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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