Classification of soils Introduction: Soil classification is the arrangement of soils into different groups such that the soils in a particular group have similar behaviour. As there are a wide variety of soils covering earth, it is desirable to systematize or classify the soils into broad groups of similar behaviour. Soils, in general, may be classified as cohesionless and cohesive or as coarse-grained and fine-grained. These terms, however, are too general and include a wide range of engineering properties. Hence, additional means of categorization are necessary to make the terms more meaningful in engineering practice. These terms are compiled to form soil classification systems. Soil Classification The need: Natural soil deposits are never homogeneous in character; wide variations in properties and behaviour are commonly observed. Deposits that exhibit similar average properties, in general, may be grouped together, as a class. Through classification of soils one can obtain an appropriate, but fairly accurate, idea of the average properties of the soil group or a soil type, which is of great convenience in any routine type of soil engineering project. From engineering point of view, classification may be made based on the suitability of a soil for use as a foundation material or as a construction material. For complete knowledge of soil behaviour of soils, all the engineering properties are determined after conducting a large number of tests. However, an approximate assessment of the engineering properties can be obtained from the index properties after conducting classification tests. A soil is classified according to index properties, such as particle size and plasticity characteristics. A classification system thus provides a common language between engineers dealing with soils. It is useful in exchange of information and experience between the geotechnical engineers.
It is important to note that soil classification is no substitute for exact analysis based on engineering properties. For final design of large structures, the engineering properties should be determined by conducting elaborate tests on undisturbed samples. Requirements for a Soil Classification System: For a soil classification system to be useful to the geotechnical engineers, it should have the following basic requirements: i) It should have a limited number of groups ii) It should be based on the engineering properties, which are most relevant for the purpose for which the classification has been made. iii) It should be simple and should use the terms, which are easily understood. Any soil classification must provide us with information about the probable engineering behaviour of a soil. Most of the classification systems developed satisfy the above requirements. Soil Classification Systems: Several classification systems were evolved by different organizations having a specific purpose as the object. A. Casagrande (1948) describes the systems developed and used in highway engineering, airfield construction etc. The two classification systems, which are adopted by the US engineering agencies and the State Departments, are the Unified Soil Classification (UCCS) and the American Association of State Highway and Transportation Officials (AASHTO) system. Other countries, including India, have mostly the USCS with minor modifications. For general engineering purposes, soils may be classified by the following systems 1. Particle size classification 2. Textural classification 3. Highway Research Board (HRB) classification
4. Unified Soil Classification 5. Indian Soil Classification 1. Particle Size Classification: The size of individual particles has an important influence on the behaviour of soils. It is a general practice to classify the soils into four broad groups, namely, gravel, sand, silt size and clay size. Note: While classifying the fine-grained soils on the basis of particle size, it is a good practice to write silt size and clay size and not just silt and clay. In general usage, the term silt and clay are used to denote the soils that exhibit plasticity and cohesion over a wide range of water content. The soil with clay-size particles may not exhibit the properties associated with clays. For eg. Rockflour has the particles of the size of the clay particles, but does not possess plasticity. It is classified as clay-size and not just clay in the particle size classification systems. Classification based on particle size is of immense value in the case of coarse-grained soils rather than fine-grained soils because the behaviour 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: i) U.S. Bureau of Soil and Public Road Administration (PRA) System Classification ii) International soil classification iii) M.I.T System iv) Indian Standard Classification (i) U.S. Bureau of Soils Classification: This is one of the earliest classification systems developed in 1895 by the U.S. Bureau of soils.
(ii) International Classification System: 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 (iii) MIT 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 m)
iv) 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 m) 2. Textural Classification: The visual appearance of a soil is called its texture. The texture depends upon the particle size, shape of particles and gradation 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.