What phenomenon is described as a sudden drop in the shear strength of a soil?

The phenomenon that describes a sudden drop in the shear strength of a soil is liquefaction. This occurs when saturated soil substantially loses strength and stiffness in response to applied stress, often during events like earthquakes. In this state, the soil behaves more like a liquid than a solid, which can lead to significant ground failures and destabilization of structures built upon it.

Liquefaction typically affects cohesionless soils, such as sand, that are saturated with water and can be especially pronounced during seismic events. The increased pore water pressure during such events reduces the effective stress within the soil, leading to a dramatic reduction in shear strength.

Settlement, erosion, and compaction relate to different soil behaviors and processes. Settlement refers to the gradual sinking of the ground due to load or removal of support, erosion involves the displacement of soil or rock by natural forces, and compaction is the process of densifying soil to increase its strength. While these processes affect soil behavior, they do not describe the sudden loss of shear strength characteristic of liquefaction.