Highway Research Board(HRB) Classification of soil
The system of classification is based on both the particle size distribution as well as the plasticity characteristics of the soil. This system is mostly used for pavement construction.
In this system, soils are divided into 7 primary groups, designated as A-1, A-2…..A-7. Group A-1 is divided into two sub-groups and group A-2 is divided into four sub-groups.
A characteristic group index is used to describe the performance of soils when used for pavement construction. A high value of the group index indicates an impoverishment of the material.