Index Properties of Soils

Soil index properties refer to the properties of the soil that help to classify and identify the properties of soil for purposes of engineering. For example, clay is generally cohesive in nature and sand and gravel are described as granular and non-cohesive. Cohesive soil is also known as plastic soil, and non-cohesive soil is also known as non-plastic soil. 

Soil index properties

The properties of soil index which are non-cohesive in nature are:

• Particle size distribution 
• Particle shape
• Relative density 
• Consistency 
• Clay and clay mineral content 
• Cohesive soil index properties are
• Consistency 
• Particle shape and its orientation 
• Clay or clay mineral content.
• water content in the soil.

Distribution of article size: the size and distribution of particles in soil are described by this property. This is considered the most basic soil index property. Soil that consists of particle sizes of a wide range is called “well-graded soil.” On the other hand, soil that has a limited number of particle sizes is regarded as soil that is poorly graded. 

Particle shape: The concept of particle shape depends on how closely the soil particles are packed. In particular, the coarse-grained soil density of the soil indicates the soil’s stiffness and strength. 

The formula of relative density: the soil’s relative density is referred to as the ratio of the bulk density and the highest possible soil density. Relative density indicates the potential increase in density of the soil and what deformation may take place under different circumstances. Soils can be graded via sieve and sediment analysis. 

Consistency: the consistency of the soil refers to the soil’s potential resistance to damage, rupture and deformation. The whole idea of the consistency of the soil is derived from a hypothesis that the soil can exist in 4 states depending on the state of moisture. Initially, the soil is a thick liquid which is resistance less or shear has strength of its own. While maturing, its content of moisture gets reduced and it starts to attain some shear strength, but it can be very easily molded. This phase is also known as the solid phase of plastic. With drying, the feature of being moulded gets drastically reduced, and cracks begin to form. Over time, the soil becomes so dry that it becomes brittle. Compression strength is also used as an indicator of soil strength and consistency. To categorise the soil, different terms such as “soft,” “medium, “stiff, “very stiff” and “hard” are used. In the case of non-cohesive soil, consistency primarily depends on the distribution of size and shape of soil particles. whereas in the case of cohesive soil, consistency depends mainly on the content of water. 

Clay and clay mineral content: Clay and clay mineral content is another important soil index property. Clay minerals are fine silicate minerals and can be distinguished by mineral property and particle size. The main silicate minerals are silicate tetrahedron and aluminium or magnesium octahedron, which are both very plastic in nature. As mentioned earlier, the clayey soils are plastic in nature, and this nature solely depends on the silicate content. So, soils that don’t contain any clay minerals have no property of plastic and the moisture content also varies greatly from liquid to semi-solid state. The most common three groups of clay minerals are kaolinite, illite, and montmorillonite. 

Water content: The next soil index property is water content. For example, the properties of soil that are fine-grained vary greatly with the concentration of water. The four states of soil are liquid, plastic, solid, and semi-solid, and water content is an important determinant of these states. Moisture content is used to calculate dry density from the determining bulk density. Soil water is not pure water and many organic and inorganic substances are dissolved in this water. 

Soil classification: In general, coarse-grained soil is classified using the soil classification letter G (G for gravel) or letter S (S for sand). The letter W is used to refer to well-graded soil, and the letter P is used to classify poorly-graded soil. If the coarse-grained soils contain more than 12% silt or clay, the second letter, M, is used to indicate silt and C, clay. The same designation is applied to fine-grained soils also. The second letter, H (high) or L (low), is used to indicate the plasticity of the soil. For example, in the gravel and sand mixture, GW denotes well-graded and clean soil, and on the other hand, SW denotes well-graded and clean sandy soil. In the case of fine-grained soils of low plasticity, the word ML denotes high content of silt, and for fine-graded soils of high plasticity, MH also represents silt. 

The formula for soil index property is

The density index is referred to as the following: void ratio (maximum): void ratio of soil in a loose state; maximum void ratio: minimum void ratio of the soil. 

Conclusion

The index properties of soil are fundamental to understanding soil structure and are very important in civil engineering. The soil’s consistency, resistance, and phase are all determinants of its suitability for constructing buildings and other engineering purposes.