Soil Mechanics / Particle size, description and classification
Particle sizes and soil description
From boulders to clay platelets — and why the fines change everything.
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 notesWhy this matters
The first thing an engineer does with a soil is describe it: what sizes of particle it contains, and in what proportions. That description already predicts a great deal — how it drains, how it packs, whether it is plastic — before a single strength test is run.
Soils are grouped by particle size into gravel, sand, silt and clay, from coarsest to finest. The great divide is between coarse-grained soils (gravels and sands), whose behaviour is dominated by the size and packing of visible grains, and fine-grained soils (silts and clays), whose behaviour is dominated by the water held around microscopic platelets.
The size boundaries are set by the classification standard in use. The current UK framework (the BS EN ISO 14688 / BS 5930 family) places the sand/gravel boundary at about 2 mm and the silt/sand boundary at about 0.063 mm; clay is the finest fraction. These are standard-dependent conventions — confirm the exact boundaries and the current edition before quoting them on a project.
Coarse fractions are measured by sieving — shaking a sample through a stack of sieves and weighing what each retains. Particles too fine to sieve reliably are sized by sedimentation (measuring how fast they settle in water, since finer particles settle more slowly). Together these give the full particle-size distribution.
Predict first
Two sands are identical except that one has 15% fine clay mixed in. How does the clay most affect its behaviour?
Summary
- Soils grade gravel → sand → silt → clay, split into coarse-grained and fine-grained.
- Coarse soils are governed by grain size and packing; fine soils by water around platelets.
- Size boundaries are standard-dependent conventions — use the current UK framework and confirm the edition.
- Coarse fractions are sieved; fine fractions are sized by sedimentation.