Plastic Limit

Published: August 10, 2020 | Last updated: July 5, 2023

What Does Plastic Limit Mean?

Plastic limit (PL) is the moisture content at which a fine-grained soil cannot be remolded without cracking. The plastic limit test requires repeated rolling of a soil sample into a thread until it reaches a point where it crumbles.

Plastic limit (PL) along with shrinkage limit (SL) and liquid limit (LL) comprise the Atterberg limits.

The PL is expressed as a percentage of the weight of the oven-dry soil, at the boundary of plastic and semi-solid consistency. It is the moisture content at which the rolled soil will begin to crumble when rolled into a thread 3mm or 1/8th inch in diameter.

Trenchlesspedia Explains Plastic Limit

The plastic limit test determines the plastic limit (PL) of a soil sample. The apparatuses required for the test include:

  • Evaporation dish or flat glass plate.
  • Spatula.
  • Ground-glass plate for rolling.
  • Balance of 0.01-gram sensitivity, rod – 3 mm in diameter, and 100 mm long.

For the test procedure, the soil is taken in an evaporating dish or glass plate and mixed with distilled water so that the soil becomes plastic and easily moldable with fingers.

A part of this wet soil is taken, made into a ball, and rolled on the glass plate with the palm to make a thread of uniform diameter of 3 mm.

The soil is then kneaded again and formed into a uniform mass and rolled again. This process is repeated until the thread begins to crumble at 3 mm in diameter.

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