The Chartership Forum

Soil Mechanics / module 2

Geology, soil formation and fabric

Where soil comes from and why that matters: weathering, transport and deposition, the clay minerals, and the particle arrangement (fabric) that controls how a soil behaves.

What you will understand

  • Explain how soils form by physical and chemical weathering of rock
  • Distinguish residual from transported soils and read a transport history from a soil
  • Describe the main clay minerals and why smectite swells while kaolinite does not
  • Define activity and relate it to clay mineralogy
  • Explain soil fabric and structure, and why they make natural soil anisotropic and sensitive to disturbance

Lessons

  1. How soils form

    Weathering, transport and deposition — and the fingerprints they leave on a soil.

  2. Clay minerals and why they matter

    Kaolinite, illite and smectite, specific surface, activity and shrink–swell.

  3. Soil fabric and structure

    How particles are arranged, and why that makes natural soil anisotropic and disturbance-sensitive.

Module summary
  • Soil is weathered rock; physical weathering makes coarse grains, chemical weathering makes clay minerals.
  • Transport agent (water, wind, ice, gravity) sets sorting, shape and layering — readable from the grading.
  • Clay minerals (kaolinite, illite, smectite) differ hugely in surface and activity, controlling plasticity and swelling.
  • Fabric and structure make natural soil anisotropic, often fissured, and sensitive to sampling disturbance.
Teaching basis and review status

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

Check your understanding

Check yourself

A chaotic deposit containing everything from clay to boulders, poorly sorted and dense, was most likely transported by:

Practice

A clay has PI = 18% and a clay fraction of 30%. What is its activity, and is it closer to kaolinite or smectite?