Effect of wetting on cohesive soils: When cohesive fine-grained soils become wetted (increase in water content toward saturation), which of the following typically occurs to undrained shear strength?

Introduction / Context:
Moisture change strongly affects the engineering behavior of cohesive soils. Construction sequences, rainfall infiltration, or leakage can move the natural water content closer to the liquid limit. Understanding the trend in shear strength is vital for slope stability and bearing capacity in the short term.

Given Data / Assumptions:

• Fine-grained, cohesive soil (clay or clayey silt).
• Increase in water content without time for drainage (undrained conditions).
• Structure and fabric not cemented.

Concept / Approach:

Undrained shear strength of clays generally decreases as water content increases toward and beyond the liquid limit. Added water reduces effective stress and weakens interparticle bonding and suction, leading to lower undrained strength. Conversely, drying increases undrained strength up to a point.

Step-by-Step Solution:

Verification / Alternative check:

Empirical correlations show undrained shear strength roughly proportional to (LL − w) over the plastic range; as w approaches LL, strength reduces.

Why Other Options Are Wrong:

(a) and (b) contradict the observed trend; (c) implies no strength change; (e) “immediately liquefy” overgeneralizes and ignores structure and loading rate.

Common Pitfalls:

Assuming lubrication increases strength; ignoring drainage time effects and confining stress changes.

Final Answer:

They decrease their shear strength