Atterberg

Atterberg limits are a group of soil-consistency tests used to characterize fine-grained soils as their water content changes. Named for Swedish soil scientist Albert Atterberg, they describe the transitions between liquid, plastic, and solid behavior in soils.

The three primary limits are the liquid limit (LL), the plastic limit (PL), and the shrinkage limit (SL). The liquid limit is the water content at which soil changes from plastic to liquid, typically determined with the Casagrande cup method (with fall-cone methods used as an alternative in some laboratories). The plastic limit is the water content at which soil begins to crumble when rolled into a thread about 3 millimeters in diameter. The shrinkage limit is the water content at which further drying causes no additional shrinkage of the soil mass, marking the transition toward a solid state.

The plasticity index, IP, is defined as LL minus PL (IP = LL − PL). It is used in soil classification to distinguish clayey soils (for example, CL or CH in the USCS) and to infer plastic behavior and mineralogical characteristics. The Atterberg limits are most informative for fine-grained soils; coarse-grained soils exhibit limited changes in consistency with moisture variation.

In practice, Atterberg limits are standardized in many regions, with the ASTM D4318 family covering LL, PL, and IP determinations. These limits support assessments of volume change, compressibility, and workability, and they inform foundation design, earthwork, and pavement subgrades alongside other geotechnical tests.