Clay is what type of soil?

Clay is classified as cohesive soil, which is a fundamental and significant characteristic of this type of material. Cohesive soils are defined by their ability to stick together due to the fine particles they contain, which create plastic and cohesive properties when wet. Clay particles are particularly small, often less than 0.002 millimeters in diameter, and they exhibit these cohesive properties because of their plate-like shape and the electrical charges on their surfaces.

When subjected to water, clay can retain moisture, which enhances its cohesion and allows it to be molded into different shapes. This behavior is important in various engineering applications, such as foundation design and earthworks, as it affects stability and load-bearing capacities.

Other types of soil mentioned in the options have different properties and are not applicable to clay. Solid rock refers to unweathered rock which does not exhibit the same characteristics as clay. Cohesionless soil, such as sand or gravel, lacks the ability to stick together, and organic soil contains significant amounts of decomposed plant materials, which does not align with the characteristics of clay. Thus, the classification of clay as cohesive soil is accurate and reflects its fundamental engineering behavior.