In which scenario would you see a grain size curve with two humps and a flat area?

• A. When the soil is well-graded
• B. When a certain grain size is absent
• C. For uniform soils
• D. For cohesive mixtures

Answer: (B)

A grain size curve featuring two humps and a flat area indicates a specific distribution of particle sizes in the soil. This pattern often arises when there is a notable absence of a particular grain size, which can create gaps in the distribution. The two humps represent the presence of two dominant sizes within the sample, while the flat area signifies the lack of intermediate sizes, leading to a non-continuous transition between the two.

In contrast, well-graded soils would exhibit a more continuous and smooth grain size distribution without gaps, typically creating a curve that ascends and descends gradually rather than displaying distinct peaks or flat sections.

For uniform soils, the grain size curve would be narrower and not present the characteristics of two distinct size distributions. Instead, uniform soils are characterized by a consistent particle size throughout, leading to a single peak on the curve.

Cohesive mixtures generally consist of a balanced distribution of both fine and coarse particles, and their grain size curves are more likely to reflect this mix rather than a distinct two-hump pattern resulting from absent grain sizes.

Thus, the observed grain size curve with two humps and a flat area is indicative of a specific scenario where a certain grain size is indeed absent, affirming the accuracy of