If the liquid limit of a soil mass is 20% and its plastic limit is 25%, then the plasticity index of this soil will be
- A5
- B–5
- C0Correct
- D45
Solution
Concept
Plasticity index
Formula
PI = LL − PL
Step-by-step solution
1. LL − PL = 20 − 25 = −5. 2. A negative value means the soil has no plastic range, so it is non-plastic (PI = 0 or NP).
Explanation
The plasticity index cannot be negative. When the plastic limit is equal to or higher than the liquid limit the soil is non-plastic and PI is reported as zero (NP).
Shortcut
If PL ≥ LL, report PI = NP (0).
Common mistake
Reporting −5.
Correct answer: C) 0