Planum

Also known as formation level

thumb|right|Layers in the construction of a mortarless pavement: A.) Subgrade B.) Subbase (pavement)|Subbase C.) [[Base course D.) Paver base E.) Pavers F.) Fine-grained sand]] 450px|thumb|right|Section through railway track and foundation showing the sub-grade In transport engineering, subgrade is the term used in the US for the native material underneath a constructed road, pavement or railway track (US: railroad track). In British English it is called formation level.