What is subgrade?

• A. A drainage feature
• B. The prepared ground layer that supports subbase and pavement
• C. The top surface of the ground
• D. A type of soil wrapper

Answer: (B)

Subgrade is the prepared ground layer that supports the layers above it, like the subbase and the pavement itself. It can be natural soil or engineered soil that is compacted to a specified density and moisture content to form a stable foundation. This layer underbearloads from traffic and spreads them into the soil below, so its strength and proper compaction are crucial for pavement performance. If the subgrade isn’t adequately prepared, the pavement can crack, settle, or deform prematurely. It isn’t a drainage feature, it isn’t the top surface, and it isn’t a soil wrapper—the subgrade lies beneath the upper layers and provides the foundation for them.