Index Properties of Soil and Its Formation

Soil is the substance existing on the Earths surface that may be dug or ploughed. In simple terms soil is a substance existing on the Earths surface, which grows and develops plant life. Soil can be also termed as, the un-aggregated or un-cemented deposits of minerals or fragments covering large portion of Earth’s crust.

Properties of soil can be divided into two types index properties and engineering properties. Index properties of soil plays a vital role in defining the behaviour of the soil and also its engineering properties. More over index properties of soil are used for identification and classification of soil.

index properties of soil

Formation of Soil 

The formation of soil takes place due to chemical decomposition of rocks or due to mechanical disintegration of rocks.

When a rock surface gets exposed to atmosphere to a specific time it gets disintegrated or decomposes forming small fine particles of soil.

This process can be termed as ‘Geological Cycle’ in which soil is considered as incidental material obtained due to this process and this process goes on continuously in the nature.

Index Properties of Soil

The key index properties of soil that are mainly considered are as follow :

  • Particle Size Distribution
  • Atterberg Limit
  • Specific Gravity & Unit Weight

Particle Size Distribution :

Particle size distribution is among one of the most important index properties of soil. It can be defined as, the determination of percentage of various sizes of particles in given mass of soil is called as ‘Particle Size Distribution’.

These various sizes are classified based on their sizes ranging from gravel, sand, silt and clay. Soil behaviour such as permeability, compaction, shear strength relatively depends upon the distribution of the soil particles.

Modern techniques such as sieve analysis and hydrometer analysis are used to carry out the particle size distribution.

Atterberg Limit :

Water contents at which the soil transition takes place from one state to another is called as atterberg’s limit. There are mainly four states in which soil can exist namely : liquid state, plastic state, semi- solid state, solid state.

atterbergs limit - index properties of soil

Liquid limit : It represents the water or moisture content at which the soil changes form liquid state to plastic state. Liquid limit can be easily determined by a laboratory test called Casagrande method. In this test method, soil sample is mixed with water till it reaches the consistency where it flows. At this moment, the moisture content of soil is recorded as the liquid limit.

Plastic limit : When the soil starts losing its plasticity and becomes brittle due to the decrease in moisture content is called as plastic limit. A simple lab method to identify the plastic limit is to roll the soil sample into thin threads without crumbling it. The moisture content at which the soil starts crumbling and cannot be rolled out further are recorded as the plastic limit.

Shrinkage limit : At this limit, the reduction in the water content of soil does not affect them much as compared to other two limits. Only a minimal volume change can be witnessed.

Specific Gravity and Unit Weight :

Density of soil can be determined by the specific gravity and the unit weight of the soil. Specific gravity can be defined as the ratio of the mass of a given volume of soil solids to the mass of an equal volume of water at a specified temperature.

Specific gravity of soil can be easily determined in the laboratory using the pycnometer method. Unit weight can be defines as, weight of the soil per unit volume, including both the solids and void spaces.

These index properties are very crucial for determining the load- bearing capacity of the soil which lead in estimating the soil settlement behaviours.

Importance of Index Properties of Soil

Index properties of soil are critical in construction projects such as foundation construction, embankment construction, and slope stability analysis.

They play a practical role in various efforts, as seen by case studies. Engineers can reduce potential risks and improve the chances of success in mega construction projects by taking these soil properties into account.

Understanding and properly applying index properties is critical for engineers in order to meet project objectives.

Also Read : Sensitivity of Soil
Also Read : Thixotropy of Soil

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