Soil Mechanics / Particle size, description and classification

Plasticity, classification and its limits

How fine soils are classified, and why classification is only the start.

Lesson review: expert verification remains open

This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.

Checks needed for project use

A geotechnical reviewer must check laboratory interpretation, classification conventions and the limits of drained, undrained and one-dimensional idealisations.
Mineralogy/formation descriptions remain simplified: wind transport and clay identification are not unique; flocculated/dispersed fabric, activity and consistency should be reviewed against specialist references. Current classification standard text has not been fully verified.

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

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.