Compression index (Cc) and Atterberg limits: How does the compression index of a fine-grained soil typically vary with its liquid limit?

Introduction / Context:
The compression index Cc governs primary consolidation settlement in clays. Empirical correlations link Cc with index properties such as liquid limit (LL) and plasticity index (PI), aiding preliminary design when oedometer data are unavailable.

Given Data / Assumptions:

• Soil is cohesive (clayey/silt with plasticity).
• Liquid limit and plastic limit are measured reliably.
• Typical empirical trends for natural soils apply.

Concept / Approach:
Higher liquid limit generally indicates higher water affinity, greater compressibility, and more open microstructure. Numerous correlations show Cc increasing with LL (for example, linear relations of the form Cc ≈ a*(LL − b) for given soil families). Thus, as LL increases, expected consolidation compression also increases, all else equal.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check with plasticity index PI or natural water content; soils near or above LL typically show pronounced compressibility consistent with larger Cc values.

Why Other Options Are Wrong:

• “Decreases with LL” contradicts well-established empirical observations.
• Independence from limits is not true; index properties often correlate with compressibility.
• Plastic limit alone has less direct correlation than LL for Cc trends.

Common Pitfalls:
Applying a correlation outside its valid soil type; always verify with oedometer tests whenever feasible.

Final Answer:
Increases with an increase in the liquid limit