AASHTO M 145-91 (2021) PDF Download

The AASHTO M 145-91 (2021) standard provides guidelines for classifying soils and soil-aggregate mixtures used in highway construction.

The standard outlines a procedure for classifying soils into seven groups based on their particle-size distribution, liquid limit, and plasticity index. This classification is important for determining the suitability of the soil material for various applications in highway construction, such as embankments, subgrades, subbases, and bases.

The classification process involves calculating a "group index" for each soil group using an empirical formula. This group index is a measure of the relative quality of the soil material and can help in assessing its performance in the field. However, it is important to note that for the detailed design of significant structures, additional data on strength and performance characteristics of the soil under actual field conditions may be necessary.