Why clays compress – mechanisms of compressibility: The observed compressibility of fine-grained clays during one-dimensional consolidation is primarily due to which mechanisms?

Introduction / Context:
Clay compressibility under load is central to settlement analysis. Unlike sands where compression is mainly particle rearrangement, clays involve physico-chemical effects, particularly the diffuse double layer around particles and their plate-like morphology.

Given Data / Assumptions:

• Fine-grained, cohesive soil with significant surface area and adsorbed water.
• Loading is slow enough for drainage (primary consolidation) and possible structural changes (secondary compression).

Concept / Approach:
Compression arises from several coupled mechanisms: (a) Dissipation of excess pore pressure leads to expulsion of water from the inter-particle double layers, reducing interparticle repulsion. (b) Particles slide and rearrange into more stable, denser configurations as effective stress increases. (c) Clay platelets may undergo limited elastic bending or structural adjustment, contributing to secondary compression over time.

Step-by-Step Solution:

Verification / Alternative check:
Oedometer tests show primary consolidation (governed by coefficient of consolidation) plus secondary compression (C_alpha), consistent with the mechanisms described.

Why Other Options Are Wrong:

• Each mechanism acts together; isolating only one would not fully explain observed settlements.

Common Pitfalls:
Attributing all clay compression solely to water expulsion; ignoring fabric changes; overlooking secondary compression in long-term settlement estimates.

Final Answer:
All of the above