A soil sample is tested for its plasticity index and the obtained value of plasticity index is 21. In such case, the soil sample can be described as:
Try it first: pick an option, then read the explanation.
Correct answer: D. highly plastic
Detailed Explanation
Plasticity index doesn't just indicate the presence of plasticity (zero vs non-zero) — its magnitude also indicates the degree of plasticity, which is classified into named bands.
| Plasticity Index Range | Classification |
|---|---|
| Ip < 7 | Low plastic |
| 7 ≤ Ip ≤ 17 | Medium plastic |
| Ip > 17 | Highly plastic |
Why "highly plastic" Is Correct
A plasticity index of 21 exceeds the medium-plasticity upper bound of 17, placing the soil in the "highly plastic" category. This same classification scheme is used again in Q47 with slightly different numeric labels (Atterberg's own medium-plasticity band, 7 ≤ PI ≤ 17).
Key Point
Ip > 17 is the threshold for "highly plastic" soil — memorize this boundary alongside the low/medium boundary at Ip = 7.
Key Concepts for Students
- Highly plastic soils generally correspond to clays with significant fine, active clay mineral content.
- This plasticity classification is distinct from (though related to) the activity classification in Q21-23 — one uses raw Ip, the other uses Ip normalized by clay percentage.
- Non-plastic soil (Ip = 0, see Q40/Q51) sits below even the "low plastic" band, as its own separate category.
- This classification directly affects engineering behavior expectations: highly plastic soils typically show greater volume change potential (shrink-swell) and lower permeability.