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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 ofa 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-grainedsoil 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 x15cm
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. Thisperiod 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 itsplastic limit. Knead the soil to reduce the water content androll it into a thread again.
iii) Repeat the process of alternate rolling and kneading until the thread crumbles.
iv) Collect and keep the piecesof crumbled soil thread in the container used to determine the moisture content.
v) Repeat the process at leasttwice more with fresh samples of plastic soil each time.
The plastic limit should be determined for at least three portions of the soil passing through 425μm IS Sieve. The average water content to thenearest whole number should be reported.
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