Rate of compression and pore fluid: Compression (settlement) of a soil mass occurs most rapidly when the voids are filled with which pore fluid?

Introduction / Context:
The rate of soil compression depends on how quickly pore fluid can leave the voids. Air is far less viscous and more compressible than water, so settlements under partially saturated conditions can occur rapidly compared with saturated conditions that require time-dependent drainage governed by permeability.

Given Data / Assumptions:

• Same soil skeleton and stress change; only pore fluid condition varies.
• Air can escape readily through interconnected voids.
• Saturated clays require consolidation (drainage of water) for volume change.

Concept / Approach:
In saturated soils, primary consolidation is rate-limited by k and Hdr; the process requires excess pore-water pressure dissipation. In unsaturated or dry soils, compression involves immediate rearrangement and expulsion of air, which is fast; often the response is essentially immediate settlement (elastic + plastic) with negligible time lag.

Step-by-Step Solution:

Verification / Alternative check:
Oedometer tests on partially saturated samples show smaller time lags; field observations after compaction in partially saturated fills confirm rapid settlement response.

Why Other Options Are Wrong:

• Water-filled voids (Option B) require consolidation time; slower.
• “Partly air and partly water” may still be faster than saturated, but the most rapid occurs when air dominates (Option A).
• “None of these” is incorrect.

Common Pitfalls:
Assuming the same magnitude of settlement regardless of saturation; ignoring suction effects in partially saturated soils.

Final Answer:
Air (dry or partially saturated conditions with easy air escape)