Question

The moisture content at which soil ceases to be plastic, becomes semi-fluid and tends to flow like a liquid:
(A). Plastic limit
(B). Plasticity number
(C). Liquid limit
(D). Shrinkage limit
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (C) only.
• C. (A), (B), (C) and (D).
• D. (B), (C) and (D) only.

Hint:

Remember the soil state transitions with increasing water content:
Solid \( \xrightarrow{\text{Shrinkage Limit}} \) Semi-solid \( \xrightarrow{\text{Plastic Limit}} \) Plastic \( \xrightarrow{\text{Liquid Limit}} \) Liquid.
The transition to flowing like a fluid is always the Liquid Limit.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Soil consistency transitions through distinct states—solid, semi-solid, plastic, and liquid—as moisture content increases.
Albert Atterberg established specific boundary moisture thresholds, known as Atterberg Limits, to define these transitions.

Step 2: Detailed Explanation:
Let us analyze the Atterberg limits:
- Shrinkage Limit (D): The moisture content below which any further loss of water does not cause a decrease in soil volume.
- Plastic Limit (A): The moisture content at which soil transitions from a semi-solid to a plastic state, defined as the minimum moisture at which soil can be rolled into threads of \( 3 \, \text{mm} \) diameter without crumbling.
- Liquid Limit (C): The moisture content at which the soil transitions from a plastic state to a liquid state.
At this moisture level, the soil loses its shear strength, ceases to behave plastically, becomes semi-fluid, and begins to flow like a liquid under a small applied force (measured using Casagrande's apparatus).
- Plasticity Index/Number (B): The numerical difference between the liquid limit and the plastic limit, representing the range of moisture over which soil exhibits plastic behavior:
\[ \text{Plasticity Index } (I_p) = \text{Liquid Limit } (w_L) - \text{Plastic Limit } (w_P) \]
Therefore, the specific moisture threshold described is the Liquid Limit.

Step 3: Final Answer:
The described moisture content is (C) only (Liquid limit).