A surface of earth or rock levelled off to receive a foundation (as of a road)

The sub grade must be able to support loads transmitted from the pavement structure. This load bearing capacity is often affected by degree of compaction, moisture content, and soil type. A sub grade that can support a high amount of loading without excessive deformation is considered good.

The sub-grade layer of a pavement is, essentially, the underlying ground. It is also known as the "Formation Level", which can be defined as the level at which excavation ceases and construction starts.