what are Atterberg limits of soil?

Atterberg Albert is a Swedish scientist and conducted various experiments on the plasticity of soil in around 1911 and concluded atterberg limits of soil.

These limits help us to find and differentiate soil based on the moisture content present in the soil.

In this article, we will discuss test procedures and the importance of all Atterberg limits of soil.

Atterberg limits of soil

atterberg limits of soil
figure-1

The atterberg limits are mainly three types namely

  • shrinkage limit
  • plastic limit
  • liquid limit

In the graph, we plot as in a similar pattern but while doing experiments we have to do from LL to SL as the moisture content is more in the liquid stage.

Liquid limit

The limit where soil changes from a plastic state to a liquid state with an increase in moisture content is known as the liquid limit.

At this state moisture content is more, So gradually soil loses it’s shear strength and becomes zero.

Liquid limit is determined with procedure given by American Society for Testing and Materials (ASTM) ASTM D4318

Casagrande’s Method and cone penetration method is used to find the liquid limit of soil. But due to simple procedures and easy judgment, we prefer Casagrande’s method.

Determination of Liquid limit by Casagrande’s method

In this method, Casagrande’s liquid limit device is used and given blows for finding liquid limit.

Apparatus

  • casagrande’s liquid limit device
  • Grooving tools
  • Evaporating dish or glass plate
  • Spatula
  • 425 micron IS sieve
  • Weighing balance
atterberg limits of soil liquid limit
figure-2

Test procedure

  1. Take a soil sample about 120g air-dried and passing through IS sieve 425 microns.
  2. Now mix water thoroughly till soil sample becomes a bit slurry.
  3. After sometime Place the mix in a cup of a device with the help of the spatula. Remove extra layer above the cup and maintain minimum depth of 1cm from the bottom.
  4. Now with a grooving tool make a vertical symmetrical groove along the axis.
  5. With the help of the handle give required number of blows to the cup so that the bottom layers separated by groove will come in contact.
  6. Also, separate some soil sample from the plate for knowing moisture content.
  7. Note the number of blows and conduct this experiment for at least 3-10 times with a decrease in moisture content by evaporating the moisture in the soil sample.
  8. Draw a graph between log N on the x-axis and water content on the y-axis.
  9. For 25 blows on x-axis find water content which is known as Liquid limit of soil sample

Test data sheet

Sl. No.Observations and Calculations Test sample
1
Test sample
2
3..
Observation
1Number of blows (N)
2Water content can No.
3Mass of empty can (M1)
4Mass of can + wet soil (M2)
5Mass of can + dry soil (M3)
Calculations
6Mass of water = M2 – M3
7Mass of dry soil sample = M3 – M1
8Water content of soil sample
(w) = [(6) / (7)] x 100

Graph

An example of plotting a graph

atterberg limits of soil liquid limit graph

Result

From the graph between log N and water content. plot the water content or 25 blows and know as the liquid limit of soil sample

Significance of liquid limit

  • The value of the liquid limit is used to classify fine-grained soils
  • It gives consistency and engineer properties of soil.
  • At liquid limit Shear strength for most of the cohesive soils is 2.7 kN/m^2 and becomes zero as moisture content increases.
  • The liquid limit is used to find derived atterberg limits of soil.
  • Derived limits are the Plasticity Index (PI = LL-PL), Liquidity index {LI=(W-PL)/(LL-PL)}, Consistency index {CI = (LL-W)/(LL-PL)}.

Plastic limit

In the atterberg limits of soil, the determination of the plastic limit is easy and important.

It is the barrier between the semi-solid state and plastic state so it is known as the plastic limit. From this state, the properties of soil change to plastic, and there will be huge differences in the engineering properties of soil.

Plastic limit is determined with procedure given by American Society for Testing and Materials (ASTM) ASTM D4148

Determination of plastic limit for soil sample

For this there is no need to conduct experiment from the beginning we can use the same sample after conducting the liquid limit test.

plastic limit apparatus

Apparatus

  • Glass plate
  • soil sample from LL
  • Bins
  • 3.2 mm small rod.

Test Procedure

  1. Take 20-30 g soil sample from the liquid limit test
  2. Leave the soil sample to open-air till the moisture content evaporates so that it doesn’t stick to the hand while rolling.
  3. Now take 1-2 g from the soil sample and roll it with the help of hand till it reaches 3.2 mm diameter. ( keep the rod aside and check dia )
  4. Small pieces break from the rolled sample and keep them aside and again make a mold then roll the same sample to 3.2 mm and again separate.
  5. keep the separated pieces in the bin and find water content.
  6. Repeat the experiment with 3-4 samples and the average of all moisture contents give plastic limit.
plastic limit hand rolling

Test data sheet

Sl. No.Observations and Calculations Test sample
1
Test sample
2
3..
Observation
2Water content can No.
3Mass of empty can (M1)
4Mass of can + wet soil (M2)
5Mass of can + dry soil (M3)
Calculations
6Mass of water = M2 – M3
7Mass of dry soil sample = M3 – M1
8Water content of soil sample
(w) = [(6) / (7)] x 100

Result

From all the moisture contents obtained from the test samples, the average gives the plastic limit of the soil.

Full PDF file of test procedure of plastic limit – download

Significance of plastic limit

  • It is used to find the consistency of the soil
  • Also used to find the Consolidation properties of soil while calculating the bearing capacity and settlements of the soil.
  • This is the limit where the soil is in both semi-solid and plastic state.
  • With an increase in moisture, compressibility Increases results in a decrease in the strength of the soil and vice-versa.
  • The plastic limit is used to find derived atterberg limits of soil.
  • Derived limits are the Plasticity Index (PI = LL-PL), Liquidity index {LI=(W-PL)/(LL-PL)}, Consistency index {CI = (LL-W)/(LL-PL)}.

shrinkage limit

Shrinkage limit is the phase where there is no further reduction in volume with a decrease in water content. with an increase in water content volume increases and solid becomes a semi-solid state.

In this stage all the pores of the soil is just filled by moisture content.

The procedure or test to determine the shrinkage limit is ASTM D4943.

For detailed test procedure visit this site : Click here

Conclusion

From this article, we learned about atterberg limits of soil ( shrinkage limit, plastic limit, liquid limit.)

Definitions, test procedures and significance.

For more information about the Importance of atterberg limits of soil visit: click here

with the results of liquid limit,plastic limit, shrinkage limit we can find all the derived limits, so that the engineering properties of soil are found easily.

Question for you:

what is the name of scientist who discovered these limits ? Comment below

what are Atterberg limits of soil?

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