What characteristic of cohesionless soil makes it vulnerable to collapse?

Cohesionless soils, such as sands and gravels, primarily rely on friction and particle interlock for their stability. This characteristic makes them particularly vulnerable to collapse under certain conditions. When these soils are subjected to loading or disturbances, such as increased water content or vibration, they can lose the frictional forces that keep the particles in place, leading to a potential failure or collapse.

In a cohesionless state, the soil lacks any binding forces that would provide additional support or stability. Therefore, when the external conditions change—such as during excavation, construction, or the application of loads—this reliance on friction becomes a critical weakness. As the effective stress decreases due to pore water pressure increases, the ability of the soil to maintain its structure is compromised, making it susceptible to collapsing.

The other characteristics mentioned, such as drainage, compressive strength, and clay content, do not inherently signify vulnerability in a cohesionless soil context. Lack of drainage can lead to saturation and increased pore pressure but does not directly relate to the fundamental instability of cohesionless soils. High compressive strength applies more to cohesive soils, where binding forces exist. Lastly, cohesionless soils typically have minimal clay content, so this factor is less relevant to their vulnerability.