PLASTIC LIMIT TEST
LEARNING OUTCOMES
At the end of this experiment, students should be able to:
1.1

Conduct the plastic limit experiment.

Identify the plastic limit value for soil.

Identify the importance and application of plastic limit test.
INTRODUCTION
The plastic limit is defined as the moisture content at which soil begins to behave
as a plastic material. A plastic material can be molded into a shape and the
material will retain that shape. If the moisture content is below the plastic limit, it
is considered to behave as a solid, or a non-plastic material.
1.2
OBJECTIVES
The objectives of this experiment are to determine the lowest moisture content
at which the soil behaves plastically.
1.3
THEORY
Plastic limit (P w) of soil is defined as the water content at which a soil will just
begin to crumble when rolled into a thread of approximately 3 mm in diameter. It
is water content at the boundary between the plastic and semi-solid states of
consistency of the soil. Plasticity index (PI or IP) is the numerical difference of the
liquid and plastic limit, and indicates the range of water content through which the
soil remains plastic.
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PI = LL – PL
For fine grained soils, determining the natural water content (the water content of
a soil in an undisturbed condition in the ground) and relating it to the plastic and
liquid limits can provide an indication of the soil’s consistency and/or sensitivity
potential. One such relationship is the liquidity index.
IL 
w  PL
PI
Consistency index is defined as the ratio of the liquid limit minus the natural water
content to the plasticity index of a soil.
Cr 
LL  w
PI
Table 1.1: Classification of soil according to plasticity.
Plasticity Index (IP
or PI)
0
<7
7-17
> 17
Degree of Plasticity
Type of Soil
Non-Plastic
Low-Plastic
Medium Plastic
Highly
Sand
Silt
Silty clay or clayey silt
Plastic Clay
Table 1.2: Classification of soil according to liquidity indices.
Liquidity Index (IL or LI)
< 0.0
0.0-0.25
0.25-0.50
0.50-0.75
0.75-1.00
>1.00
Consistency
Desiccated (dry) hard soil
Stiff
Medium to soft
Soft
Very soft
Liquid s
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1.4
1.
APPARATUS
The most important piece of apparatus for this test is the hand of operator,
which should be clean and free from grease.
2.
Evaporating dish.
3.
A separate glass plate reserved for rolling of threads. This should be smooth
and free from scratches, and about 300 mm square and 10mm thick. The
surface condition of the plate can affect the behaviour of rolled threads, and
the use of unscratched glass reduces the likelihood of discrepancies. An
alternative is to reserve one side of the mixing plate for thread rolling, and
avoid mixing the soil on this area.
4.
Two palette knives or spatulas.
5.
A short length (say 100 mm) of 3mm diameter metal rod.
6.
Standard moisture content apparatus (container, balance and oven)
Figure 1.1: Apparatus for plastic limit test
1.5
1.
PROCEDURES
Selection and preparation of sample
(a) Take about 20 g of the prepared soil paste and spread it on the glass
mixing plate.
(b) Mix occasionally to avoid local drying out.
(c) It is convenient to set aside this sample just before carrying out the liquid
limit test.
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Figure 1.2: Prepared soil paste.
2.
Rolling into a Ball
(a) When the soil is plastic enough, it is well kneaded and then shaped into
a ball.
(b) When slight cracks begin to appear on the surface, divide the ball into
two portions each of about 10 g.
(c) Further divide each into four equal parts, but keep each set of four parts
together.
Figure 1.3: Rolling into ball and four equal part of soil after divide.
3.
Rolling into a Threads
(a) Using a steady pressure, roll this mass into a thread between the fingers
of one hand and the surface of the glass plate.
(b) The pressure should reduce the diameter of the thread from 6 mm to
about 3 mm after between five and ten back-and-forth movements of the
hand.
(c) It is important to maintain a uniform rolling pressure throughout; do not
reduce pressure as the thread approaches 3 mm diameter.
(d) Dry the soil further by moulding between the fingers again, not by
continued rolling which gives a dried crust.
(e) Form it into a thread and roll out again as before.
(f) Repeat this procedure until the thread crumbles when it has been rolled
to 3 mm diameter.
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(g) The metal rod serves as a reference for gauging this diameter.
(h) The first crumbling point is the plastic limit
Figure 1.4: The soil specimen is rolled under the fingers into a thread 3 mm
diameter.
4.
Moisture content measurement
This is placed in a numbered moisture content container, which is weighed,
oven dried and weighed as in the standard moisture content procedure
according to BS 1377: Part 2: 1990.
(a) Weight the empty moisture content container
(b) Take a moisture content sample of about 10 g from the area penetrated
by the cone, using the tip of a small spatula.
(c) Weight the wet sample
(d) Dry in the oven for about 24 hours (overnight)
(e) Weight the dry sample
(a)
(b)
(c)
(e)
(d)
Figure 1.5: Process to take moisture content.
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1.6
CALCULATION EXAMPLE
1.
Results Calculation and plotting
(a) The moisture content of the soil from each penetration reading is
calculated from the wet and dry weighing as in the moisture content test.
(b) The plastic limit and plasticity index are usually reported with the liquid
limit.
Table 1: Example of Data Sheet.
Location : RECESS
Loc. No. : 00123
Soil description: Silty CLAY
Sample No. :
Sample type : Undisturbed
Depth of Sample : 1.5 m
Operator: A . Z. S
Date Started :
Test Number
Can Number
Mass of can + moist soil
(Mcws)
Mass of can + dry soil (Mcs)
Mass of can (Mc)
Mass of dry soil (Ms)
Mass of water (Mw)
Water content
Plastic limit
Unit
s
gram
gram
gram
gram
gram
%
%
6
1
2
3
4
11
46.76
21
57.20
32
63.60
41
71.72
32.51
8.31
24.20
14.25
58.88
38.31
8.35
29.96
18.89
63.05
41.64
8.26
33.38
21.96
65.79
45.78
8.29
37.49
25.94
69.19
64.27
Calculation of moisture content;
w
1.7
1.
mw 14.25

