What Is Soil? Functions Of Soil Subgrade

The unconsolidated mineral material of earth crust is known as soil.
According to IS: 2809-1972, soil is defined as, "sediments or other unconsolidated accumulation of solid particles produced by the physical and chemical disintegration of rocks which may or may not contain organic matter.
The soil may also contain air, water, organic matter, consisting of more or less decomposed remains lof plants and animal organism and other substance which remain dispersed through out the mineral particles of the soil. Thus soil is non-homogeneous, porous and extremely variable in its composition as well as in properties.
Soil not only forms the foundation of  road pavements but also is the principal material used for their construction. Since there is a wide variation in soil types, an adequate knowledge of the properties of  different types of soil is, therefore, essential for proper design and construction of road pavements.

Soil Subgrade

The importance of soil as a highway subgrade lines in the fact that it act as an integral part of road pavement. the soil as a highway subgrade serves the following functions :-

1. To provide an adequate support to the road pavement.
2. To provide stability to the road pavement.
3. To provide good drainage of rain water percolating through the road pavement.

The various defects such as waves, corrugations, ruts, cracks, etc. occurring  in the road pavements are mainly due to poor subgrade soil conditions. the subgrade soil should, therfore, be well compacted and stabilized so as to prevent these defects and  to keep the road pavement stable and serviceable for a longer period.