The single-grained structure is a characteristic of coarse-grained soils.

Coarse-grained soils are composed of particles that are relatively large in size, typically greater than 0.075 mm. These soils include sands and gravels. On the other hand, fine-grained soils are composed of particles that are smaller in size, typically less than 0.075 mm. These soils include silts and clays.

A single-grained structure refers to a soil structure where individual particles are separate and distinct from each other, with little or no cohesion between them. This means that the soil particles are not bound together and can easily move and shift relative to one another.

Let's understand this in more detail:

1. Soil Structure: Soil structure refers to the arrangement and organization of soil particles in a soil mass. It can be classified into different types based on the arrangement of particles, such as single-grained, flocculated, and dispersed structures.

2. Single-Grained Structure: In a single-grained structure, each soil particle exists as a separate entity. There is no bonding or cohesion between particles, and they can freely move and shift relative to one another. This structure is typically found in coarse-grained soils, where the individual particles are large enough to have minimal contact with each other. As a result, the soil has low shear strength and exhibits granular behavior.

3. Coarse-Grained Soils: Coarse-grained soils, such as sands and gravels, have relatively large particles. These particles are typically well-rounded or angular in shape. Due to their large size, there is limited surface area for contact between particles, resulting in a single-grained structure. The absence of cohesion between particles makes coarse-grained soils permeable and allows water to flow through relatively easily.

4. Fine-Grained Soils: Fine-grained soils, such as silts and clays, have smaller particles. These particles are typically more angular and have a larger surface area compared to coarse-grained soils. The increased surface area allows for stronger attractive forces between particles, leading to the formation of aggregates or clusters of particles. Fine-grained soils often exhibit flocculated or dispersed structures, where particles are bound together by electrochemical forces or water films.

In conclusion, the single-grained structure is a characteristic of coarse-grained soils, which are composed of relatively large particles with minimal cohesion between them. This structure contributes to the granular behavior and permeability of coarse-grained soils.