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Difference between Subbase and Subgrade

What is a Subgrade in Construction?

 structures over subgrade is a thick compacted layer of native soil or improved soil material to support the upcoming layers of it. It can be defined as a layer of soil above which the foundation layers are placed. It is a layer that is used mainly for the construction of roads and pavements. 

Pavement Construction
Image Credits: Flexible Pavement Design - PE Continuing Education - PDH for Engineers (cedengineering.com)

A subgrade that is composed of impermeable soil must require a drainage facility to let water drain away without resulting in the collapse of the subgrade and the above layers of the pavement. A proper grade with respect to the elevation must be provided to let the water drain off.

Also Read: What is Camber in Road Construction?



Cross-Section of  a Rigid Pavement

Heavier is the loads coming over the subgrade, higher is the thickness of the subgrade. A thickness of 12 inch or more can be expected for normal highway pavement. Compared to other surface material, the cost of subgrade material is less.

The subgrade material can be improved when it do not have adequate load bearing strength. Generally, geotextiles are installed to help improve the strength and bearing capacity. These help prevent the mixing of soft or inadequate material that can impair the structural capacity of the subgrade.

Stabilizing the Subgrade Using Geosynthetics

What is a Subbase in Construction?

A subbase is a layer of gravel or soil that comes over the subgrade. Over the subbase comes the base course layer. Subbase is considered the main load bearing layer of a pavement.

Some of the subbase material used for construction are recycled concrete, granular fill. manufactured aggregate, lean concrete, recycled materials like brick or concrete materials, and crushed rock.

A subbase layer:

  1. Impart strength and support to overlying pavement
  2. Help workers to stay out of mud
  3. Improved drainage and frost protection
  4. Can be reused
  5. Provide a workable surface
  6. Reduce the construction cost
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