When the plastic limit of a soil is greater than the liquid limit, then the plasticity index is reported as

Explanation:

• Plasticity Index (PI) = Liquid Limit (LL) – Plastic Limit (PL)

• Normally, LL > PL, so PI is positive.

• However, if PL > LL, this is physically unrealistic, often due to testing errors or soil peculiarities.

• As per IS 2720 (Part 5) and general practice, in such cases, the Plasticity Index is reported as zero, not negative.

Additional InformationLiquid Limit (LL):

• The minimum water content at which a soil changes from a plastic state to a liquid state.

• It represents the boundary between liquid and plastic behavior.

Plastic Limit (PL):

• The minimum water content at which a soil can be rolled into 3 mm diameter threads without breaking.

• It marks the boundary between plastic and semi-solid states.

Plasticity Index (PI):

• The numerical difference between the liquid limit and the plastic limit:
  PI = LL − PL

• It indicates the range of moisture content over which the soil remains plastic.