Consistency of soil | plastic limit | shrinkage limit

 Consistency of soil

In this article we discussed:-

What is consistency of soil ?

What is shrinkage limit ?

What is plastic limit ?

What is liquid limit

What is plasticity index

What is consistency index ?

Thixotropy of clay 

Activity of clay and sensitivity of clay etc.

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Consistency it is the external work done required to convert the soil from one state to another. 

Consistency limit

I. It is the water content at which soil changes form one state to another 

II. It is also known as Atterberg’s limit

a) Liquid limit 

b) Plastic limit

c) Shrinkage limit 

Liquid Limit

I. It is the minimum water content at which the soil converts from plastic state to liquid state 

II. At liquid limit, all the soils contains very small shear strength

Plastic limit

I. This minimum water content at which the soil is converted from semi solid to 

plastic state.

Shrinkage Limit

i. it is the minimum water content at which a soil is convert from semi-solid state to 

solid

ii. it is the water content below which volume of soil is constant

iii. At shrinkage limit & above water content, the soil is fully saturated. 

iv. Below shrinkage limit ,the soil is partially saturated hence lies b/w 0 to 100.

1). Plasticity Index

Plasticity index represents the range of water content in which soil contains plastic properties.

IP = WL-WP

i. WL= WP -----> IP=0  Non-plastics soil e.g. sand

ii. WL> WP ----->  IP = +Ve

iii. WL< WP ----->  IP≠ -Ve  ----->   IP=0 

Ip= plasticity index

WL = liquid limit

Wp = plastic limit

W = natural water content

IP Soil type Example

0 Non plastic soil sand

0-7 Low plastic soil Silt

7-17 Medium plastic Salty clay

>17 Highly plastic soil Clay

2). Consistency Index

Ic=WL-W/ IP

 Ic=  WL-WP /WL-WP

i. IC = 0 ------- >WL= W  ------- > Soil is at liquid Limit

ii. IC = 1  ------- > WP = W  ------- > Soil is at plastic limit 

iii. 0<IC<1  ------- > soil is at plastic state

iv. IC>1  ------- > W<WP  ------- > Semi-solid/solid

v. IC<0  ------- > W>WL ------- > Liquid state

3). Liquidity Index or Water plasticity Ratio

IL = W-Wp/Ip  = W-Wp /WL-Wp  

• IC+IL=1

Activity of Clay

• Activity of Clay is the ratio of plasticity index to percentage of particles less  than 2µ.

This term is defined only for clay particle  

• Activity of clay is the slope of curve b/w plasticity index & percentage particles less than 2µ.  

• A= IP / (%C)

• Unit ------- > unit less 

Activity

0-0.75  - inactive

0.75-1.25 - Normal

>1.25 Active (max shrinkage & swelling)(black cotton soil)

Sensitivity of clay –

 It is the ratio of shear strength of soil for undisturbed condition to the shear strength of soil for disturbed condition  

 undisturbed /disturbed  =Sr

Sensitivity of soul is always greater than 1

If Sr = 1  ------- >Insensitive  soil

Thixotropy of clay

Thix ------ > To touch 

• The process of strength loss & strength gain without any change in water content of soul is known as Thixotropy.  

• The gain in strength is due to reformation of molecular crystals & structures.