liquid limit experiment

Plastic Limit and Liquid Limit test for soil

In this article, we explained lab test procedure of the plastic and liquid limit test. Theory behind the plastic limit and liquid limit test, What is the aim of PL nad LL test, Apparatus, Procedure of Liquid limit and plastic limit test, and so more.

Table of Contents

Aim of plastic limit and liquid limit test:

The object of the experiment is to determine the liquid limit and plastic limit of expansive nature soil.

Apparatus and Materials:

• Casagrande’s standard liquid limit device,
• Grooving tools
• Evaporating discs
• Balance accurate to 0.01 g.
• Crucibles to determine water content.
• Electric oven (thermostatically controlled),
• Wash bottle containing distilled water
• Glazed glass desiccators
• 425-micron sieve etc.

Theory of Plastic limit and liquid limit test:

The consistency is generally measured by Atterberg’s system which recognizes four stages,

A fine-grained soil such as clay will pass through these stages as the moisture content changes. Refer fig. below:

When sufficient water is added to a soil mass, the result is a mass that will deform freely without breaking. The soil has no shear strength.

Also Read: Causes of Foundation Failure

Liquid limit:

During the drying process, initially, the liquid state reaches a consistency at which the “Soil ceases to behave like a liquid and begins to exhibit the behavior of a plastic deformation but no friction under constant load).

At this stage, the shear strength begins to appear. The small shearing strength against flowing can be measured by a standard liquid limit device.

The water content at this stage is called Liquid Limit (L.L).

In other words, it is water content corresponding to the arbitrary limit between liquid and plastic state of consistency of soil.

Plastic limit:

As the drying process continues, the soil volume and the plasticity decreases.

The plastic state reaches a consistency at which the soil ceases to behave as plastic and begins to break apart is Plastic Limit (P.L) the water content corresponding to an arbitrary limit between the plastic and semi-plastic state of consistency of soil.

It is defined as the minimum water content at which a soil will begin to crumble when rolled into a thread approximately 3 mm diameter on a glazed plane glass plate.

Shrinkage limit:

Finally, the water content at which the soil ceases to shrink is denoted as the Shrinkage Limit (S.L.).

In other words, shrinkage limit is defined as the maximum water content at which a reduction in water content will not cause a decrease in the volume of the soil mass.

Also Read: Foundation in Black Cotton Soil

Procedure of Liquid Limit Test :

• Take about 30 to 40 gm of given soil passing through 425 microns IS sieve in evaporating disc.
• Add water to form a paste. Leave the soil for sufficient time so that water may permeate throughout the soil mass.
• Take a portion of the paste with the spatula and put it into the cup of a liquid limit device which is adjusted for 10 mm blows.
• Cut the vertical groove with the standard grooving tool. (Casagrande’s tool for clays and ASTM too l for sandy soils).
• Give blows by rotating crank of the standard device at the rate of 2 rotations per seconds and note the no, of blows for 12 mm closer of the groove at the bottom.
• Collect a representative slice of soil with the help of a spatula for water content. determination.
• Repeat the process changing the consistency for the different numbers of blows varying from 15 to 35 and find the Moisture content of corresponding samples collected.
• Plot the graph of Moisture content against no. of blows on semi-log paper having no. of blows on X-axis with log scale and water content on Y-axis with arithmetical scale.
• Find the liquid limit as the water content for 25 no. of blows from the graphs.

Also Read: Types of Pile Foundation

Precautions should be taken during liquid limit test:

• See that the closing of groove is by flowing of soil and not. by sliding
• Clean the cup every time for subsequent observation.

Procedure of Plastic Limit Test:

• Take about 8 gm of plastic soil, make a ball of it, and roll it on the glass plate with the palm of the hand with just sufficient pressure to roll the mass into a thread of uniform diameter throughout the length. Continue rolling until the thread starts crumbling. At the time of breaking or crumbling if the diameter is bigger than 3 mm, add slight water and knead it again to roll.
• Continue this until the thread just crumbles at 3 mm diameter.
• Collect the crumbled soil thread in the crucible and keep it for water content determination.
• Repeat the process for two or three determination to get the average value of P.L.

Practical Utility of Plastic limit and Liquid limit test:

The practical significance of the Liquid limit and Plastic limit lies in their ability to reflect the types and amounts of clay minerals present in the fine fraction.

The high value of Liquid Limit and plasticity index indicate that the soil has a high percentage of clay and colloidal sizes of more active type minerals, and such soil possesses a poor foundation or traffic load-bearing capacity.

