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In plastic limit test the soil ball is rolled on a glass plate into a thread of ....... mm diameter.
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The plastic limit of a soil is defined as the moisture content at which the soil begins to crumble when rolled into a thread of exactly 3 mm in diameter. This test is a fundamental procedure in geotechnical engineering used to classify fine-grained soils by determining the transition state between plastic and semi-solid consistency.
The plastic limit of a soil is defined as the moisture content at which the soil begins to crumble when rolled into a thread of exactly 3 mm in diameter. This test is a fundamental procedure in geotechnical engineering used to classify fine-grained soils by determining the transition state between plastic and semi-solid consistency.
PL=(MsMw)×100 — Calculation of Plastic Limit where Mw is mass of water and Ms is mass of oven-dried soil at the crumble point.
As the soil thread is rolled on a glass plate, it loses moisture due to evaporation and absorption by the plate. At 3 mm diameter, the surface area exposed to evaporation relative to the soil volume reaches a critical threshold where the cohesive forces are no longer sufficient to maintain the plastic shape, leading to cracking or crumbling.
Test conducted according to IS: 2720 (Part 5).
Soil must be air-dried or oven-dried to pass through 425-micron sieve before testing.
If the soil crumbles at a diameter greater than 3 mm, the moisture content is below the plastic limit.
The test is repeated multiple times to ensure consistency of the moisture content determination.
Simple and inexpensive equipment required.
Essential for calculating the Plasticity Index (PI=LL−PL).
Operator dependent (subjective judgment of crumbling).
Slow process due to the requirement of constant rolling.
Classification of fine-grained soils.
Estimation of swelling potential for expansive soils (e.g., Black Cotton Soil).
Standard: IS 2720 Part 5.
The plastic limit is the lower boundary of the plastic state and the upper boundary of the semi-solid state in Atterberg Limits.
B is correct — The soil is rolled into a thread of 3 mm diameter to determine the threshold moisture content where plastic behavior ceases.
Always remember that the Liquid Limit (LL) test uses a Casagrande apparatus, whereas the Plastic Limit (PL) test is a manual rolling process. The difference LL−PL defines the Plasticity Index (IP), which indicates the degree of plasticity.