AASHTO M 145-91 (2021)

This recommended practice describes a procedure for classifying soils into seven groups based on laboratory determination of particle-size distribution, liquid limit, and plasticity index. Evaluation of soils within each group is made by means of a “group index,” which is a value calculated from an empirical formula. The group classification, including group index should be useful in determining the relative quality of the soil material for use in earthwork structures, particularly embankments, subgrades, subbases, and bases. However, for the detailed design of important structures additional data concerning strength or performance characteristics of the soil under field conditions will usually be required.