1.0 OBJECTIVE

To Determine the Lowest Moisture Content at Which the Soil Behaves

Plastically.

2.0 LEARNING OUTCOME

At the end of this experiment, students are able to:

Conduct the plastic limit experiment.

Identify the plastic limit value for soil.

Identify the importance and application of plastic limit test.

3.0 INTRODUCTION

The following moisture conditions - liquid unlit, plastic limit, along with

shrinkage limit are referred to as the "Atterberg Limits", after the originator of the

test procedures. Semi-Solid .StagySolid State Atterbera Limits and Indices. The

plastic limit, also known as the lower plastic limit, is the water content at which a

soil changes from the plastic state to a semisolid state. A plastic limit set is an

apparatus that is used to determine the plastic limit of soil.

The change from a semisolid state to a plastic state of soil is a gradual

process. According to a standard empirical definition, the plastic limit is the water

content at which a soil passing a 0.425mm sieve can be rolled into threads one-

eighth inch in diameter without the soil breaking into pieces. Generally, the terms

plastic limit and liquid limit are used together for soil identification and

classification.

Definition

The method described herein is based upon A_ASHTO Designation T89

which has been modified for New York State Department of Transportation

use. The liquid limit of a soil is the moisture content, expressed as a percentage of

the weight of the oven-dried soil. at the boundary between the liquid and plastic

states of consistency. The moisture content at this boundary is arbitrarily defiled

as the water content at which two halves of a soil ca1Le will flow together, for a

distance of l,- in.(12.7 min) along the bottom of a groove of standard dimensions

separating the two halves, when the cup of a standard liquid limit apparatus is

dropped 25 times from height of 0.3937 in. (10 nun) at the rate of two

drops/second.

4.0 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.

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.

Consistency index is defined as the ratio of the liquid limit minus the natural

water content to the plasticity index of a soil.

Plasticity Index Degree of Plasticity Type of Soil

(IP or PI)

0 Non-Plastic Sand

< 7 Low-Plastic Silt

7-17 Medium Plastic Silty clay or clayey silt

> 17 Highly Plastic Clay

Classification of soil according to plasticity.

Liquidity Index (IL or LI) Consistency

< 0.0 Desiccated (dry) hard soil

0.0-0.25 Stiff

0.25-0.50 Medium to soft

0.50-0.75 Soft

0.75-1.00 Very soft

>1.00 Liquid s

Classification of soil according to liquidity indices.

4.0 TEST EQUIPMENTS

1. 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

behavior of rolled threads, and the use of unscratched glass reduces

the like lihood 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)

5.0 PROCEDURES

1) First of all a soil paste is prepared and a part of it is placed on the glass

mixing plate.

2) The soil is allowed to dry to such an extent that it becomes plastic and can

be shaped into a ball.

3) The ball is then moulded by pressing gently between fingers and palms so

that it dries up and cracks appear on the soil.

4) The soil is then divided into two sub-samples and separate determination

process is carried out for both the samples. Moisture is evenly spread on

both soil samples by pressing them between fingers. A thread is formed of

about 6mm diameter between the first finger and thumb of each hand.

5) The thread is rolled between the fingers, from the finger-tip to the second

joint, of one hand and the surface of the glass rolling plate.

6) The process is repeated until the thread shears both longitudinally and

transversely when it has been rolled to about 3mm diameter, as gauged by

a rod.

7) 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.

i. Weight the empty moisture content container

ii. Take a moisture content sample of about 10 g from the area

penetrated by the cone, using the tip of a small spatula.

iii. Weight the wet sample

iv. Dry in the oven for about 24 hours (overnight)

v. Weight the dry sample

(a) (b)

(c)

(d) (e)

Process to take a moisture content.

