Plastic limit test:-
This test is done to determine the plastic limit of soil as per
IS: 2720 (Part 5) – 1985.The plastic limit of fine-grainedsoil is the water
content of the soil below which it ceases to be plastic. It begins to crumble
when rolled into threadsof 3mm dia.
The apparatus used:
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 thesoil with distilled water in an
evaporating dish and leave the soil mass for 24hrs.Procedure to determine The
Plastic Limit of Soil.
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 90strokes 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 watercontent 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 result
The plastic limit should be
determined for at least three portions of the soil passing through 425µm IS
Sieve. Theaverage water content to the nearest whole number should be reported.
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