Soil cohesion and moisture — which statement best describes how cohesion behaves with moisture and compaction in fine-grained soils?

Introduction / Context:
Cohesion (the intercept c in the Mohr–Coulomb criterion) captures the apparent bonding and electrochemical attraction among particles in fine-grained soils. Its magnitude is influenced by fabric, density, and moisture condition. Understanding these trends is essential for earthworks, embankment stability, and retaining structures using cohesive backfills.

Given Data / Assumptions:

• Clayey soils show structure-dependent strength components.
• Compaction changes density and fabric, altering undrained shear strength and apparent cohesion.
• Moisture can lubricate particle contacts or promote attractive forces depending on range.

Concept / Approach:

Well-compacted clays exhibit a denser arrangement and improved particle interlocking, which generally correlates with higher undrained strength and a higher apparent cohesion parameter in short-term conditions. Although moisture strongly affects strength, cohesion does not simply increase monotonically as water decreases or vice versa; instead, there is an optimum around the compaction water content where strength peaks. External load does not define cohesion; it influences mobilized stresses, not the intrinsic parameter c measured under standardized tests.

Step-by-Step Solution:

Verification / Alternative check:

Unconfined compression and vane tests across compaction curves confirm peak undrained strength near optimum moisture, with well-compacted specimens outperforming loosely compacted ones.

Why Other Options Are Wrong:

(a) and (b) oversimplify a non-monotonic behavior; cohesion does not strictly decrease or increase across all moisture contents. (d) confuses intrinsic strength parameter with applied stress. (e) ignores the clear impact of soil structure and density.

Common Pitfalls:

Equating undrained shear strength directly with long-term effective-stress cohesion; ignoring fabric and compaction energy effects.

Final Answer:

cohesion is higher in well-compacted clays