ASTM D4318

Liquid Limit, Plastic Limit, and Plasticity Index of Soils

Applicability: Applies to fine-grained soils, soil-aggregate mixtures, and other geotechnical materials where the Atterberg limits are used for classification, compaction control, and assessment of engineering properties. Required for soil classification under the Unified Soil Classification System (USCS) and AASHTO soil classification, and used in geotechnical investigations for foundations, embankments, pavement subgrades, and earthwork projects.

Purpose: Determines the liquid limit, plastic limit, and plasticity index of soils through standardized laboratory procedures. The liquid limit is measured using the Casagrande percussion cup or the fall-cone method, while the plastic limit is determined by rolling soil threads to a defined diameter. The plasticity index, calculated as the difference between liquid limit and plastic limit, characterizes the range of water content over which a soil exhibits plastic behavior. These indices are used to predict compressibility, shear strength, and swell potential, and to evaluate the suitability of soils for engineering applications.