Which soils are described as cohesive and plastic?

• A. Gravel
• B. Sand
• C. Silt
• D. Clay

Answer: (D)

Cohesive and plastic soils are primarily characterized by their fine particle size, which allows them to exhibit properties that enable them to stick together and deform without cracking. Clay is an excellent example of such soils because it contains a high proportion of tiny particles that contribute to both its cohesion and plasticity.

The cohesion in clay soils arises from the electrical attraction between the particles, which can hold them together even under stress. This characteristic is crucial in various engineering applications, as cohesive soils tend to shift under load differently compared to non-cohesive soils. Moreover, plasticity in clay allows it to undergo considerable deformation without breaking, which is beneficial when considering its behavior in construction and soil mechanics.

In contrast, soils such as gravel and sand are mainly composed of larger particles, which do not have the same cohesive properties; they tend to be more granular and do not exhibit significant plasticity when wet. Silt, while finer than sand, does not reach the same levels of cohesion or plasticity as clay, primarily due to its unique particle shape and smaller surface area compared to clay particles. Thus, while silt can show some cohesiveness and plasticity, it generally lacks the same robust properties seen in clay.