In geotechnical site investigations, a vane shear tester is normally used to determine the in-situ undrained shear strength of which soil type?

Introduction / Context:
The field vane shear test is a rapid, minimally disturbing method for assessing undrained shear strength, especially where sampling quality is difficult to maintain. Choosing the correct soil type for which the vane test is appropriate is crucial for obtaining reliable design parameters for foundations, embankments, and excavations.

Given Data / Assumptions:

• Tool: field vane shear tester (four-bladed vane).
• Measurement: undrained shear strength (su) in-situ.
• Environment: saturated fine-grained soils vs. coarse or granular soils.

Concept / Approach:

The vane test measures the torque required to cause shear failure along a cylindrical surface in the soil around the vane. It relies on the soil’s cohesion and is valid when drainage is negligible during the short test duration. This aligns with soft, saturated clays where undrained conditions prevail. In sands or gravels, disturbance, drainage, and particle interlock invalidate the test; in stiff clays, remolding and rate effects can lead to scatter, and alternative tools (e.g., CPTU correlations or high-quality sampling with laboratory triaxial) may be preferable.

Step-by-Step Solution:

Verification / Alternative check:

Standard practice recommends the vane test primarily for very soft to soft clays and organic clays/silts; reported sensitivity can also be obtained by comparing peak and remolded strengths.

Why Other Options Are Wrong:

• Sand/gravel: Drained, non-cohesive; vane readings are unreliable.
• Stiff clays: Possible but less reliable; sample disturbance and strain-rate dependency complicate interpretation.
• Silty sand: Predominantly frictional; vane test is unsuitable.

Common Pitfalls:

• Testing in layered soils where sand seams cause premature failure planes.
• Not correcting for plasticity or sensitivity when interpreting results.

Final Answer:

soft clays