The level ground under a road or a railroad track.
Subgrades are important for making sure roads and railroad tracks are strong and safe.
Examples:
The workers are fixing the subgrades under the road.
The subgrades need to be flat and strong.
The subgrades help the road stay in place.
The subgrades are like the road's bed.
The subgrades are the road's foundation.
The subgrades are under the road, like the ground under a house.