Subgrade vs Subbase
Subgrade is the prepared soil or native surface; subbase is a placed layer above the subgrade that supports pavement, slabs, or other construction.
Subbase
- Aggregate support layers
- Drainage and load distribution
- Pavement or slab buildup
Key differences
| Point | Subgrade | Subbase |
|---|---|---|
| Material | Existing or prepared soil. | Placed aggregate or engineered layer. |
| Location | Below the base or slab assembly. | Above subgrade and below the finished system. |
| Quality control | Moisture, density, proofrolling, and compaction. | Thickness, gradation, compaction, and drainage. |
Example in context
A warehouse slab may require compacted subgrade first, then a granular subbase before vapor barrier and concrete placement.