What occurs to saturated cohesionless sands when shear strength drops to zero?

When saturated cohesionless sands experience a drop in shear strength to zero, they enter a state of liquefaction. This phenomenon is characterized by a dramatic loss of strength and stiffness, causing the sand to behave like a liquid rather than a solid. In this state, the sand particles lose contact with each other due to the high pore water pressure generated, which allows individuals particles to move more freely and flow, similar to how liquid behaves.

This is particularly significant during events such as earthquakes or sudden loading, where the dynamic stresses can cause increased pore pressures in the saturated sands. As a result, the effective stress, which is crucial for maintaining stability, drops, allowing the sands to lose their ability to support loads. This transformation can lead to serious consequences in engineering applications, including ground failure, instability of structures, and hazardous conditions if not properly addressed in design and construction.

The other options suggest behaviors that do not accurately represent the response of saturated cohesionless sands under these conditions. For instance, compaction does not occur when the shear strength reaches zero, nor do sands retain their original strength or become unmanageable solely due to density increases. Understanding liquefaction is critical for civil engineering and geotechnical assessments in areas prone to such failures.