Also, the results of the test are of great use for the classification of soils, design of footings, and predicting consolidation settlement.

References : IS-2720 (Part-5)-1985 (Method of test For soil)

1 thought on “Plastic Limit and Liquid Limit test for soil”

Why is liquid limit obtained from the graph and not by average of the moisture content. why is the formular for Plasticity Index =Liquid limit – Plastic limit

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Determine liquid limit of soil specimen by casagrande method.

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Equipment for Liquid Limit Test on Soil

• Casagrande’s liquid limit device
• Grooving tools of both standard and ASTM types
• Evaporating dish or glass sheet
• 425 micron IS sieve
• Weighing balance accuracy 0.01g.
• Wash bottle.

Fig: Casagrande’s Liquid Limit Apparatus

Fig: Casagrande’s Apparatus Details and Tools

Procedure of  Liquid Limit Test on Soil

Data sheet for liquid limit test,   result of  liquid limit test.

Fig: Liquid Limit Flow Curve

Gopal Mishra

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Liquid Limit Device ‑ a mechanical device consisting of a brass cup suspended from a carriage designed to control its drop onto a hard rubber base. The device may be operated by either a hand crank or electric motor.

Cup ‑ brass with mass (including cup hanger) of 185 to 215 g.

Cam ‑ designed to raise the cup smoothly and continuously to its maximum height, over a distance of at least 180o of cam rotation, without developing an upward or downward velocity of the cup when the cam follower leaves the cam.

Flat Grooving Tool ‑ a tool made of plastic or non‑corroding metal having specified dimensions.

Gage ‑ A metal gage block for adjusting the height of the drop of the cup to 10 mm.

Ground Glass Plate ‑ used for rolling plastic limit threads.

Significance and Use

This testing method is used as an integral part of several engineering classifications systems to characterize the fine‑grained fractions of soils and to specify the fine‑grained fraction of construction materials. The liquid limit, plastic limit and plasticity index of soils are also used extensively, either individually or together, with other soil properties to correlate with engineering behavior such as compressibility, permeability, compactibility, shrink‑swell and shear strength.

This test method covers the determination of the liquid limit, plastic limit and plasticity index of soils. The liquid and plastic limits of soils are often referred the as the Atterberg limits .

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Procedure for Liquid Limit Test

Place a portion of the prepared sample in the cup of the liquid limit device at the point where the cup rests on the base and spread it so that it is 10mm deep at its deepest point. Form a horizontal surface over the soil. Take care to eliminate air bubbles from the soil specimen. Keep the unused portion of the specimen in the storage container.

Form a groove in the soil by drawing the grooving tool, beveled edge forward, through the soil from the top of the cup to the bottom of the cup. When forming the groove, hold the tip of the grooving tool against the surface of the cup and keep the tool perpendicular to the surface of the cup.

Lift and drop the cup at a rate of 2 drops per second. Continue cranking until the two halves of the soil specimen meet each other at the bottom of the groove. The two halves must meet along a distance of 13mm (1/2 in).

Record the number of drops required to close the groove.

Remove a slice of soil and determine its water content, w.

Repeat steps 1 through 5 with a sample of soil at a slightly higher or lower water content. Whether water should be added or removed depends on the number of blows required to close the grove in the previous sample.

Note:    The liquid limit is the water content at which it will takes 25 blows to close the groove over a distance of 13 mm. Run at least five tests increasing the water content each time. As the water content increases it will take less blows to close the groove.

Observations & Calculations:

Liquid Limit, LL

Plot the relationship between the water content, w, and the corresponding number of drops, N, of the cup on a semi‑logarithmic graph with water content as the ordinates and arithmetical scale, and the number of drops on the abscissas on a logarithmic scale. Draw the best fit straight line through the five or more plotted points. Take the water content corresponding to the intersection of the line with the 25 drop abscissa as the liquid limit, LL, of the soil.

Precautions

• After performing each test the cup and grooving tool must be cleaned.
• The number of blows should be just enough to close the groove.
• The number of blows should be between 10 and 40.

Applications

• The value of liquid limit helps in classification of fine grain soil.
• The values of liquid limit are required to calculate flow index, toughness index etc.

Limitations of Liquid Limit Test

Procedure for determination of the plastic limit.

From the 20g sample select a 1.5 to 2 g specimen for testing.

Roll the test specimen between the palm or fingers on the ground glass plate to from a thread of uniform diameter.

