Which statement is true about the relationship between the Liquid Limit and Plastic Limit?

• A. The Liquid Limit is typically greater than the Plastic Limit.
• B. The Plastic Limit is typically greater than the Liquid Limit.
• C. The Plastic Limit equals the Liquid Limit.
• D. The Plasticity Index equals the sum of LL and PL.

Answer: (A)

In soils, the Liquid Limit is the water content at which the soil begins to flow and behave like a liquid, while the Plastic Limit is where it changes from plastic to semisolid as it is rolled. Since increasing water content makes the soil softer, the liquid state occurs at a higher moisture content than the plastic state. Therefore, the Liquid Limit is typically greater than the Plastic Limit.

The Plasticity Index, which measures how much moisture the soil can remain plastic, is the difference between LL and PL (PI = LL − PL). That’s why LL is greater than PL and why the other statements don’t fit: PL isn’t typically greater than LL, they aren’t usually equal, and PI isn’t the sum of LL and PL.