The determination of plasticity index and the percentage by weight of clay mineral finer than 2 μ present in the soil were done for two soil samples A and B. The ratio of plasticity index to the percentage of clay size particle for the samples A and B are given in the table. The two soil samples A and B are classified as: *(Table: Sample A ratio is Less than 0.75; Sample B ratio is Greater than 1.25)*
Try it first: pick an option, then read the explanation.
Correct answer: A. Sample A: Inactive; Sample B: Active
Detailed Explanation
This question gives the activity ratio for two samples directly (rather than the raw plasticity index and clay percentage) and asks you to classify each one using the standard activity ranges.
| Activity Value | Classification |
|---|---|
| Ac < 0.75 | Inactive |
| 0.75 ≤ Ac ≤ 1.25 | Normal |
| Ac > 1.25 | Active |
Why "Sample A: Inactive; Sample B: Active" Is Correct
Sample A's ratio is given as less than 0.75, which falls in the Inactive range. Sample B's ratio is given as greater than 1.25, which falls in the Active range. Neither sample falls in the 0.75–1.25 "Normal" band.
Key Point
This question only requires reading the given ratios against the standard activity table — no new calculation is needed once you have the classification ranges memorized.
Key Concepts for Students
- This is the same activity classification table used in Q21 and Q22, just applied by reading given ratios rather than computing them.
- An inactive clay (low Ac) is generally less prone to swelling/shrinking problems than an active clay (high Ac) at the same clay content.
- Questions that supply a ratio/table directly (rather than raw limits) are testing whether you've memorized the classification boundaries, not your arithmetic.
- Always double check which sample corresponds to which given ratio before matching to the answer choices — swapping A and B is an easy mistake here.