Vane Shear Test – In-situ Measurement of Undrained Strength The field vane shear test is primarily used for in-situ determination of the undrained shear strength of which intact, fully saturated soil type?

Introduction / Context:
The vane shear test provides a quick in-situ measure of undrained shear strength without the need for large sampling disturbance. It is especially useful for soft to medium clays where sampling for laboratory shear tests may be problematic or unrepresentative.

Given Data / Assumptions:

• Undrained loading conditions apply (short-term behavior).
• Soil is intact (not remolded before testing) and fully saturated.
• Field apparatus consists of a cruciform vane pushed and rotated in the ground.

Concept / Approach:

In clays, undrained shear strength governs short-term stability. The vane test mobilizes soil resistance on cylindrical surfaces formed during rotation and correlates torque to undrained strength. Cohesionless soils like sands and gravels cannot be reliably tested by vanes in situ because they lack stability under undrained torque—grains rearrange and drainage prevents undrained conditions.

Step-by-Step Solution:

Verification / Alternative check:

Field correlations between vane strength and unconfined compressive strength of cohesive soils support the method; repeat tests (remolded strength) also yield sensitivity estimates.

Why Other Options Are Wrong:

Sands and gravels require different in-situ tests (e.g., CPT, SPT). Highly organic soils may give erratic vane results due to fibrous structure and anisotropy.

Common Pitfalls:

Testing fissured or highly anisotropic clays without correction; ignoring rate effects or temperature dependence on torque readings.

Final Answer:

Clays