Soil classification systems serve the purpose of grouping soils based on their performance under specific physical conditions. It is important to note that soils which perform well under one set of physical conditions may not necessarily perform the same under a different set of physical conditions. As a result, numerous classification systems have been developed, each with a specific intended purpose. The soil engineer benefits greatly from soil classification, as it provides empirical guidelines based on the field experience of others.
The usefulness of soil classification cannot be overstated. It enables the soil engineer to obtain general guidelines for the appropriate use of soils in various applications. This is accomplished by grouping soils based on their performance characteristics under specific physical conditions. Soil classification has a long history and has been refined over time to meet the needs of engineers and other professionals who work with soils.
Due to the diverse physical conditions that soils may be subjected to, a variety of classification systems have been developed. Each classification system is designed to
Different Classification of Soils for Engineering Purpose
Soil can be categorized in various ways, and one such way is based on the size of its grains. Another classification is the textural classification. Additionally, there are two widely used classification systems, namely the AASHTO classification system and the Unified soil classification system. These systems help to identify and distinguish soil based on their physical and engineering properties. The AASHTO classification system is typically used for highway construction purposes, while the Unified soil classification system is commonly used for general engineering purposes. By using these classification systems, it becomes easier to analyze and understand the behavior of soil and select the appropriate methods for construction or engineering projects.
(i) Grain Size Classification System for Soils
Grain size classification systems are utilized to categorize soil particles based on their size. The terms clay, silt, sand, and gravel are used in these systems to indicate the particle size and do not signify the nature of the soil type. Several classification systems are being used, but the most frequently used ones are presented here.
(ii) Textural Classification of Soil
Soil can be classified based on the distribution of particle sizes using a system called textural classification. This method involves identifying the percentage of sand, silt, and clay present in the soil and assigning a specific name to the soil based on these percentages. To facilitate this process, triangular charts are used to classify the soil. Figure 1 provides an example of a typical textural classification system. By using this system, it is possible to categorize different types of soil based on their physical characteristics and particle size distribution.
Fig-1: Textural Classification of U.S. Public Roads Administration
(iii) AASHTO classification system of Soil
The AASHTO classification, also known as the PRA classification system, was originally created in 1920 by the U.S. Bureau of Public Roads for the purpose of categorizing soil for use in highway subgrades. This system is based on the particle size and plasticity characteristics of the soil mass. After undergoing revisions, the AASHTO adopted the system in 1945. The classification divides soils into seven major groups, with some groups containing subgroups. To classify a soil, one must follow the chart from left to right to determine which group the soil test data fits into. Soils with fine fractions are further classified based on their group index. The group index is calculated using the equation: Group index = (F – 35)[0.2 + 0.005 (LL – 40)] + 0.01(F – 15)(PI – 10). A higher group index value indicates poorer quality material.
(iv) Unified Soil Classification System
The Unified Soil Classification System, initially known as the airfield classification system, was created by Casagrande in 1948. The system was later modified and adopted by the U.S. Bureau of Reclamation and the U.S. Corps of Engineers, and subsequently by the ISI for general engineering purposes under IS 1498-1970. The system categorizes soil based on its grain size and plasticity characteristics. Soil is divided into three major groups: coarse-grained, fine-grained, and organic soils, as well as miscellaneous soil materials. Coarse-grained soils have more than 50% of their material larger than 0.075mm size and are classified further into gravels (G) and sands (S), which are then divided into four categories based on gradation, silt or clay content. Fine-grained soils have more than 50% of soil finer than 0.075 mm sieve size and are subdivided into silt (M), clay (C), and organic salts and clays (O). Based on their plasticity nature, they are added with L, M, and H symbols to indicate low plastic, medium plastic, and high plastic, respectively. Fine-grained soils are also sub-divided into three subdivisions of low, medium, and high compressibility instead of the original two sub-divisions in the Unified Soil Classification System. Table-3 shows the classification system, and Table-2 lists the group symbols for the soils in Table-3.
| Soil | Soil Component | Symbol |
| Coarse Grained | Boulder | None |
| Cobble | None | |
| Gravel | G | |
| Sand | S | |
| Fine Grained | Silt | M |
| Clay | C | |
| Organic Matter | O |
Table – 3
| Soil | Soil Component | Symbol |
| Peat | Peat | Pt |
| Applicable to Coarse grained Soils | Well graded | W |
| Poorly Graded | P | |
| Applicable to Fine grained soils | Low compressibility WL<35 | L |
| Medium compressibility (WL 35 to 50) | I | |
| High compressibility (WL>50) | H |
The standard suggests that if a soil has properties of two different groups, either in terms of its particle size distribution or plasticity, then it should be designated using a combination of group symbols. In order to identify such soils in the field, certain tests are recommended.
For fine-grained soils, the following tests are recommended for visual identification: visual examination, dilatancy test, toughness test, dry strength test, organic content and colour, as well as other identification tests.
By using these tests, it becomes possible to accurately identify and categorize soils that may have properties of multiple groups. This is important for various applications, such as engineering projects or agricultural land management, where the properties of the soil must be known in order to make informed decisions.
Indian Standard Classification System for Soil
The Bureau of Indian Standards has adopted the Indian Standard Classification System (ISC) which is quite similar to the Unified Soil Classification (USC) system. Under the ISC, soils are broadly categorized into three divisions based on their grain size. Coarse grained soils are those in which 50% or more of the total material by weight is retained on a 75 micro IS sieve. On the other hand, fine grained soils are those in which more than 50% of the total material passes through the 75 micron IS sieve.
Soils that have a high organic content and consist of a significant percentage of decomposed vegetation particles are placed in a separate category called peat (Pt). In total, there are 18 different groups of soils classified under the ISC. These groups are further divided into 8 groups of coarse grained soils, 9 groups of fine grained soils, and one group of peat.
Fig.2: Indian Standard Classification Plasticity ChartClick to view following ISC Charts