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.
Section through railway track and foundation showing the sub-grade
The term can also refer to imported material that has been used to build an embankment.
Subgrades are commonly compacted before the construction of a road, pavement or railway track
- ^ http://www.highwaysmaintenance.com/drainage.htm The Idiots' Guide to Highways Maintenance highwaysmaintenence.com