In highway engineering, subgrade is the native material underneath a constructed pavement. It is also called formation level.
Subgrades are commonly compacted before the construction of a pavement, and are sometimes stabilized by the addition of asphalt, soil cement, portland cement or lime. It is the foundation of the pavement structure, on which the subbase is laid.
Preparation of the subgrade for construction usually involves digging, in order to remove surface vegetation, topsoil and other unwanted material, and to create space for the upper layer of the pavement. This process is known as subgrade formation or reduction to level.
The load-bearing strength of subgrade is measured by California Bearing Ratio (CBR) test, falling weight deflectometer backcalculations and other methods.
subgrade in German: Planum
subgrade in French: Planum
subgrade in Italian: Planum