Question

The minimum water content at which the soil just begins to crumble when rolled into thread of 3 mm in diameter is known as:

• A. Liquid limit
• B. Plastic limit
• C. Shrinkage limit
• D. Permeability limit

Hint:

The key values to remember for the plastic limit test are:
- Thread diameter: \( 3 \text{ mm} \).
- Characteristic behavior: The thread just begins to crumble.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In soil mechanics, the Atterberg limits define the boundaries of the solid, semi-solid, plastic, and liquid states of fine-grained soils as a function of their water content.
These limits are used to classify cohesive soils and predict their engineering behavior.

Step 2: Detailed Explanation:
The plastic limit (\( w_p \)) is the boundary between the semi-solid and plastic states of a soil.
It is determined in the laboratory using a standardized test:
1. A small sample of soil is mixed with water and rolled by hand on a glass plate to form a thin thread.
2. The water content of the soil decreases as it is rolled and handled.
3. The plastic limit is defined as the lowest water content at which the soil can be rolled into a thread of \( 3 \text{ mm} \) (approximately \( 1/8 \text{ inch} \)) in diameter without crumbling.
4. If the soil crumbles before reaching a diameter of \( 3 \text{ mm} \), its water content is below the plastic limit.
Let us review the other options:
- Liquid limit (\( w_l \)): This is the water content that separates the plastic state from the liquid state, determined using the Casagrande cup or cone penetrometer.
- Shrinkage limit (\( w_s \)): This is the maximum water content at which further loss of moisture does not cause a decrease in the volume of the soil mass.
Therefore, the description provided matches the definition of the plastic limit.

Step 3: Final Answer:
The correct option is (B) Plastic limit.