How is the plasticity index expressed?

The plasticity index is expressed as the difference between the liquid limit and the plastic limit of a soil and is used to assess the plasticity characteristics of the material. It indicates the range of moisture content over which the soil remains plastic. In the context of the options provided, expressing the plasticity index as a percentage of the mass of an oven-dried soil reflects how the soil behaves under varying moisture conditions, which is crucial for determining its suitability for construction purposes.

While the liquid limit and plastic limit are both critical parameters in this calculation, the plasticity index directly relates to the moisture content in reference to the mass of the soil, emphasizing its importance in soil classification and engineering practices. This understanding aids soil testing technicians in evaluating soil properties essential for effective construction and engineering applications.