Generally, soil with a plasticity index of what value or greater should be considered as expansive?

A plasticity index (PI) is a measure of the plasticity of soil, which indicates how much it can deform without cracking or collapsing when wet or dry. The plasticity index is an important characteristic when assessing soil behavior under varying moisture conditions.

A plasticity index of 15 is commonly cited in geotechnical engineering as a threshold for considering soil expansive. Soils with a PI of 15 or greater have a higher tendency to undergo significant volume changes with fluctuations in moisture content. This characteristic can lead to issues such as heaving or settling in structures built on such soils, making it critical to identify and manage these soils in construction projects.

In contrast, soils with lower plasticity indexes (such as those below 15) are less likely to exhibit significant expansive behavior, which is why understanding this specific index helps contractors and engineers make informed decisions about foundation design and soil treatment. This makes the identification of a plasticity index of 15 as a critical threshold for expansive soils a well-documented and accepted practice in the field.