Undrained Shear in Plastic Clays – Dominant Source of Shear Strength For a plastic, fully saturated clay tested in undrained conditions (short-term loading), the mobilized shear strength arises primarily from which component?

Introduction / Context:
Short-term stability of clayey foundations, embankments, and excavations often depends on undrained shear strength. In undrained loading, pore pressures change but water does not have time to drain, so the effective stress pathway differs from long-term drained conditions.

Given Data / Assumptions:

• Clay is plastic and fully saturated.
• Loading is rapid (undrained), so drainage is negligible.
• Analysis follows total stress (undrained) framework.

Concept / Approach:

In undrained conditions, the shear strength of clays is represented by su (often treated as a cohesive intercept in total stress Mohr–Coulomb), and the friction angle in total stress is frequently taken as zero for short-term analysis (φ_u ≈ 0). Thus, the apparent strength comes primarily from cohesion. Over longer times with drainage (effective stress analysis), frictional resistance governed by φ′ becomes relevant.

Step-by-Step Solution:

Verification / Alternative check:

Field vane, unconfined compression, and CU triaxial tests (with pore pressure measurement) all provide measures of su consistent with this concept for short-term conditions.

Why Other Options Are Wrong:

(a) and (b) describe frictional mechanisms important in drained, effective stress terms; they are not dominant in undrained plastic clay short-term behavior.

Common Pitfalls:

Using drained strength parameters for rapid loading; ignoring rate effects and structure sensitivity (thixotropy).

Final Answer:

Cohesion (undrained shear strength, su)