AASHTO M 145-91 (2017) PDF Download

The AASHTO M 145-91 (2017) standard provides a procedure for classifying soils based on laboratory tests. These tests include determining the particle-size distribution, liquid limit, and plasticity index of the soil.

The standard divides soils into seven groups, and each group is evaluated using a "group index" calculated through an empirical formula. This classification system helps in assessing the relative quality of soil materials for use in various highway construction purposes, such as embankments, subgrades, subbases, and bases. However, it should be noted that for the detailed design of significant structures, additional data on the strength and performance characteristics of the soil under field conditions may be necessary.