Soils are highly plastic when the liquid limit is greater than 50 and have low plasticity when the liquid limit is less than 50. What lab test is used on fine-grained materials to determine the liquid limit?

• A. Proctor
• B. Atterberg
• C. Permeability
• D. Unconfined compressive test

Answer: (B)

This tests the plasticity of fine-grained soils, which is captured by the Atterberg limits. The liquid limit is the water content at which a soil transitions from plastic to liquid as it is sheared. For fine-grained materials, this is determined with the liquid limit test, part of the Atterberg limits, most commonly using the Casagrande apparatus. A soil paste is placed in a brass cup with a groove, and a standardized set of blows is used to close the groove. The water content at which 25 blows close the groove is defined as the liquid limit. This method targets plasticity in fine-grained soils; the other tests measure different properties—compaction effort (Proctor), hydraulic conductivity (permeability), or strength (unconfined compression)—and do not determine the liquid limit.