What does "subgrade" refer to in earthwork?

In earthwork, "subgrade" refers to the compacted soil layer that serves as a foundation for a pavement structure. This layer is critical as it provides support and stability for the roadway or pavement system being built on top of it. A properly prepared and compacted subgrade is essential for ensuring the long-term performance of the pavement, preventing issues such as cracking or settling.

The subgrade must be adequately assessed and prepared to bear the loads of traffic and environmental conditions. Engineers often perform tests on the subgrade to determine its load-bearing capacity, moisture content, and compaction level to ensure it meets the necessary standards before any additional layers of pavement are applied.

The other choices do not accurately describe the subgrade: the soil layer at the surface of the ground is more general and doesn't specifically pertain to the engineered context, while the layer of soil removed during excavation and the top layer before alteration refer to different stages and conditions of the soil rather than its function as a supportive layer for construction.