The liquid limit and plastic limit of clay materials is 45% and 25% respectively, what is the activity of clay materials if 20% by weight of particles of size less than 2 microns?
Try it first: pick an option, then read the explanation.
Correct answer: D. 1
Detailed Explanation
Given: liquid limit WL = 45%, plastic limit WP = 25%, particles finer than 2 microns = 20% by weight.
Step 1 — Plasticity index: Ip = WL − WP = 45 − 25 = 20%
Step 2 — Activity: Ac = Ip / (% clay-sized particles) = 20/20 = 1
Why "1" Is Correct
Dividing the plasticity index (20%) by the percentage of clay-sized particles (20%) gives exactly 1, placing this soil right in the middle of the "Normal" activity range (0.75–1.25).
| Activity Value | Classification |
|---|---|
| Ac < 0.75 | Inactive |
| 0.75 ≤ Ac ≤ 1.25 | Normal |
| Ac > 1.25 | Active |
Key Point
Always compute Ip first from the given limits before dividing by the clay percentage — skipping straight to a ratio of the raw limits is a common setup error.
Key Concepts for Students
- This is a direct two-step application of Ac = Ip/(% clay) — first get Ip, then divide.
- An activity of exactly 1.0 is a clean, easy-to-remember benchmark value sitting mid-way through the "Normal" range.
- Watch for percentages given inconsistently (as whole numbers vs decimals) — here both Ip and clay % were left as whole-number percentages, which conveniently cancel cleanly.
- This is the same activity classification used in Q21 and Q23 — worth memorizing once for all three.