Subgrade

In transport engineering, subgrade is the native material underneath a constructed road, pavement or railway track (US: railroad track). It is also called formation level.

The subgrade provides support to the subbase level and acts as an integral load-bearing layer. Failure of the subgrade can cause depressions and rutting of the upper base and surface courses. These in turn can lead to water pooling in deformations and cause vehicle aquaplaning among other issues.

The term can also refer to imported material that has been used to build an embankment.

Construction
Subgrades are commonly compacted before the construction of a road, pavement or railway track. This is to ensure their ability to absorb the loads being transferred down from the upper layers, increasing the life and wear of the surface courses.