Which type of soil can be rolled into threads without crumbling, according to the plasticity test?

The correct answer is cohesive soil. This type of soil has the property of plasticity, allowing it to be molded and rolled into threads without crumbling when moist. Cohesive soils, such as clay, contain fine particles that stick together due to their electrochemical properties, providing the necessary adhesion that facilitates shaping.

While clay soil is often a significant component of cohesive soils and may also exhibit the same plasticity characteristics, the term "cohesive soil" encompasses a broader range of soil types that share this property. In practice, soils that are cohesive will demonstrate the ability to retain form under compression and manipulation, which makes them suitable for a variety of engineering applications.

Granular and sandy soils, on the other hand, do not possess the same cohesive properties and tend to fall apart when wet, failing to form threads during the plasticity test. They lack the fine particles necessary to create the stickiness found in cohesive soils and do not have the same level of plasticity.