Module 4 · Lesson 4.3
Plasticity, classification and its limits
How fine soils are classified, and why classification is only the start.
Why this matters
Fine soils cannot be classified by size alone — two clays of the same particle size can behave very differently. Their plasticity (from the Atterberg limits met earlier) is what classifies them, and it hints at their compressibility and how they respond to water.
Fine soils are classified using their liquid limit and plasticity index — plotted on a plasticity chart that separates silts from clays and low- from high-plasticity soils. A high-plasticity clay holds more water, is more compressible and swells and shrinks more with moisture change than a low-plasticity silt. (The precise chart lines are set by the classification standard; use the current UK framework and confirm the edition.)
Check yourself
Two clays classify identically (same grading, same plasticity). Which factor could still make them behave very differently?
Summary
- Fine soils are classified by plasticity (liquid limit and plasticity index), not size.
- Higher plasticity means more water held, more compressibility and more swell/shrink.
- Classification captures index properties only — not stress history, fabric or state.
- Use it to communicate and choose tests, then measure the parameters the design needs.
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint