Which of the following statements is/are true for plastic limit? i. Plastic limit is the water content above which the soil stops behaving as a plastic material ii. At plastic limit, soil begins to crumble when rolled into a thread of a soil of 3 mm diameter
- AOnly (i)
- BOnly (ii)Correct
- CBoth (i) and (ii)
- DNeither (i) nor (ii)
Solution
Concept
Definition and properties of plastic limit
Formula
NONE
Step-by-step solution
1. Atterberg limits describe the boundaries between different physical states of fine-grained soil. 2. The plastic limit is defined as the water content at which a soil thread of 3 mm diameter starts to crumble. 3. Therefore, statement ii is correct. Answer: B
Explanation
The plastic limit is defined as the lowest water content at which soil can be rolled into threads of 3 mm diameter without crumbling. Below this water content, the soil behaves as a semi-solid rather than a plastic material, meaning statement i incorrectly describes the boundary behavior. Therefore, only statement ii is correct, making option B the right answer.
Why the other options are wrong
A) Statement (i) describes the liquid limit rather than the plastic limit. C) Statement (i) is false, making the combination of both statements incorrect. D) Statement (ii) is true because soil crumbles at 3 mm thread diameter at the plastic limit.
Shortcut
Plastic limit = moisture content at which soil crumbles when rolled into a 3 mm thread (lower bound of plastic state).
Common mistake
Selecting option C by confusing statement (i) with the definition of liquid limit rather than plastic limit.
Correct answer: B) Only (ii)