Geotechnical Engineering

Plasticity Index

The range of water content over which a fine-grained soil is plastic: liquid limit minus plastic limit.

The plasticity index PI = LL − PL spans the water contents at which a clay or silt can be moulded without crumbling or flowing. A high PI marks a clay that swells and shrinks a great deal with moisture and loses strength when wet.

On the plasticity chart used by the Unified Soil Classification System, the A-line PI = 0.73(LL − 20) separates clays above it from silts below, and a liquid limit of 50 divides low from high plasticity. Lime stabilisation is used largely to bring a high PI down.

How much of the FE Civil exam is Geotechnical Engineering?