What is the definition of 'subgrade' in highway construction?

In highway construction, the term 'subgrade' refers to the underlying layer of soil or material that provides the foundational support for the pavement structure. It is essentially the surface upon which the roadbed and pavement layers are constructed. A well-prepared subgrade is crucial because it influences the overall stability, strength, and longevity of the road.

The importance of the subgrade cannot be overstated, as any imperfections or weaknesses in this layer can lead to premature pavement failure, such as cracking or heaving. Therefore, proper compaction, moisture control, and material selection for the subgrade are essential practices in highway construction to ensure a durable and effective roadway.

The other choices do not accurately describe the subgrade. Landscaping areas are above the pavement and unrelated to foundational support. The layer above asphalt would refer to surface material, which is distinct from the subgrade's role. Similarly, the structure supporting traffic signs does not pertain to the layered construction of the roadway itself.