The size of individual particles has an important influence on the behavior of soils. It is a general practice to classify the soils into four broad groups, namely; gravel, sand, silt size and clay size.
Classification based on particle size is of immense value in the case of coarse-grained soils rather than fine-grained soils because the behavior of such soils depends mainly on the particle size, whereas fine-grained soils depend on the plasticity characteristics. Some of the classification systems based on particle size alone are:
iii) M.I.T System
U.S. Bureau of Soils Classification
This is one of the earliest classification systems developed in 1895 by the U.S. Bureau of soils. In this system, the soils below the size 0.005mm are classified as clay size in contrast to 0.002mm in the other systems. The soils between 0.005mm and 0.05mm are classified as silt clay. Sandy soils between 0.05mm and 1.0mm are subdivided into four categories very fine, fine, medium and coarse sands. Fine gravels are in the range of 1.0 to 2.0mm
Fig 1; US Bureau of Soil Classification
International soil classification
This system was proposed for general use at the international Soils Congress held at Washington in 1927. This was known as the Swedish Classification system before it was adopted as International system. In this system, an additional term Mo (Majla) has been used for soil particles in the size range between sand and silt.
Fig 2; International Soil Classification System
Prof. G. Gilboy at Massachusetts Institute of Technology in U.S.A developed MIT system of classification of soils. In this system, the soil is divided into four groups:
(a) Gravel, particle size greater than 2 mm
(b) Sand, particle size between 0.06 mm to 2 mm
(c) Silt size, particle size between 0.002 mm to 0.06 mm
(d) Clay size, particle size smaller than 0.002 mm (2 µm)
The soils in sand and silt-size range particles are further divided into three categories; coarse, medium, fine. The M.I.T System only uses two integers 2 and 6 which is easy to remember.
Fig 3; M.I.T System of Classification
Indian Standard Classification
As per I.S. Classification (IS: 1498-1970) the soil is divided into six groups:
(a) Boulders, particle size greater than 300 mm
(b) Cobble, particle size between 80 mm to 300 mm
(c) Gravel, particle size between 4.75 mm to 80 mm
(d) Sand, particle size between 0.075 mm to 4.75 mm
(e) Silt (size), particle size between 0.002 mm to 0.075 mm
(f) Clay (size), particle size smaller than 0.002 mm (2 µm)
Fig; Indian Standard Classification
TEXTURAL CLASSIFICATION SYSTEM
The visual appearance of a soil is called its texture. The texture depends upon the particle size, shape of particles and gradation of particles. The textural classification incorporates only the particle size. Here, the term texture is used to express the percentage of the three constituents of soils, namely; sand, silt and clay.
According to the textural classification system, the percentages of sand (size 0.05 to 2.0 mm), silt (size 0.005 to 0.05 mm) and clay (size less than 0.005 mm) are plotted along the three sides of an equilateral triangle. The equilateral triangle is divided into 10 zones; each zone gives a type of soil. The soil can be classified by determining the zone in which it lies.
For e.g. If a soil contains 30 % sand, 20 % silt and 50 % clay, it is shown by point P in the figure. The point falls in the zone labeled clay. Therefore, the soil is classified as clay.
The textural classification system is useful for classifying soils consisting of different constituents. The system assumes that the soil does not contain particles larger than 2-mm size. However, if the soil contains a certain percentage of soil particles larger than 2 mm, a correction is required in which the sum of the percentages of sand, silt and clay is increased to 100 %. For e.g. if a soil contains 20 % particles of size larger than 2-mm size, the actual sum of the percentages of sand, silt and clay particles is 80 %. Let these be respectively 12, 24, and 44 %. The corrected percentages would be obtained by multiplying with a factor of 100/80. Therefore, the corrected percentages are 15, 30 and 55 %. The textural classification of the soil would be done based on these corrected percentages.
UNIFIED SOIL CLASSIFICATION SYSTEM
This system was developed by Casagrande in 1948. This system has also been adopted by American Society of Testing Materials. The various symbols are given below;
Unified soil classification system.
The soils are classified into 15 groups. The soils are first classified into two categories.
If the coarse-grained soils contain less than 5% fines and are well graded (W), they are given the symbols GW and SW, and if poorly graded (P), symbols GP and SP. The criteria for well-grading are given below. If coarse-grained soils contain more than 12% fines, these are designated as GM, GC, SM or SC, as per criteria given. If the percentage of fines is between 5-12% dual symbols such as GW-SM, SP-SM, are used.
Fine-grained soils- They are further divided into two groups;
The exact type of soil is determined from the plasticity chart. The A-line has the equation Ip=0.73(Wl-20). It separates the clays from silts
The inorganic soil ML and MH and the organic soils OL, OH plot in the same zone of the plasticity chart. The distinction between the inorganic and organic is made by oven-drying.
Highly organic soils are termed peat ( Pt).
September 23, 2019