For which of the following soil, plasticity index is maximum?
Try it first: pick an option, then read the explanation.
Detailed Explanation
Plasticity index generally increases as particle size decreases, because finer particles (with much larger surface area per unit volume) can hold and retain more bound water across a wider range of the plastic state.
Why "Clay" Is Correct
Clay-sized particles (finer than 0.002 mm / 2 microns) exhibit the highest plasticity index among common soil types, exceeding silt, sand (a distractor labeled "gravel"/"cobble" here are clearly coarser and effectively non-plastic). Clay's extremely fine particle size gives it very low permeability (well below 10⁻⁶ cm/sec) and a cohesive, flocculated structure that holds water tightly across a wide plastic range.
| Soil Type | Relative Plasticity | Relative Permeability |
|---|---|---|
| Clay | Highest | Very low (<10⁻⁶ cm/s) |
| Silt | Medium | Low |
| Sand | Low / near zero | High |
| Gravel / Cobble | None (non-plastic) | Very high |
Key Point
Plasticity and permeability move in opposite directions across the grain-size spectrum: the finest soils (clay) have the highest plasticity and lowest permeability, while the coarsest (gravel) have essentially zero plasticity and the highest permeability.
Key Concepts for Students
- Clay's flocculated structure (from inter-particle cohesive forces) is what gives it both its plasticity and its very low permeability.
- Cobble and gravel are coarse enough to be essentially non-plastic — plasticity index isn't a meaningful concept for them at all.
- This inverse plasticity-permeability relationship is a recurring theme across several questions in this topic (see also Q1's note that higher Ip indicates lower permeability).
- "Impermeable soil" is sometimes used informally to describe clay specifically because of how low its permeability is relative to coarser soils.