Coefficient of Uniformity (Cu)
The coefficient of uniformity (Cu) is a measure used in geotechnical engineering to describe the gradation of soil particles in a soil sample. It is a dimensionless parameter that indicates the range of particle sizes present in the soil.

Definition of Coefficient of Uniformity
The coefficient of uniformity is defined as the ratio of the particle size D60 to the particle size D10. The particle size D60 refers to the diameter of the soil particles below which 60% of the soil mass is finer, and the particle size D10 refers to the diameter below which 10% of the soil mass is finer.

Interpretation of Coefficient of Uniformity
The coefficient of uniformity provides information about the range of particle sizes present in a soil sample. A higher value of Cu indicates a wider range of particle sizes, while a lower value indicates a narrower range.

Explanation of the Correct Answer
The correct answer to the question is option 'C' - Uniform soil. When the coefficient of uniformity is less than 3, it signifies that the soil is uniform. This means that the soil sample has a relatively narrow range of particle sizes.

Reasoning behind the Correct Answer
When the coefficient of uniformity is less than 3, it indicates that the soil sample has a relatively narrow range of particle sizes. This implies that the soil is composed of particles that are similar in size, resulting in a more uniform soil structure.

A narrow range of particle sizes in the soil can have both advantages and disadvantages. On the positive side, a uniform soil can have improved engineering properties such as higher shear strength and better compaction characteristics. This makes it suitable for certain construction applications.

However, there are also potential drawbacks to a uniform soil. It may have reduced permeability, which means that water may not be able to flow through it easily. This can lead to drainage problems and potentially affect the stability of structures built on or with the soil.

Conclusion
In summary, a coefficient of uniformity less than 3 indicates that the soil is uniform, meaning it has a relatively narrow range of particle sizes. While a uniform soil can have certain advantages in terms of engineering properties, it is important to consider the potential drawbacks such as reduced permeability.