Plastic Limit Test of Soil Procedure [Update-2019]

Plastic limit test of soil is required to find out the moisture content in which soil losing it’s plastic state. We described plastic limit test of soil procedure with atterberg limits.


What is Plastic Limit?

In easy word plastic limit of soil is the moisture content in which soil crumbles when it is being rolled in 3 mm diameter by pressing hand.

Plastic limit test is the part of atterberg limit test.


Importance of Plastic Limit Test

Purpose of Plastic Limit Test


Why Plastic Limit Test is Done 

Purpose of plastic limit test or why plastic limit test is done, same meaningful questions and here is the answer.

  • To know the limit of moisture content in which soil can hold it’s plastic state.
  • To know the class of soil.
  • To design the Highway road or foundations.


Plastic Limit of Soil IS code

IS code for plastic limit of soil is IS2720 part 5.


Atterberg Limit

Atterberg limit indicate liquid limit and plastic limit.

Plasticity index = Liquid limit – Plastic limit


Plastic Limit Test Apparatus

  1. Evaporating dish ( dia – near about 120 mm ).
  2. Glass plate ( minimum thickness – 1 centimeter to mix the soil with water ).
  3. Spatula to mix.
  4. Containers to take sample.
  5. Weighing balance ( minimum capacity – 500 gram, least count – 0.01 gram ).
  6. Hot air oven to maintain the temperature between 100 to 110 degree centigrade.
  7. Rod to check plastic roll ( length – near about 100 mm, thickness – 3 mm ).
  8. IS Sieve 425 micron to sieve sample.


Precautions Before Proceeding The Test

Rod, plate, and spatula must be clean and dry before the test.


Plastic Limit Test of Soil Procedure

  • Before starting the test soil sample must be dried in a hot air oven for 24 +/- 0.5 hours at temperature 100 to 110-degree centigrade.
  • Allow soil sample to be cool in the atmosphere until soil sample temperature reaches similar to the atmosphere.
  • Take 425-micron sieve and sieve the soil sample with it.
  • Take near about 20 gram of passed soil through 425 microns.
  • Add water in this soil and mix properly in the evaporating dish by using a spatula.
  • Mix the soil paste until it comes in a plastic state and you can make a rounded ball with it. Water content should be added carefully for bring the soil in the plastic state.
Important Tip
  • If you are going to test a clayey soil then you must keep this mixed soil for 24 hours in an airtight container for a good distribution of moisture.
  • Take near about 8-gram mix from the mixed soil (20-gram mixed ball ) and start making a ball between fingers.
  • After making a ball, Start rolling on a glass plate by pressing finger.
  • Press the soil while rolling forward and backward to make it a thread to all its length.

Plastic Limit Test

  • The speed of rolling must be between 80 to 90 strokes per minute.
  • One stroke means when you roll forward and return back to the starting point.

Plastic Limit Test of Soil

  • Keep rolling until it’s dia reaches 3 mm, check by a rod which dia is 3 mm. If it is not crumbled then knead this roll together and mate it a ball.
  • Start rolling it again.
  • Repeat these procedures until soil thread crumb before reaching 3 mm in dia.
  • Then take the crumbled part of the soil and keep it in the container.
  • Note down the weight of the container and rolled soil.
  • Two tests must be done for plastic limit.
  • Keep sample containers in a hot air oven for 24 hours in temperature 100 to 110-degree centigrade.
  • After 24 hours, allow it to be cool for 15 minutes in atmosphere temperature then note down the weight.

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Plastic Limit Test Lab Report

container no1213
empty weight of container (gm) 40.66 36.15
cont. wt. + wet soil weight (gm) 68.1266.08
cont. wt. + oven dry soil wt. (gm)63.6061.23
water content (gm)4.524.85
weight of dry soil (gm)22.9425.08
moisture content ( % )19.7019.34

Formula to Calculate Plastic Limit of Soil

So Plastic limit is the average of both moisture content

Plastic limit = ( 19.70 + 19.34 ) / 2 = 19.52 %



Atterberg Limits

Plastic Limit of Soil Range/Value

ActivityLiquid Limit ( max )Plasticity Index ( max )
Embankment / Subgrade7045
Grainular sub-base256
Wet Mix macadam6


Viva Questions on Plastic Limit Test

Q.1  What is plastic limit of soil?

Ans. Plastic limit test of soil is the moisture content in which soil losing its plastic state.


