PLASTIC LIMIT of Soil

Updated: May 1, 2020

AIM

To determine the plastic limit of soil as per IS: 2720 (Part 5)

- 1985.

PRINCIPLE

The plastic limit of fine-grained soil is the water content of the

soil below which it ceases to be plastic. It begins to crumble

when rolled into threads of 3mm dia.

APPARATUS

i) Porcelain evaporating dish about 120mm dia.

ii) Spatula

iii) Container to determine moisture content

iv) Balance, with an accuracy of 0.01g

v) Oven

vi) Ground glass plate - 20cm x 15cm

vii) Rod - 3mm dia. and about 10cm long

PREPARATION OF SAMPLE

Take out 30g of air-dried soil from a thoroughly mixed sample of

the soil passing through 425µm IS Sieve. Mix the soil with

distilled water in an evaporating dish and leave the soil mass for

naturing. This period may be upto 24hrs.

PROCEDURE

i) Take about 8g of the soil and roll it with fingers on a glass

plate. The rate of rolling should be between 80 to 90 strokes

per minute to form a 3mm dia.

ii) If the dia. of the threads can be reduced to less than 3mm,

without any cracks appearing, it means that the water

content is more than its plastic limit. Knead the soil to

reduce the water content and roll it into a thread again.

iii) Repeat the process of alternate rolling and kneading until the

thread crumbles.

iv) Collect and keep the pieces of crumbled soil thread in the

container used to determine the moisture content.

v) Repeat the process at least twice more with fresh samples

of plastic soil each time.

REPORTING OF RESULTS

The plastic limit should be determined for at least three portions

of the soil passing through 425µm IS Sieve. The average water

content to the nearest whole number should be reported.

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