Consider different types of soils, i.e. Fine sand (F), Homogeneous clay (C), Coarse gravel (G), Silty clay (S). Arrange the soils in the increasing order of their permeability (low to high values).
Try it first: pick an option, then read the explanation.
Detailed Explanation
Permeability describes how easily water can flow through a soil's void spaces, and it is strongly controlled by particle size and specific surface area (SSA) — the total surface area of particles per unit volume/mass.
Why "C, S, F, G" Is Correct
Permeability (k) is inversely proportional to the square of specific surface area: k ∝ 1/(SSA)². Homogeneous clay (C) has the finest particles and therefore the largest SSA, giving it the lowest permeability. Coarse gravel (G) has the coarsest particles and smallest SSA, giving it the highest permeability. Silty clay (S) and fine sand (F) fall in between, in that order, since silty clay still has significant fine content while fine sand is coarser and more free-draining.
| Soil | Relative SSA | Relative Permeability |
|---|---|---|
| Homogeneous Clay (C) | Highest | Lowest |
| Silty Clay (S) | High | Low |
| Fine Sand (F) | Low | High |
| Coarse Gravel (G) | Lowest | Highest |
Key Point
Permeability and specific surface area always move in opposite directions: more surface area per unit volume (finer particles) means lower permeability, and vice versa.
Key Concepts for Students
- Specific surface area (SSA) captures why particle shape and fineness — not just average particle size — control permeability so strongly.
- This same C < S < F < G ordering logic (by increasing permeability) generalizes to virtually any grain-size comparison in soil mechanics.
- Clay's very low permeability is exactly why it's often used as a liner material to control seepage in dams, ponds, and landfills.
- This relationship, k ∝ 1/(SSA)², is a useful qualitative rule even where exact SSA values aren't given — you can usually rank permeability just from relative fineness.