Q40: Which of the soil possesses zero plasticity index?
Correct Answer:
C. Sand
📝 Detailed Explanation
Plasticity index depends heavily on particle size and mineralogy (see Q28, Q38) — coarse, cohesionless soils lack any meaningful plastic behaviour at all.
✅ Why “Sand” Is Correct
Sandy soil has a plasticity index of zero, since it is a coarse, cohesionless material that cannot exhibit plastic (mouldable) behaviour the way fine-grained clays and silts can. As soils get progressively finer, their plasticity index tends to increase.
🔑 Key Point
Zero plasticity index (“non-plastic”) is characteristic of sand and other coarse, cohesionless soils — this is the same underlying idea as Q24’s low/medium/high plasticity classification, just at its lower limit.
💡 Key Concepts for Students
- Plasticity index = liquid limit − plastic limit; for non-plastic soils, both limits are essentially undefined or coincide, so Ip reduces to zero.
- Clayey silt and silty soils (the other options here) still contain enough fines to show some measurable plasticity, unlike pure sand.
- See Q24 and Q51 for how a zero plasticity index is formally classified (“non-plastic”).
- This is a recurring theme across the whole topic: plasticity generally increases as particle size decreases, from sand up through silt to clay.
