Subgrade refers to the soil or material that lies beneath a road, pavement, or foundation. It serves as the base layer that supports the weight of the structure above. Properly prepared subgrade is crucial for ensuring stability and preventing issues like settling or cracking in the surface layer. The quality of the subgrade can significantly affect the performance and longevity of the construction. Engineers often assess the subgrade's strength and drainage properties before building. Techniques such as compaction and stabilization may be used to enhance the subgrade, ensuring it can adequately support the load of vehicles and structures above it.