What phenomenon describes clay soils that become more fluid when worked?

The correct phenomenon that describes clay soils becoming more fluid when worked is sensitivity. Sensitivity in the context of clay soils refers to the change in shear strength and behavior of a clay when it is disturbed or remolded. Essentially, when the clay is mechanically disturbed, such as through digging or mixing, it can lose its structure and become more fluid or workable. This is particularly important in geotechnical engineering, as it affects how the soil behaves under various conditions and can impact construction practices.

In contrast, plasticity refers to the property of a material to deform without breaking and retain that deformation, which can apply to clay but does not specifically highlight the change induced by working the soil. Viscosity pertains to the resistance of a fluid to flow, which is a different concept more aligned with fluids rather than soils. Elasticity describes a material’s ability to return to its original shape after being deformed, which also does not capture the behavior of clay soils when they are disturbed. Understanding sensitivity is crucial in geotechnical applications where the behavior of clay under stress or disturbance can significantly influence structural stability and design.