Soil Mechanics / Geology, soil formation and fabric
How soils form
Weathering, transport and deposition — and the fingerprints they leave on a soil.
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
A soil is not a manufactured material with a specification — it is the product of a geological history. Reading that history tells you what to expect before you ever test a sample: whether the ground is likely to be uniform or erratic, strong or soft, and where the surprises may lie. Geology is the engineer's first and cheapest source of insight.
What you should already know
- No previous geology assumed
- The idea of particle size (Module 4 develops it further)
Soil is rock broken down by weathering:
- Physical weathering breaks rock into smaller pieces without changing its minerals — frost prising open cracks, temperature cycling, abrasion. It makes the coarse, chemically-unchanged grains of gravels, sands and silts.
- Chemical weathering alters the minerals themselves — feldspars and micas reacting with water and carbon dioxide to form new, tiny clay minerals. It makes the fine, chemically-new material of clays.
Which dominates depends on climate: cold, dry places weather physically; warm, wet places weather chemically and deeply.
Once formed, soil either stays put or is moved:
- A residual soil remains where its parent rock weathered, grading down into rock beneath. Its character is inherited from the parent.
- A transported soil is carried away and deposited elsewhere, and the transport agent leaves an unmistakable signature:
- Water (alluvial, marine) sorts grains by size — fast water drops gravel, slow water drops silt and clay — giving well-sorted, often layered deposits.
- Wind (aeolian) carries only fine sand and silt, giving very uniform, well-rounded deposits (dune sand, loess).
- Ice (glacial till) sorts nothing — it dumps everything from clay to boulders together, giving a chaotic, well-graded, often dense deposit.
- Gravity (colluvium) moves material downslope, giving loose, angular, variable deposits.
Practice
A soil sample is very uniform, with well-rounded fine sand grains and no gravel or clay. Which transport agent does this most suggest? Enter 1 for water, 2 for wind, 3 for ice.
Summary
- Soil is weathered rock: physical weathering makes coarse grains, chemical weathering makes clay minerals.
- Residual soils stay put and grade into parent rock; transported soils are moved and deposited.
- The transport agent (water, wind, ice, gravity) sets the sorting, shape and layering.
- Grading and grain shape reveal the geological history before any test.