![]() The system was also adopted by the American Society for Testing and Materials (ASTM). The unified soil classification system was first developed in 1948 by Arthur Casagrande and was later modified in 1952 by the US Bureau of Reclamation (USBR) and the US Corps of Engineers. Similarly, the group index of soil group A-4 depends only on %fines, and it increases by 1 for every 5% increment in %fines above 35, as shown in Table 6.9. Further, group index for soil groups A-2-6 and A-2-7 depends only on the plasticity index, and it increases by 1 for every 5% increment in the plasticity index above 10. It is interesting to note that the group index is always zero for soil groups A-1, A-3, A-2-4, and A-2-5. The group index is to be specified even if its value is zero. For example, when a soil is classified based on %fines, LL, and I p as A-4, and if its group index is 3, the soil is finally designated as A-4 (3). In showing the classified soil, the group index is to be mentioned within parentheses after the soil group. However, in the revised AASHTO classification, there is no specific maximum value for the group index. Only the soil group A-7 can have maximum group index of up to 20. The soil groups A-1-a, A-1-b, A-3, A-2-4 and A-2-5 have zero group index. The minimum possible value of the group index is zero. ![]() The lower the group index, the more suitable is the soil as a highway material or subgrade. A-7 is the least suitable of all soil groups. Thus, A-1 is the most suitable soil group, followed by A-2, A-3, A-4, A-5, and A-6 in decreasing order of their suitability as a highway material or as subgrade. The lower the number of the group, the more suitable is the soil as a highway material or subgrade. The textural classification classifies soils on the basis of the proportion of clay, silt, and sand-sized particles in the soil as follows: Textural Classification of Soil:Īs soils occurring in the nature consist of a mixture of particles of various sizes, the textural classification recommends classification of soils based on particle size distribution, instead of particle size. In addition to the grain size distribution, it is also necessary to consider the liquid limit and plasticity index of the soil for classifying the soil. Hence, to classify a given soil, it is necessary to determine its grain (or particle) size distribution. Thus, every soil consists of clay, silt, sand, and gravel in some proportion. The soil in nature consists of a mixture of particles of different sizes rather than soil particles of single size. Plasticity characteristics represented by the liquid limit and the plasticity index. The following two criteria are found to be significant for soil classification after years of research and experience with wide-ranging soils, all over the world: Scientific Basis for Soil Classification: The initial soil classification systems were thus based on particle size. Hence, the larger the particle size, the higher the stability and strength of the soil. ![]() As soils are formed by weathering, large soil particles are those that have sustained the disintegrating effects of weathering. The size of soil particles is found to have a significant influence on the engineering characteristics of the soil. ![]() Thus, soil classification serves as a language of communication to describe soils, thereby avoiding possible errors due to the use of local terminology. Soil classification provides standard terms to describe different groups of soils with distinct characteristics, among different engineers at the national and international level. This enables the prediction of its behavior and its suitability as a foundation soil or construction material. The purpose of soil classification is to identify the group to which a given soil belongs. It is more convenient to study the behavior of groups than that of individual soil. Soil classification is a system of arrangement of soils into different groups so that soils within a group have similar characteristics and behavior. In this article we will discuss about:- 1. ![]()
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