Index Properties of Soil

Index properties of soil are those properties which help to access engineering behaviour of soil and also helps in the determination of its classification.
Index properties include indices which help in determining the engineering behaviour such as

1. Strength
2. Load bearing capacity
3. Swelling and shrinkage
4. Settlement

These properties either relates to individual soil grain or aggregate soil mass.
The properties of individual soil grains can be determined from a disturbed or remoulded sample as they depend upon the individual grains and their mineralogical composition, size and shape are independent of soil formation.
The properties of aggregate soil mass depend upon the mode of soil formation, soil history and soil structure. These properties should be determined from undisturbed samples.

Types of soil	Index Properties
Coarse Soil	Grain Shape, Particle size, Relative Density
Fine Soil	Atterberg's Limit and Consistency

Grain Shape

1. The shape of sand and gravel is bulky which are further classified as angular, subangular, subrounded, rounded and well rounded.


2. The high angularity of soil grain leads to higher shearing strength

Classification of bulky grain is done on the basis of Sphericity (S)

$S=\frac{{D}_{e}}{L}$
Where,
${D}_{e}=$ diameter of equivalent spherical particle
$L=$ Length of the particle

Grain size Analysis

Grain size analysis is a method of speration of soils into different fractions based on the particle size.
It expresses quantitatively the proportions by mass of various sizes of particles present in a soil. It is shown graphically on a particle size distribution curve.

Particle Size	Type of Soil
> 300 mm	Boulder
300 to 80 mm	Cobbles

Particle Size	Type of Soil
80 mm to 4.75 mm	Gravel	Coarse Grain soil
4.75 mm to 2 mm	Coarse sand
2 mm to 0.475 mm	Medium sand
0.475 mm to $75 \mu$	Fine sand
$75 \mu$ to $2 \mu$	Silt	Fine Grained Soil
less than $2 \mu$	Clay

Grain size analysis of coarse grained soil is carried out by sieve analysis and fine grained soil is analysed by sedmentation analysis using hydrometer method or pipette method.


Consistency Limits

Whenever water is added to the soil, soil deforms and changes its state. Consistency represents the relative ease with which a soil can be deformed. The term consistency is important for clayey soil and is related to its water content.

Atterberg classified the consistency in 4 stages,

1. Solid state
2. Semi-solid state
3.  Plastic state
4. Liquid state

where
${w}_{l}=$ Liquid limit water content
${w}_{p}=$ PLastic limit water content
${w}_{s}=$ Shrinkage limit water content
${V}_{L}=$ Volume of soil at liquid limit
${V}_{P}=$ Volume of soil at plastic limit
${V}_{dry}=$ Volume of soil at shrinkage limit.


The boundary between the liquid state and plastic state is called Liquid Limit which is the minimum water content at which soil has a tendency to flow. All soils at liquid limit have a similar shear strength (around $2.7$ $KN/m^2$) which is negligible.

Click here to see how Liquid Limit is determine.

The boundary between the plastic state and semi-solid state is called Plastic Limit which is the minimum Water content at which soil is in the plastic stage. At plastic limit water content, a soil when rolled into a thread of 3 mm starts to crumble.

Click here to see how Plastic Limit is determine

The boundary between the semi-solid state and solid state is called Shrinkage Limit which is that maximum water content at which further reduction in water content does not cause any reduction in the volume of the soil sample.
When water is reduced to shrinkage limit, the soil particles are so closely packed that volume reduction will not take place and the void space starts getting occupied by air instead of water (See image below). It is the minimum water content at which the soil is saturated.

Click here to see how Shrinkage Limit is determine.

Shrinkage Limit, ${w}_{s}$ can also be determine as follows:

${w}_{s}=\frac {{W}_{w}}{{W}_{s}}= \frac {({V}_{d}-{V}_{s}){\gamma}_{w}}{{W}_{s}}$

${w}_{s}= \frac {({V}_{d}-\frac{{W}_{s}}{G {\gamma}_{w}}){\gamma}_{w}}{{W}_{s}}$

Shrinkage Ratio is defined as volume change in soil above shrinkage limit expressed as the percentage of dry soil per unit change in water content above shrinkage limit.

$SR=\frac{{{\gamma }_{d}}}{{{\gamma }_{w}}}=\frac{\frac{{{V}_{1}}-{{V}_{d}}}{{{V}_{d}}}}{{{W}_{1}}-{{W}_{2}}}\times 100$

Volumetric Shrinkage $=({w}_{1}-{w}_{s})SR$
Relative Density

Degree of denseness and coarseness of natural deposits of coarse grained soil is measured by its relative density. It is equivalent to relative consistency of clayey soil.

${{D}_{r}}=\frac{{{e}_{\max }}-{{e}_{nat}}}{{{e}_{\max }}-{{e}_{\min }}}$

Relative Density	Classification
<15	Very Loose
15-35	Loose
35-65	Medium
65-85	Dense
>85	Very Dense

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