Q27: The coefficient of permeability of clay is not more than:
📝 Detailed Explanation
Clay sits at the extreme low-permeability end of the soil spectrum already introduced in Q14 — this question asks for the specific upper bound on clay’s coefficient of permeability.
✅ Why “1 × 10⁻⁷ cm/sec” Is Correct
The coefficient of permeability of clay is not more than about 1×10⁻⁷ cm/sec — reflecting clay’s extremely fine particle size and correspondingly enormous specific surface area, which severely restricts water flow through its pore structure.
| Soil Type | Typical K Range (cm/sec) |
|---|---|
| Coarse sand | 1.0 – 0.01 |
| Fine sand | 0.01 – 0.001 |
| Silt | 0.001 – 0.00001 |
| Clay | < 0.000001 |
🔑 Key Point
Clay’s permeability upper bound, ~1×10⁻⁷ cm/sec, sits roughly seven orders of magnitude below coarse sand’s typical value (~1 cm/sec) — this enormous range (see Q14) is worth keeping in mind whenever comparing soil types by permeability.
💡 Key Concepts for Students
- This exact clay permeability figure directly reuses the range table introduced in Q14 — worth recognizing as the same reference data, not a separate new fact to memorize independently.
- Clay’s extremely low permeability is exactly why it’s the material of choice for engineered barriers — landfill liners, dam cores, and cut-off walls — where minimizing seepage is the design goal.
- The source material for this specific question notes some historical inconsistency between different reference texts regarding the exact figure (10⁻⁷ vs. 10⁻⁹ cm/sec in some accounts) — the value used here, 10⁻⁷ cm/sec, matches the standard, commonly cited figure and the given answer key.
- See Q28 for the physical factors (viscosity, unit weight of pore fluid) that further modulate permeability within any single soil type’s typical range.
📚 Going Deeper: Why Clay Is So Impermeable
Clay’s extraordinarily low permeability — often four to seven orders of magnitude below sand’s — comes down to two compounding physical effects, both already touched on in earlier topics and this one’s Kozeny-Carman discussion (Q11).
| Effect | Consequence for Permeability |
|---|---|
| Extremely small particle size | Creates very narrow, tortuous pore channels — physically restricting flow |
| Very high specific surface area | Per the Kozeny-Carman relationship (k ∝ 1/Ss²), permeability drops sharply as surface area increases |
| Bound/adsorbed water films | Clay particles hold tightly-bound water layers that further constrict the effective pore space available for free-flowing water |
These combined effects are exactly why clay is deliberately chosen as a barrier material in engineering practice — the same properties that make it a poor choice for drainage layers make it an excellent choice anywhere seepage needs to be minimized, such as landfill liners, canal linings, and the impermeable cores of earth dams.