are sand, they will settle down. Silt has a slightly darker color than sand. Hence, Squeeze between fingers and lab tests are used.

Dispersion Test

Pouring a spoonful of sample into a jar of water is part of this test. When the material is sand, it will settle down in a few minutes, but if it is silt, it may take fifteen to one hour. In both cases, nothing will stay in the suspension.

3) Silt from clay

Only in the laboratory, the particles can be examined microscopically. Visual inspection is impossible. Moist soil taken and squeezed between fingers: if moisture comes out it’s silt, if the soil also along with moisture comes out it’s clay.

  • Soapy touch →clayey,
  • Rough→ Sand
  • Moisture comes out when squeezed → Silt

In the absence of one, some basic tests are conducted.

a) Shaking test (Dilatancy test) 

When performing this test, a portion of soil is shaken in the palm of the hand after being mixed with water to a very soft consistency. Water will rise quickly to the surface if the soil is silty, giving it a shiny, glistening appearance.

When it is clay, the water cannot move easily, so it remains dark. Noting whether the reaction is fast, slow, or nonexistent can be used to estimate the relative proportions of silt and clay in an unknown soil mixture.

b) Dry strength

The strength of soil in a dry condition is a representation of its cohesion and, therefore, its characteristics. It can be estimated by crushing a dried fragment of 3 mm size between the forefinger and thumb. Then a silt fragment can be crushed easily, a clay fragment can only be broken with great effort.

c) Rolling test (Toughness test)

A thread is attempted to be made out of a sample of moist soil with a diameter of approximately three millimeters. When the material is silt, it is impossible to make such a thread without it cracking and breaking down. When clay is used, such a thread can even grow to 30 cm in length and be held at its ends by its own weight.

d) Dispersion test

A spoonful of soil is mixed with a bucket of water. If it is silt, the particles will settle in 15 to 30 minutes. If it is clay, it will form a suspension that will stay there for hours or even days if there is no flocculation.

Descriptive Soil Classification

Descriptively soil are classified as

  • boulders
  • cobbles
  • gravels
  • sand
  • silt and
  • clay

Particle size Classification

S.NSoil typesSize (mm)
1.Clay<2µ (0.002)
 Fine silt0.002-0.006
 Medium silt0.006-0.02
 Coarse silt0.02-0.06
 Fine sand0.06-0.2
 Medium sand0.2-2.0
 Coarse sand2.0-4.75
  • soil is of basic type sand or gravel (coarse soil), if after removal of boulders and cobbles, over 65% of the materials is in the sand and gravel range.
  •  A soil is of basic type silt or clay (fine-grained soil) when over 35% of the soil is in the silt and clay range.
  • Mixtures containing over 50% boulders and cobbles are referred to as very coarse soil.

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