When the liquid limit or plastic limit cannot be determined, how should the plasticity index be reported?

The plasticity index is a numerical representation of the plasticity of a soil, defined as the difference between the liquid limit and the plastic limit. When the liquid limit or plastic limit cannot be determined, it indicates that the soil may not exhibit plastic behavior within the testing limits, signaling either a lack of cohesive properties or that the soil is granular.

In such cases, the appropriate designation for the plasticity index is to classify the soil as "Nonplastic." This designation of NP signifies that the material does not have sufficient plasticity to be characterized by a liquid limit and plastic limit, thus eliminating the possibility of having a defined plasticity index.

The other choices do not appropriately describe the situation where neither limit can be determined. "Liquid Plastic," "Alkali Plastic," and "Dry Plastic" do not align with the classification standards of soil mechanics when referring to plasticity indices, especially in scenarios where the soil does not behave plastically. The classification of NP serves to clearly communicate the nonplastic nature of the soil, which is crucial for engineers and technicians in understanding and evaluating the soil’s behavior for construction and design purposes.