Define 'subgrade' in highway construction.

The term 'subgrade' refers to the natural ground or the layer that has been improved which serves as the foundation for a pavement structure in highway construction. This foundational layer is critical as it directly impacts the performance and durability of the roadway. Proper preparation and stabilization of the subgrade are essential to ensure that it can support the weight of the traffic and withstand changes in weather conditions.

In contrast, other terms associated with highway construction describe different parts of the pavement structure. For instance, the layer of asphalt above the base layer pertains to surfacing and durability but does not define the subgrade. Similarly, the surface layer of concrete refers to a specific type of paving material and does not encompass the foundational role the subgrade plays. The layer of soil immediately below the road surface could be interpreted in various ways, but it lacks the emphasis on the natural ground or its improvement that is central to the definition of subgrade.