What does the term 'subgrade' refer to in highway construction?

In highway construction, 'subgrade' specifically refers to the layer of soil or material that is directly beneath the pavement and supports it. The quality and composition of the subgrade are critical to the overall performance of the pavement structure because it provides the necessary strength and stability. A well-prepared subgrade ensures that loads from vehicles are distributed evenly and reduces the risk of pavement failure.

The upper layer of pavement, while important, is not defined as the subgrade; instead, it involves the wearing surface and layers above the subgrade. Similarly, materials used for backfill pertain to soil or rock materials used to fill excavations or around structures, distinct from the subgrade’s role in supporting the road. The finished grade of the road surface represents the final contour and elevation of the pavement, which is also not synonymous with the subgrade.

By understanding the definition and function of the subgrade, as well as how it integrates into the overall highway structure, one can appreciate the significance of proper subgrade preparation and its impact on road longevity and safety.