Consolidation terminology: The reduction in soil volume caused by squeezing out water from the voids under sustained load is termed:

Introduction / Context:
Settlements in saturated fine-grained soils manifest as immediate (elastic) settlement, primary consolidation, and secondary compression. Correctly naming the mechanism guides prediction and mitigation measures in design.

Given Data / Assumptions:

• Sustained static load applied to saturated soil with drainage paths available.
• Time-dependent dissipation of excess pore pressure.
• Volume change due to water expulsion, not elastic distortion alone.

Concept / Approach:
Primary consolidation is the process by which excess pore pressure dissipates and effective stress increases, causing a reduction in void ratio. Secondary compression then continues at approximately constant effective stress due to viscous adjustment of the soil skeleton. Immediate settlement is nearly instantaneous elastic strain without significant pore-pressure dissipation in coarse soils or at very small strains.

Step-by-Step Solution:

Verification / Alternative check:
Oedometer curves show a characteristic primary consolidation segment followed by a near-linear secondary compression tail in e–log t space.

Why Other Options Are Wrong:

• Secondary compression: occurs after primary consolidation, not the initial water expulsion phase.
• Immediate settlement: largely elastic and rapid.
• Elastic rebound: recovery upon unloading, opposite effect.
• Creep under constant effective stress: describes secondary time effects, not primary consolidation.

Common Pitfalls:
Using “primary compression” loosely; precise term in soil mechanics is “primary consolidation.”

Final Answer:
Primary consolidation