Causes of over-consolidation in clays: identify the correct drivers Which of the following leads to over-consolidation (preloading higher than present overburden) in natural clays?

Introduction / Context:
Over-consolidation means the current effective overburden is less than the maximum historical effective stress. Recognizing proper causes helps interpret preconsolidation pressure from oedometer tests and predict settlement behaviour.

Given Data / Assumptions:

• Over-consolidation occurs due to past higher effective stresses.
• Mechanisms include material removal, desiccation, aging/cementation, and glacial loading/unloading.
• Changes that only reduce current effective stress without a prior higher stress do not “cause” over-consolidation by themselves.

Concept / Approach:

Both erosion of overburden and the presence and subsequent melting of ice sheets indicate a historical load higher than today’s, producing over-consolidation. A permanent rise of the water table decreases present effective stress; unless it follows a prior lower water table (higher past effective stress), it does not by itself create a higher historical stress scenario. Thus, (a) and (b) are the direct causes among the listed items.

Step-by-Step Solution:

Verification / Alternative check:

Textbook lists of over-consolidation causes feature erosion, glacial effects, desiccation, and aging; a WT rise is not a standard primary cause.

Why Other Options Are Wrong:

Option (e) includes (c), which is not generally a cause; options (a) and (b) individually are incomplete; (c) alone is incorrect.

Common Pitfalls:

Conflating “current lower effective stress” with a proven higher “historical” stress; ignoring desiccation and cementation (also common causes though not listed here).

Final Answer:

Only (a) and (b) are correct causes of over-consolidation.