 0.5888or58.88%
ms 24.20
REFERENCES
Das, Braja M., 2014. “Principles of geotechnical engineering”: Cengage
Learning. (TA710 .D37 2014)
2.
Morris, A., 2012. “Geotechnical engineering of soil”: Auris Reference.
(TA705 .G49 2012)
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3.
Braja, M.D., 2010. “Principle of Geotechnical Engineering”: McGrawHill.
(TA710.D37 2010)
4.
Cheng Liu & Jack B. Evett, 2008. “Soils and Foundations”: Prentice Hall.
(TA710.L58 2008)
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1.8
ADDITIONAL THEORY
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(10 marks)
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1.9
DATA
Data Sheet
Location :
Loc. No. :
Soil description:
Sample No. :
Sample type :
Depth of Sample :
Operator:
Date Started :
Test Number
Unit
s
Can Number
Mass of can + moist soil
(Mcws)
Mass of can + dry soil (Mcs)
Mass of can (Mc)
Mass of dry soil (Ms)
Mass of water (Mw)
Water content
Plastic limit
1
2
3
m
4
gram
gram
gram
gram
gram
%
%
Plastic limit
=
%
Liquid limit
=
%
Plasticity index
= Liquid Limit – Plastic Limit =
%
(25 marks)
APPROVED BY
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1.10
ANALYSIS & RESULT
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(25 marks)
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1.11
1.
QUESTION & DISCUSSION
What is the definition of plastic limit?
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2.
A sample of wet clay and its container weigh 102 g. After oven drying the
sample and the container weigh 60 g. What is the water content and plastic
limit for the soil?
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3.
What is the different between liquid limit and plastic limit?
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4.
A sample of wet clay weighs 176 g. After oven drying the sample weigh 60
g and its container is 7.32 g. What is the water content and plastic limit for
the soil?
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5.
Discussion
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(15 marks)
1.12
CONCLUSION & RECOMMENDATION
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1.13 REFERENCES
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(5 marks)
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