What is the primary purpose of the sub grade in road construction?

The primary purpose of the subgrade in road construction is to provide a stable foundation for the surface layers of the roadway. It is the layer of soil or material beneath the pavement, which directly supports the weight and pressure exerted by vehicles. By ensuring that the subgrade has sufficient strength and stability, it helps maintain the integrity of the road surface and prevents deformation, settlement, or cracking over time.

This foundational support is critical, as any weaknesses in the subgrade can lead to premature failure of the road. Properly compacted and prepared subgrade enhances load distribution and prevents issues caused by water infiltration or expansive soil movement, thereby ensuring a durable and reliable roadway.