本推荐规程描述了根据实验室测定的粒度分布、液限和塑性指数将土壤分为七组的程序。通过“组指数”对每组土壤进行评估，该指数是根据经验公式计算的值。在确定土方工程结构（尤其是路堤、路基、底基层和基层）中使用的土壤材料的相对质量时，包括组指数在内的组分类应该是有用的。然而，对于重要结构的详细设计，通常需要关于现场条件下土壤强度或性能特征的额外数据。

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.