Continue rolling the thread until it reaches a uniform diameter of 3.2mm or 1/8 in.

When the thread becomes a diameter of 1/8 in. reform it into a ball.

Knead the soil for a few minutes to reduce its water content slightly.

Repeat steps 2 to 5 until the thread crumbles when it reaches a uniform diameter of 1/8 in.

When the soil reaches the point where it will crumble, and when the thread is a uniform diameter of 1/8", it is at its plastic limit. Determine the water content of the soil.

Note :  Repeat this procedure three times to compute an average plastic limit for the sample.

Calculations

Plastic Limit, PL

Compute the average of the water contents obtained from the three plastic limit tests. The plastic limit, PL, is the average of the three water contents.

Plasticity Index

Calculate the plasticity index as follows: PI = LL ‑ PL where:

LL = liquid limit, and PL = plastic limit.

• The apparatus required for the experiment should be clean.
• All the readings should be noted carefully.
• Practical applications
• The value of liquid limit and plastic limit are used to classify fine grained soil.
• The values of liquid limit and plastic limits are used to calculate flow index, toughness index and plasticity index of the soil.

Observations and calculations:

Average plastic limit = (1.13 + 1.7)/2 =1.15%

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Liquid State

Plastic State

Semi Solid State and

Solid State

And consistency limits are:

Liquid Limit

Plastic Limit and

Shrinkage Limit

We learned liquid limit as the water content of the soil at which it changes its state from being liquid to plastic and stops being liquid. And also stops flowing like a liquid.

At that point shearing strength of the soil is the smallest value that can be measured in the laboratory.

The liquid limit of the soil depends upon the clay mineral present in it. Some clay minerals have larger surface area and clay particles have negatively charged surface. So greater the surface area, stronger the surface charge and greater will be the amount of absorbed water, therefore higher will be the liquid limit.

We can determine liquid limit by two laboratory methods.

1. Casagrande Method

2. Cone penetration Method

Here we will discuss casagrande’s method.

Brass cup when rotated by its handle lifts to the height of 1cm and drops freely on rubber base.

Then we take about 120 gm of this sieved soil in a dish and mix it with distilled water to form a uniform soil paste.

A portion of this soil paste is placed in the cup of Casagrande apparatus and the surface is smoothened and leveled.

Then a sharp groove is cut symmetrically through the sample using a standard grooving tool.

No, we do not use our finger as the standard tool.

But there are two types of standard grooving tools available.

One is Casagrande tool

and other is ASTM tool

This is Casagrande Tool. And this is how the soil profile will look like when the groove is cut.

This tool cuts the groove of width 2 mm at the bottom and 11 mm at the top and 8 mm deep.

ASTM tool cuts the groove of width 2 mm at the bottom, 13.6 mm at the top and 10 mm deep.

The Casagrande tool is recommended for normal fine grained soils.

After the soil pat has been cut the handle of the apparatus is turned at a rate of 2 revolutions per second, which applies blows to the grooved soil pat. And due to which two halves of the soil pat comes into contact. And when the contact length of 12 mm is achieved we stop turning the handle.

We should take care that the groove should close by flow of soil not by slippage of it. And if slippage occurs then we should discard that soil pat and re-do the whole procedure.

This contact distance of two soil halves is measured with a ruler.

About 10 gm of soil near the closed groove is taken for the water content determination. Water content is determined using any of the water content determination methods. Determined water content is noted as W1.

The liquid limit has been defined as the water content of the soil at which the groove closes to 12 mm in 25 number of blows.

That means our observation of water content w1 could be the liquid limit of our soil sample if number of blows given to soil sample N1 was 25.

But practically, it is difficult to get exactly 25 blows for the 12 mm groove closure, so we conduct the test for different water contents so as to get the number of blows in the range of 10 to 40.

Hence the experiment is repeated three more times after adding more water to the soil and each time recording the number of blows to close the groove for a distance of 12 mm and determining the corresponding water content.

Soil with low water content would yield more number of blows as the soil paste will be stiffer and soil with higher water content would yield less number of blows.

We plot a graph between these experimentally obtained values of number of blows and their corresponding water content. Number of blows are plotted on log scale.

The water content corresponding to 25 blows is read from the graph (flow curve) and is taken as the liquid limit.

The liquid limit is expressed as the nearest whole number.

Now let's look at the flow curve of two different soils. If we draw a line from 25 numbers of blows, we can see, Soil 1 would reach at liquid like state at water content LL1. And soil 2 would attain the same state at water content LL2. Clearly LL1 is higher than LL2.

