Plastic limit is the water content below which the soil stops behaving as a plastic material.The plastic limit is determined by rolling a part of a soil into thread,when the thread begins to crumble at a diameter of 3.18mm or 1/8", the water content at this stage is the plastic limit.

To determine the plastic limit of soil as per IS: 2720 (Part 5)- 1985.

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.

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

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.

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.

Plastic Limit Test

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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