Which statement describes how compaction affects soil unit weight?

• A. Unit weight increases.
• B. Decrease in unit weight.
• C. Increase in unit weight.
• D. Unit weight becomes zero.

Answer: (C)

When soil is compacted, the particles are packed more tightly and the air voids shrink. Unit weight is weight per volume, so with the same mass occupying less space, the weight per unit volume increases. In soils, this means the dry unit weight rises as the void ratio decreases, reaching a maximum at the optimum moisture content. So compaction describes an increase in unit weight. It wouldn’t become zero, and it wouldn’t decrease—the whole point of compaction is denser, heavier soil per unit volume.