Physical properties which influence permeability are:

Q28: Physical properties which influence permeability are:

A. Viscosity only

B. Unit weight only

C. Both viscosity and unit weight

D. None of the above

Correct Answer: C. Both viscosity and unit weight

📝 Detailed Explanation

Beyond the soil’s own structural properties (particle size, void ratio), the properties of the PORE FLUID itself (almost always water, in these problems) also directly influence permeability, per the Kozeny-Carman relationship already introduced in Q11.

✅ Why “Both viscosity and unit weight” Is Correct

Per the Kozeny-Carman relationship, k ∝ γw/μ — permeability is directly proportional to the pore fluid’s unit weight (γw) and inversely proportional to its viscosity (μ). Both physical properties genuinely influence permeability, not just one or the other.

❌ Why the Other Options Are Wrong

Viscosity only: correctly identifies one genuine influencing factor, but incompletely — unit weight also plays a role, per the same k ∝ γw/μ relationship.
Unit weight only: also correctly identifies one genuine factor, but again incompletely — viscosity is equally part of the same relationship.
None of the above: incorrect, since both viscosity and unit weight of the pore fluid are confirmed, genuine influencing factors.

🔑 Key Point

k ∝ γw/μ means BOTH pore-fluid properties matter together, in opposite directions — higher unit weight increases permeability, while higher viscosity decreases it — neither one alone tells the whole story.

💡 Key Concepts for Students

This directly reinforces Q11’s Kozeny-Carman relationship, now framed as a simpler “which properties matter” recognition question rather than a full statement-evaluation problem.
See Q35 for a full numeric application: computing the percentage change in K resulting from given percentage changes in both viscosity and unit weight due to a temperature rise.
Temperature is the most common practical reason pore-fluid viscosity and unit weight change in the field or lab — warmer water is both less viscous and slightly less dense.
See Q11’s Going Deeper section for the complete Kozeny-Carman formula and how void ratio and specific surface area factor in alongside these fluid properties.

📚 Going Deeper: How Pore-Fluid Properties Interact to Affect Permeability

While particle size and void ratio (the soil’s own structural properties) set the basic geometry of the flow pathways, the pore fluid’s own physical properties determine how easily it can actually move through that fixed geometry.

Viscosity (μ) represents the fluid’s internal resistance to flow — a more viscous fluid experiences more internal friction moving through the same pore channels, directly reducing K. Unit weight (γw) represents the driving force per unit volume available to push the fluid through those same channels — a heavier fluid is driven through more readily under the same pressure gradient, increasing K. Both effects act simultaneously and in the SAME formula (k ∝ γw/μ), which is exactly why both must be considered together, and why a change in temperature — which affects both properties, in different directions — produces a combined, calculable net effect on K (see Q35).