What does the term "base course" refer to in road construction?

The term "base course" in road construction specifically refers to the layer directly beneath the pavement. This layer plays a crucial role in supporting the load of traffic and distributing it to the underlying subgrade. The base course is typically composed of high-quality materials that provide adequate strength and stability, ensuring the integrity and durability of the pavement structure above. It serves as an essential component in the road design, contributing to the overall performance of the pavement by facilitating proper drainage and providing resistance against deformation under load.

The other options pertain to different layers of road construction. The top layer of pavement material serves as the wearing surface, while the layer above the subgrade soil, which is separate from the base course, could refer to either a subbase or another structural layer. The drainage layer, which is also distinct, serves to manage water within the pavement structure to prevent damage from moisture accumulation. Each of these layers has its own specific role and characteristics that contribute to the overall performance of a road, but the base course is uniquely defined as the layer directly beneath the pavement.