In-situ Shear Strength of Soft Clay – Most suitable field test Which field test is most appropriate for directly determining the in-situ undrained shear strength of soft, saturated clays?

Introduction / Context:
Soft cohesive soils require careful evaluation of undrained shear strength for stability analyses of excavations, embankments, and foundations. A rapid, disturbance-minimizing, and directionally appropriate in-situ test is preferred to capture the intact structure of sensitive clays.

Given Data / Assumptions:

• Soil: soft, saturated clay with low effective stress strength during rapid loading.
• Objective: undrained shear strength su in situ (short-term conditions).
• Minimal sample disturbance desired; rapid test acceptable.

Concept / Approach:

The field vane shear test uses a cruciform vane inserted into the clay and rotated at a controlled rate. The peak torque at failure gives the undrained shear strength su based on cylindrical shear along the failure surface. The method is direct, quick, and especially well-suited to very soft to soft clays where other tests (e.g., SPT) are unreliable and cause significant disturbance.

Step-by-Step Solution:

Verification / Alternative check:

Comparisons with high-quality UU triaxial tests on block samples and with CPT correlations confirm that vane shear gives reliable su in soft clays where sampling is difficult.

Why Other Options Are Wrong:

Plate load test yields bearing/settlement parameters, not direct su. CPT provides correlations (indirect). SPT is unsuitable in very soft clays (N-values very low/erratic). Pressuremeter gives undrained moduli/limits but is more involved and not the simplest choice for su alone.

Common Pitfalls:

Not correcting for rate effects or anisotropy, ignoring thixotropic recovery between tests, and misapplying correlations outside soft clay range.

Final Answer:

Vane shear test