What Is Soil?
Soil is a mixture of organic materials, minerals, organisms, air, and water that provide the medium for plant growth. Soil is also important for construction as it is the material upon which infrastructure is built. In this article, we’ll discuss the classification of soil.
After analyzing the soil behavior, if they seem to be similar, they are grouped into one; this helps understand the character and properties of different soils.
These soil classification systems help engineers decide the suitability of different soils for different uses and applications. Soil is classified based on specific requirements such as:
• The classification of soil must be based on engineering properties.
• The classification must contain only a limited number of groups with similar characters and properties.
• It should be simple and easy to understand.
Classification Of Soil
The classification of soil is as follows:
- Massachusetts Institute of Technology System (MIT)
- Textural Classification of Soils
- Unified Soil Classification System
- Indian Standard System of Soil Classification
- AASHTO System of Soil Classification
1. Massachusetts Institute Of Technology System (MIT)
The MIT soil classification system was first developed at Massachusetts Institute in the USA by Prof. G. Gilboy. Here, soils are classified based on grain size. These grain sizes are allowed certain names or terms that specify a particular size range as shown below.
Following are the 4 types of soil based on grain size:
- Gravel (> 2mm).
- Sand (0.06mm to 2mm).
- Silt (0.002mm to 0.06mm).
- Clay (< 0.002mm).
2. Textural Classification Of Soil
Soil texture classification is based on particle size and percentage distribution. These are represented or plotted along the three axes of an equilateral triangle.
Their boundary lines are drawn to indicate different percentages of sand, silt, and clay-sized particles, as shown in the below image.
The textural classification of soils is most preferred for classifying coarse-grained soils, and the U.S. Bureau of Soils first developed this.
3. Unified Soil Classification System
This system was created by A. Cassagrande and adopted by the U.S. Corps of Engineers in 1942 as the ‘Airfield Classification.’ It was later adapted for universal use and reworked in 1957 as the “Unified Soil Classification.”
This soil classification is based on the grain size and plasticity characteristics of the soil. These are classified into 3 main groups as follows:
- Coarse-grained soils with up to 50% passing No. 200 ASTM Sieve
- Fine-grained soils with more than 50% pass No. passing No. 200 ASTM Sieve
- Organic Soils
According to the uniform soil classification system, the soil is coarse-grained if more than 50% of the soil remains in a sieve of 0.075 mm. Coarse-grained soil is further divided into two types:
Gravel (G) – If more than 50% of the coarse particles are retained on the 4.75mm sieve, it is known as Gravel.
Sand (S) – If more than 50% of the coarse particles are not retained on the 4.75mm sieve, it is known as Sand.
According to the uniform soil classification system, fine-grained soils are those for which more than 50% of soil passes through 0.075 mm sieve size. The plasticity chart provided by the unified soil classification system is based on the data on the liquid limit and the plasticity index.
A line is drawn on the chart. With the help of this plasticity chart, we can determine silt, clay, and organic soils. Based on their liquid limit (W.L.) and plasticity index (Ip), the fine-grained soil is further divided into two types:
i) Silt (M) – Fine-grained soils are said to be slit when the data lies below the A-line.
ii) Clay (C) – Fine-grained soils are said to be clay when the data on the L.L. and P.L. plot both lie above the A-line.
iii) Organic Soil (0) – Fine-grained soils are said to be organic if the drying of the silt reduces the L.L. by 30% or even more, the soil is organic else the soil is inorganic silt.
Organic soils are fine-grained soils that possess organic properties such as high compressibility, dark color, strong odor, prominent visible organic materials, etc.
Organic soils are further classified into three types:
- Low-plastic organic soils (O.L.)
- Highly plastic organic soils (OH)
- Peat (Pt)
4. Indian Standard System Of Soil Classification
The Indian Standard Classification System (ISC) is a similar type of Unified soil classification system developed according to Indian standards. The Indian Standards Bureau first adopted this.
Here, the soils are further classified into three different types:
i) Coarse-grained soil – If 50% or more of the total weight of the material is stored in a 75 micro I.S. screen.
ii) Fine-grained soils – If more than 50% of the total material passes through a 75-micron I.S. sieve.
iii) Organic soils – Soils possess organic properties such as high compressibility, dark color, strong odor, prominent visible organic materials, etc.
5. AASHTO Classification System Of Soil
This type of soil classification is also known as the PRA classification system. The U.S. Bureau of Public Roads first developed it in 1920 to classify soil used in highway subgrade.
Here the soils are classified based on the particle size and plasticity characteristics of soil mass. After multiple revisions, It was adopted by the American Association of State Highway and Transportation Officials (AASHTO) in 1945.
The soils are further divided into eight groups: A-1 to A-7, with an additional group A-8 for peat. The suitability of soil is determined according to the group index.
Group Index (GI) = 0.2a + 0.005ac + 0.01bd
a = Soil percentage passing the 75 µ sieve greater than 35 and not exceeding 75.
b = Soil percentage passing the 75µ sieve greater than 15 and not exceeding 55.
c = Soil liquid limit greater than 40 and not greater than 60.
d = Soil plasticity index greater than 10 and not exceeding 30.
Soil with a lesser group index is more desirable soil for highway construction. Hence group index 0 indicates a good subgrade material, whereas group index of 20 or more indicates a very poor subgrade material.