Last reviewed 16 Sept 2026 · 6 min read
Soil in engineering
To a civil engineer, soil is any uncemented or weakly cemented accumulation of mineral particles formed by the weathering of rocks, with voids filled by water and/or air. It supports foundations, forms embankments, dams and road subgrades, and is retained by walls. Soil mechanics studies its physical and engineering behaviour; geotechnical engineering applies it.
Weathering
Rocks break down into soil by weathering:
Physical (mechanical) weathering
Disintegration without change in chemical composition. Produces coarse particles (gravel, sand, silt) that keep the parent rock's minerals.
- Temperature changes (unequal expansion of minerals), frost action (freezing water in cracks expands about 9%).
- Abrasion by water, wind and glaciers; impact of rolling boulders.
- Exfoliation (unloading), growth of plant roots, animal activity.
Chemical weathering
Decomposition that changes the mineral composition. Produces fine clay minerals.
- Hydration and hydrolysis (feldspars → clay minerals), oxidation (iron minerals), carbonation (limestone dissolves in carbonic acid), solution, leaching.
- Favoured by warm, humid climates — hence deep residual clays and laterites in tropical regions.
Residual and transported soils
| Type | Formed | Features |
|---|---|---|
| Residual soil | Remains at the place of weathering over the parent rock | Gradual transition to rock; particle size decreases upward; laterites, some red soils |
| Transported soil | Carried away from the parent rock and deposited elsewhere | Classified by the transporting agent |
Transported soils by agent
| Agent | Deposit | Characteristics |
|---|---|---|
| Running water | Alluvial (flood plains, deltas, fans) | Stratified, sorted by size; Indo-Gangetic plains |
| Still water — lakes | Lacustrine | Fine, often soft layered clays and silts (varved clays) |
| Sea | Marine | Soft, compressible clays with organic matter and salts (coastal regions) |
| Glaciers | Glacial till (boulder clay), outwash | Unsorted mixture of all sizes; moraines |
| Wind | Aeolian: loess (silt), sand dunes | Loess is uniform silt, stands in vertical cuts but collapses when wetted; dunes of uniform fine sand (Thar desert) |
| Gravity | Colluvial (talus) | Angular fragments at the foot of slopes |
Organic soils (peat, muck) form where vegetation accumulates in swamps — highly compressible, poor foundations.
Major soil deposits of India
| Soil | Region | Engineering behaviour |
|---|---|---|
| Alluvial soils | Indo-Gangetic–Brahmaputra plains, coastal deltas | Silty sands to clays; generally fair; liquefaction-prone saturated fine sands in seismic zones |
| Black cotton soils (regur) | Deccan plateau — Maharashtra, MP, Gujarat, Karnataka, Telangana | Rich in montmorillonite; high swelling and shrinkage; cracks in summer, sticky in rain — needs special foundations (under-reamed piles, CNS layer) |
| Red soils | Tamil Nadu, Karnataka, Odisha, Jharkhand | Residual; generally good bearing |
| Laterite and lateritic soils | Kerala, Karnataka coast, Odisha, NE | Residual, iron- and aluminium-rich; hardens on exposure; used as building blocks |
| Desert (aeolian) soils | Rajasthan | Uniform fine sands, low cohesion, dunes |
| Marine clays | Mumbai, Kochi, Kolkata, Chennai coastal belts | Very soft, compressible, low strength |