Sub-grade is the lowest layer of the road pavement which receives the incoming stresses and transfers them to the soil underneath. It also provides strength to pavement by supporting it from beneath.
Thus, the properties of road are controlled by the properties of sub-grade soil. The properties of road are controlled by the properties of sub-grade soil. The properties such as the formation of waves,corrugations,ruts,cracks,etc are due to poor sub-grade condition. Hence,sub-grade soil is the integral parts which contributes to the strength stability and durability of road .
Requirements of sub-grade soil:
Following are some of the requirements of sub-grade soil.
- permanency in strength
- minimum change in volume and stability under adverse weather condition and ground water
- good drainage
- ease of compaction
Importance of sub-grade:
- To provide adequate support to the road pavement.
- To distribute all the wheel loads coming on the road pavement to a larger area of soil beneath.
- To prevent sub-soil water from entering the road pavement by capillary action,i.e. sub-grade should consist of self draining materials.
- To prevent formation of waves ,corrugation,ruts,cracks,pot-holes,etc in black-topped surface by remaining intact and stable under adverse climatic and stable under adverse climatic and loading condition.