Soil Mechanics – Sensitivity of clays What is the ratio of the unconfined compressive strength of an undisturbed sample to that of a remoulded sample at the same water content called?

Introduction / Context:
Sensitivity is a critical index for clays, describing the loss of strength upon remoulding. It is particularly important in assessing the stability of sensitive and quick clays, where disturbance can drastically reduce strength and cause flow slides.

Given Data / Assumptions:

• Undisturbed and remoulded specimens are tested at the same water content.
• Unconfined compression test used to measure compressive strength.
• No time-dependent thixotropic recovery considered for the definition.

Concept / Approach:

Sensitivity St is defined as St = qu(undisturbed) / qu(remoulded). High St implies large strength loss on disturbance. Typical values: St ≈ 1–4 for normal clays; St > 8 for sensitive clays; St ≫ 30 for quick clays. The index guides construction practices (excavation, sampling, embankment loading) and risk mitigation.

Step-by-Step Solution:

Verification / Alternative check:

Comparisons with vane shear tests often corroborate sensitivity trends; soils with pronounced structure or bonding show high St.

Why Other Options Are Wrong:

Activity relates PI to clay fraction; damping is a dynamic characteristic; plasticity is a broader behavior attribute; liquidity is part of liquidity index, not this strength ratio.

Common Pitfalls:

Not matching water content between tests, allowing thixotropic strength gain before remoulded testing, or confusing sensitivity with liquidity index.

Final Answer:

Sensitivity