Which statement correctly distinguishes subgrade from subbase?

• A. Subbase is the granular or stabilized layer placed on subgrade to provide structural support.
• B. Subgrade is the top asphalt layer; Subbase is the underlying soil.
• C. Subgrade is a drainage layer; Subbase is the surface crust.
• D. Subgrade is the prepared soil surface for pavement; subbase is the granular or stabilized layer placed on subgrade to provide structural support.

Answer: (D)

The key idea is understanding how pavement is built from the bottom up: the subgrade is the prepared foundation soil that will carry the pavement loads, while the subbase is a separate layer placed on top of that foundation to add stiffness and distribute loads.

Subgrade is the prepared soil surface for the pavement. It is the foundation that must be compacted to a specified density and moisture content so it can support traffic without excessive deformation. Subbase, on top of that foundation, is the granular or stabilized layer that provides additional structural support, helps spread loads, and can improve drainage.

The other descriptions mix up these roles. For example, the top asphalt layer is the surface wearing course, not subgrade. And describing the subbase as the drainage layer or the surface crust doesn’t accurately reflect its purpose or its position above the subgrade.

So, the correct distinction is that subgrade is the prepared foundation soil, and subbase is the granular or stabilized layer placed on that subgrade to provide structural support.