Which of the following is considered an unsuitable soil for bearing capacity?

Organics are considered an unsuitable soil for bearing capacity due to their variable nature and low strength when saturated. Organic soils, such as peats and mucks, contain a significant amount of decomposed plant material, which makes them compressible and less stable under load. This can lead to excessive settlement and potential failure of structures built on such soils.

In contrast, materials like gravel, clay, and sand can exhibit better engineering properties for load-bearing applications, as they are typically denser and provide more predictable performance under stress. Gravel has good drainage properties and high shear strength; clay, while it can be problematic when wet, has high compressibility characteristics and can still provide significant bearing capacity when properly managed; and sand tends to have good load distribution characteristics. Hence, organic soils stand out as the least suitable option for structural foundations due to their tendency to deform and their general lack of reliability in supporting loads.