6.0 RESULTS / ANALYSIS   / CALCULATION

DATA SHEET

Test Number Units 1 2 3 4Can NumberMass of can + moist soil (Mcws) gram 12 12 11.5 11.8Mass of can + dry soil (Mcs) gram 11.4 11.6 11.0 11.2Mass of can (Mc) gram 9.0 9.9 9.3 9.0Mass of dry soil (Ms) gram 2.4 1.7 1.7 2.2Mass of water (Mw) gram 0.6 0.4 0.5 0.6Water content % 25 23.53 29.41 27.27Plastic limit % 26.30

Plastic limit = 26.30 %

Liquid limit = %

Plasticity index = Liquid Limit – Plastic Limit = %

a) To determine the value of mass of dry soil (M S)

Test Number Units 1 2 3 4Can Number 1 2 3 4Mass of can + dry soil (Mcs) gram 11.4 11.6 11.0 11.2

Mass of can (Mc) gram 9.0 9.9 9.3 9.0

Mass of dry soil (Ms) = [Mass of can + dry soil (MCS)] – [Mass of can (MC)]

i) Mass of dry soil (Ms) = 11.4 - 9.0 = 2.4 g

ii) Mass of dry soil (Ms) = 11.6 - 9.9 = 1.7 g

iii) Mass of dry soil (Ms) = 11.0 - 9.30 = 1.7 g

iv) Mass of dry soil (Ms) = 11.2 – 9.0 = 2.2 g

b) To determine the value of mass of water (M W)

Test Number Units 1 2 3 4Can Number 1 2 3 4Mass of can + moist soil (Mcws) gram 12 12 11.5 11.8

Mass of can + dry soil (Mcs) gram 11.4 11.6 11.0 11.2

Mass of water (MW) = [Mass of can + moist soil] – [Mass of can + dry soil ]

i) Mass of water (MW) = 12.0 - 11.40 = 0.6 g

ii) Mass of water (MW) = 12.0 – 11.60 = 0.4 g

iii) Mass of water (MW) = 11.5 – 11.00 = 0.5 g

iv) Mass of water (MW) = 11.8 – 11.20 = 0.6 g

c) To determine the value of water content, %

Test Number Units 1 2 3 4Can Number 1 2 3 4Mass of dry soil (Ms) gram 2.4 1.7 1.7 2.2

Mass of water (Mw) gram 0.6 0.4 0.5 0.6

Water content, w = x 100%

i) Water content, w = 0.6 X 100 = 25%

2.4

ii) Water content, w = 0.4 X 100 = 23.53% 1.7

iii) Water content, w = 0.5 X 100 = 29.41% 1.7

iv) Water content, w = 0.6 X 100 = 27.27% 2.2

d) To determine the value of plastic limit and plasticity index

Plastic limit = average of the water content

PL = 25 + 23.53 + 29.41 + 27.27

4

= 26.30%

Plasticity index = Liquid Limit – Plastic Limit

= 58.10% – 26.30 %

= 23.35%

7.0 DISCUSSION

8.0 CONCLUSION

9.0 QUESTIONS :

QUESTIONS 1

(a) What is the definition of plastic limit?

The plastic limits is arbitrarily defined as the water content at which a silt

or clay will just begin to crumble when rolled into a thread

approximately 3.2 mm in diameter (ASTM D 4318-98, 2000).

Both of these definitions are based on empirical concepts.

(b) 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?

Wet clay soil + container = 102 g

Dry soil + container = 60 g

Container = 6 g

So, dry soil, M2 = 60 – 6 = 54 g

And wet soil, M1 = 102 – 6 – 54 = 96 g

Therefore, plastic limit for the soil is 77.78%

QUESTIONS 2

(a) What is the different between liquid limit and plastic limit?

Different between liquid limit and plastic limit is behavior change between

the liquid and plastic states and between the plastic and semisolid states of

a silt or clay. Liquid limit is arbitrarily defined as the water content at

which a part of soil, cut by a groove of standard dimensions and

will flow together for a distance of 12.7mm under the impact of 20

blows in a standard liquid limit device. Plastic limit is arbitrarily

defined as the water content at which a silt or clay will just begin to

crumble when rolled into a thread approximately 3.2mm in diameter.

(b) 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?

Wet clay soil + container = 176 g

Dry soil + container = 60 g

Container = 7.32 g

So, dry soil, M2 = 60 – 7.32 = 52.68 g

And wet soil, M1 = 176 – 7.32 – 52.68 = 116 g

Therefore, plastic limit for the soil is 120.2