Soil 2 has liquid limit value less than that of soil 1. Soil 2 will become and behave like a liquid at lower water content compared to soil 1.

In other words we can also say that if we keep on adding water to both the soils, soil 2 loses its shear strength and becomes liquid like earlier at lower water content than soil 1.

Liquid limit of soil can also be determined by another method called Cone penetration method.

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Liquid limit (for N=25) =

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5 Atterberg Limit Test

Introduction.

The Atterberg limit refers to the liquid limit and plastic limit of soil. These two limits are used internationally for soil identification, classification, and strength correlations. When clay minerals are present in fine-grained soil, the soil can be remolded in the presence of some moisture without crumbling. This cohesiveness is caused by the adsorbed water surrounding the clay particles. At a very low moisture content, soil behaves more like a solid; at a very high moisture content, the soil and water may flow like a liquid. Hence on an arbitrary basis, depending on the moisture content, the behavior of soil can be divided into the four basic states shown in Figure 5-1: solid, semisolid, plastic, and liquid.

Practical Application

• This test method is used as an integral part of several engineering classification systems (USCS, AASHTO, etc.) to characterize the fine-grained fractions of soils and to specify the fine-grained fraction of construction materials.
• The liquid limit, plastic limit, and plasticity index of soils are also used extensively, either individually or with other soil properties to correlate with engineering behavior such as compressibility, hydraulic conductivity (permeability), shrink-swell, and shear strength.
• This method is sometimes used to evaluate the weathering characteristics of clay-shale materials. When subjected to repeated wetting and drying cycles, the liquid limits of these materials tend to increase. The amount of increase is considered to be a measure of the shale’s susceptibility to weathering.
• To determine the liquid limit (LL), plastic limit (PL), and the plasticity index (PI) of fine-grained cohesive soils.
• Casagrande’s liquid limit device
• Grooving tool
• Mixing dishes
• Texas Department of Transportation’s (TxDOT’s) recommended plastic limit rolling device

Standard Reference

• ASTM D4318: Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils
• TEX-105-E: Determining plastic limit of soils

Liquid Limit Test

• Determine the mass of each of the three moisture cans (W 1 ).
• Calibrate the drop of the cup, using the  end of the grooving tool not meant for cutting, so that there is consistency in the height of the drop.

• If N= 15 to 40, collect the sample from the closed part of the cup using a spatula and determine the water content weighing the can + moist soil (W 2 ). If the soil is too dry, N will be higher and will reduce as water is added.
• Do not add soil to the sample to make it dry. Instead, expose the mix to a fan or dry it by continuously mixing it with the spatula.
• Perform a minimum of three trials with values of N-15 to 40, cleaning the cap after each trial.
• Determine the corresponding w% after 24 hours (W 3 ) and plot the N vs w%, which is called the “flow curve”.

Plastic Limit Test

• Mix approximately 20 g of dry soil with water from the plastic squeeze bottle.
• Determine the weight of the empty moisture can, (W 1 ).

• If the soil crumbles forming a thread approximately the size of the opening between the plates (around 3 mm diameter), collect the crumbled sample, and weigh it in the moisture can (W 2 ) to determine the water content. Otherwise, repeat the test with the same soil, but dry it by rolling it between your palms.
• Determine the weight of the dry soil + moisture can, (W 3 ).
• The water content obtained is the plastic limit.

Shrinkage Limit Test

•  A reduction in the amount of moisture past the plastic limit does not decrease the volume of the soil.
• The sample changes from semi-solid to solid state at the shrinkage limit (boundary water content). Beyond this point the sample begins to dry up.
• The figure below depicts the phenomena of volume change.
• Plot point A, using the values of LL and PI determined experimentally, and extend it to meet O.

Video Materials

Lecture video.

Demonstration Video

Results and Discussions

Sample data sheet.

Sample Calculation

Blank Data Sheet

Determination of Shrinkage Limit

• Objective of the test
• Applications of the test
• Apparatus used
• Test procedures (optional)
• Analysis of test results – Complete the table provided and show one sample calculation.
• Calculate the value for liquid limit, flow index, plastic limit, plasticity index and shrinkage limit.
• Summary and conclusions – Comment on the Atterberg limit values of the given soil sample.

Properties and Behavior of Soil - Online Lab Manual Copyright © 2021 by MD Sahadat Hossain, Ph.D., P.E.; Md Azijul Islam; Faria Fahim Badhon; and Tanvir Imtiaz is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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