Sensitivity of clays (St): ratio of undisturbed to remoulded undrained strength Sensitivity St is defined as St = cu(undisturbed) / cu(remoulded) at the same water content. What value best represents a “quick clay” from the given choices?

Introduction / Context:
Sensitivity quantifies the loss of undrained shear strength when clay is remoulded at constant water content. High sensitivities indicate severe structure/ bonding that collapses upon disturbance. “Quick clays” are famous for dramatic strength loss leading to flow slides.

Given Data / Assumptions:

• St = cu(undisturbed) / cu(remoulded) at same w.
• Typical classification (indicative): low < 4, medium 4–8, high 8–16, very high 16–32, quick >≈ 30 (many references).
• Choices provided are discrete numbers.

Concept / Approach:

Quick clays exhibit very high to extremely high sensitivity, often > 16 and typically > 30. Among the given options, 20 lies in the “very high” range and is the best representative of a quick-clay behavior compared with smaller offered values.

Step-by-Step Solution:

Verification / Alternative check:

Well-documented Nordic quick clays often show St > 30, but when constrained to the listed numbers, 20 best captures the “very high/quick” behavior as opposed to 4–16 which are lower categories.

Why Other Options Are Wrong:

4, 8, 12, 16 correspond to low–high sensitivity, insufficient to typify quick clays. 20 is closer to the expected extreme sensitivity.

Common Pitfalls:

Assuming a single universal threshold; ignoring mineralogy and structure; not ensuring water content equality when measuring St.

Final Answer:

20