Q.2  What is the IS code for liquid limit test?

Ans. Liquid limit test IS code 2720 part 5.


Any Other Question? Just ask in comment. We are waiting.

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Read More:

Liquid Limit Test of Soil Procedure by casagrande 2019

Standard Proctor Test & Modified Proctor Procedure 2019

Particle Size Distribution of Soil Procedure or Soil Gradation

CBR Test of Soil Procedure, Calculation & Graph

Types of Soil, Properties & Uses in Construction

Concrete Mix Design – M40, M60 as per IS [Update-2019]


Student Corner

Weigh the container with the earth threads to an accuracy of at least 0.01 g. 

Form and add more threads according to steps 3.4, 3.5.1, if the weight of the bottom threads is less than 5 g until this weight is exceeded (weight between 5 and 7 g is suitable).

Repeat steps 3.1 to 3.9 for the other globe (the ball-shaped in step 2.3).

For floors with very low plasticity, skip step 3.10 if the plasticity of the floor is too low to properly perform the test for two balls with different water content (to test only one-floor bale). 

With regard to steps 2.1 to 2.3, for contiguous soils (mainly clays), it is recommended that at least one of the spheres have some stiffness, indicating that the moisture content is close to pl. 

For soils with low or very low cohesion (mainly silts and sandy soils), the consistency of the soil bales should be soft, but without excess water (this soft texture is required because soil soils are usually difficult to form in soils with low plasticity which the soil consistency becomes stiff).

It is important to point out that the amount of water added in these steps depends on the type of soil.

Therefore, the operator must judge at his own discretion whether the soil has the proper consistency to perform the test properly because the soil is difficult to form if the soil is too dry (it may crumble) or too wet (it may be sticky)), even if step is followed. 



The volume of the sample varies according to soil type: in fine soils (clays and silt) 100-1000 g is generally sufficient, however, in sandy and gravelly and siliceous soils may require large quantities of its own up to a few kg.

Reduce the sample by quartering it in the lab if it is too bulky (use shrapnel if necessary).

Place the sample on a tray and dry the soil at a temperature of not more than 60 ° C. 

Also, the drying step can be ignored in very fine soils if they contain a natural moisture suitable for the test (water content above the plastic limit without actually being sticky).


The plasticity index is expressed as a percentage of the dry weight of the soil sample. 

It shows the size of the range of moisture content at which the soil remains plastic. 

In general, the plasticity index depends only on the amount of clay present. 

It shows the fineness of the soil and its ability to change the shape without changing the volume. 

The consistency range in which a soil has plastic properties is referred to as a plastic area and indicated by the plasticity index. 

For sandy soils, the plastic limit should first be determined. 

If the plastic limit is equal to or greater than the liquid limit, the plasticity index is reported as zero. 

If the consistency index of soil equals one, it is at the plastic limit.

Similarly, a bottom with a CI of zero is at its liquid boundary. 

A negative consistency index indicates that the soil has a natural water content that is above the liquid limit, and therefore behaves like a liquid. 


The non-dewatered shear strength of soil from the water content around the liquid limit to the water content around the plastic limit can be determined by the falling cone test.

The determination of the liquid limit by the falling cone test is based on the assumption that the soil at the liquid boundary assumes a unique condition giving a unique shear strength. 

When the unrestrained shear strength is related to the liquid and the plastic limit, it is emphasized that the liquid limit and the plastic limit could also be clearly related. 


The liquid limit (II) is conceptually defined as the water content at which the behavior of a clayey soil change from the plastic to the liquid state.

However, the transition from plastic to liquid behavior occurs gradually over a range of water contents, and the shear strength of the soil is not actually zero at the liquid boundary. 

The precise definition of the liquid limit is based on the standard test methods described below.

Limit (ll) and plastic limit (pl) are the two most important soil consistency limits of the borders defined by atterberg in 1911. 

LL marks the boundary between fluid and plastic states and PL between plastic and semisolid states. 

LL is determined worldwide according to various standards by the Casagrande method 2,3 or the penetration test. 

For this reason, it has often been criticized because the skills and judgment of the operator play a crucial role in the test result. 

Usually such a high liquid limit results in substantial moisture content, plasticity and a pI value. 

Anyway, I would classify the material as a plastic silt, unless you have a unique geological formation. 

If you want soil compactors, we need to determine the density of the soil to be leached and the soil density. 

If you need expansive soil properties, we need to do an expansion index test.


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