What does "subgrade" refer to in highway engineering?

In highway engineering, "subgrade" specifically refers to the soil that serves as a foundation for the pavement. It is the layer of soil or rock that is prepared to support the overlying layers of the pavement structure. The subgrade must be evaluated for its strength, stability, and drainage properties, as it directly influences the performance and longevity of the pavement above it. A properly prepared subgrade is essential for distributing the loads applied to the pavement, thereby preventing deformation and cracking.

The other responses relate to different components of highway construction. The surface layer of the highway pavement is distinct from the subgrade and is designed to be durable and wear-resistant. Crushed stone can be used as a base layer or aggregate but is not the same as subgrade soil, which is the material directly beneath it. Lastly, materials added for road drainage are important for maintaining the integrity of the highway, but they do not describe the subgrade itself. Thus, the definition of subgrade is critical for understanding its role in the overall construction and performance of highway systems.