subgrade
Sign in to savethumb|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.
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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.
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.