Which statement correctly describes the Plastic Limit (PL)?

• A. The moisture content where thread breaks apart at a diameter of 1/8 inch
• B. The moisture content at which soil deforms plastically
• C. The moisture content where soil becomes liquid
• D. The moisture content where clay loses cohesion

Answer: (A)

Plastic Limit is the moisture content at which soil changes from plastic to semisolid as it dries. It is determined by rolling a sample into a thread on a glass plate until it reaches about 1/8 inch in diameter and just crumbles. The moisture content at that moment is the Plastic Limit. This specific 1/8 inch thread criterion is why the statement describing the thread breaking apart at that diameter best matches the PL. The notion of simply “deforming plastically” is too vague, the liquid limit describes the transition to a liquid state, and losing cohesion isn’t the formal boundary used in Atterberg limits. The Plastic Limit, together with the Liquid Limit, defines the Plasticity Index.