In transportation engineering, the subgrade is the native material underneath a construction road, pavement, or railway track, it is also called formation level, the subgrade may also refer to imported material that has been used to build an embankment.

The native soil is compacted to resist its overload formed by any cutting, snatching, or filling.

This can be a natural, undisturbed material, although usually natural soil will be compacted to uniformity.

Here we will learn about subgrade, properties of subgrade soil & much more.

Introduction to subgrade:

The sub-grade is made from wide range of materials, although some are much better than others.

In transportation engineering, the subgrade is the basic material for construction road, pavement, or railway track.

The term may also refer to imported material that has been used to build an embankment.

Subgrade performance depends on these basic characteristics:

  • Load-bearing capacity.
  • Moisture content.
  • Stabilization with cement or asphaltic binder.
  • Additional base layers.
  • Strength and stiffness of sub grade soil.

Properties of Subgrade Soil:


Sub grade soils must have sufficient resistance to permanent deformation under traffic loads.


The finished surface of the sub grade should be well compacted so that it prevents compressive under heavy traffic load and provides good support to road pavement.


Subgrade soil is the most important component of road pavement.

If the sub grade is weak, it affects the durability of the road pavement, hence the bearing capacity of sub grade soil should be sufficient for long term durability.

4.Drain ability:

Drain ability is also an important property of the sub grade.

The sub grade must have excellent drain capacity otherwise it affects the strength of the pavement.

Good drainage is necessary so as to prevent excessive moisture retention and minimize potential frost action.

5.Ease of compaction:

Sub grade soils must have good compacting properties.

A well-compacted subgrade surface increases the strength and durability of road pavements.

Due to ease of compaction ensures high density and strength.

Sub grade soils must have resistance to weathering therefore retain the desired support.

There should be a minimum change in the amount of stability under adverse weather conditions and imposing water.

Difference between Subgrade and Subbase:

The term sub grade soil generally means the soil beneath any types of construction.

And subbase is the layer of aggregate material placed on the subgrade on which the base course layer is located.

It is rarely used for uncontrolled filler under scenario loads that will not bear the load.

It provides support for the construction that is directly under the grade line such as unsupported floor slabs on straight grades and pavements.

The spreading footing and deep foundation occur on soils that may or may not be called residual soils depending on the continuity of this soil below the construction grade.

Advantages of subgrade:

  1. Subgrade maintains a high level of carrying capacity in a road.
  2. It avoid the initial cost in the reconstruction road.
  3. Also, increase the quality of the road.
  4. It avoids differential settlements in the ground.

Disadvantages of subgrade:

  1. Soil containing high moisture is not compacted properly.
  2. Sometimes they are highly economical.
Also read: WBM Road, Soil Classification & Road Drainage


The main function of the subgrade is to provide adequate support to the all types of pavement or structure.

The residual must be of sufficient stability under adverse climatic and loading conditions.

Hello, I'm Rahul Patil founder of, I had studied B.E. Civil. This blog provides authentic information regarding civil structures, equipment, materials, tests & much more.

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