Time-dependent volume change: The compression of a saturated soil mass under a sustained static load, caused by gradual expulsion of pore water, is called:

Introduction / Context:
Two different processes reduce soil volume: compaction (mechanical densification at nearly constant water content) and consolidation (time-dependent volume decrease in saturated soils due to drainage of pore water). Distinguishing them is crucial for predicting settlements of structures and embankments.

Given Data / Assumptions:

• Saturated soil subjected to sustained static load.
• Drainage path allows pore water to exit over time.
• Result is a gradual decrease in void ratio and settlement.

Concept / Approach:
Consolidation is governed by Terzaghi’s one-dimensional theory in many practical cases. Effective stress increases as pore pressure dissipates: sigma′ = sigma − u. As u decreases with time, soil skeleton carries more load, leading to primary consolidation; secondary compression may follow due to creep within the soil skeleton.

Step-by-Step Solution:

Verification / Alternative check:
Oedometer tests exhibit time–deformation curves with primary consolidation and secondary compression phases, matching field settlement observations.

Why Other Options Are Wrong:

• Compaction: rapid densification by mechanical effort in unsaturated soils.
• Swelling: volume increase due to wetting or unloading.
• Creep: time-dependent strain at approximately constant effective stress; not primarily water expulsion.

Common Pitfalls:
Assuming all time-dependent settlement is consolidation; in organic soils, secondary compression can dominate long-term behavior.

Final Answer:
Consolidation