Soil types and construction suitability: particle size, gradation, cohesion

One-line orientation

Particle size, gradation, density, moisture, and plasticity help describe how soil may bear, drain, and change volume. Foundation suitability comes from the full classification and site testing, not particle size alone.

Key points

• Particle-size scale (finest → coarsest): clay → silt → sand → gravel → cobbles/boulders. Cobbles and boulders are large enough to be hand-carried; generally stable but hard to compact uniformly.
• Coarse-grained soils (sand and gravel): strength comes mainly from friction and interlocking. Bearing and drainage depend on density, gradation, confinement, and moisture; coarse particle size alone does not guarantee a good foundation soil.
• Fine-grained soils need separate treatment: clay is cohesive and plastic. Silt is generally treated as granular unless it contains enough clay to become cohesive. Suitability still depends on the full classification, density or consistency, and moisture.
• Organic soils (peat, topsoil): unsuitable — highly compressible, low strength.
• Gradation:
  • Well-graded = a good mix of fine-to-coarse sizes → denser, higher shear strength, better bearing, lower permeability. Best for foundations and roadbeds.
  • Poorly graded = mostly one particle size → looser, lower shear strength, lower bearing, but better drainage.
  • Note: “graded” is the inverse of “sorted” — a well-graded soil is poorly sorted.

Mineral soil size and gradation — plus the organic-soil exception

Schematic

Particle symbols show relative size; behavior notes compare categories, not a continuous performance scale

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"Well-graded" = poorly sorted — graded and sorted are inverse terms

Confusions / comparison

	Coarse-grained (sand/gravel)	Fine-grained (silt/clay)	Organic (peat/topsoil)
Bearing	Depends on density, gradation, and confinement	Depends on silt/clay classification, consistency, and moisture	Unsuitable
Drainage	Often better	Often slower	—
Volume change with moisture	Usually lower	Clay can shrink and swell; silt varies	—
Strength comes from	Mainly friction / interlocking	Clay: cohesion; silt may behave frictionally	—
Foundation use	Preferred	Difficult	Avoid / remove

	Well-graded	Poorly graded
Particle sizes	Good mix, fine to coarse	Mostly uniform
Density / compaction	Dense, compact	Loose
Shear strength / bearing	Higher	Lower
Drainage	Lower (less permeable)	Better
Best for	Foundations, roadbeds	Where drainage is the goal

Related

→ Soil properties (this module): bearing capacity, shear strength, Atterberg limits quantify these soils · Soil liquefaction (this module): why loose granular soils fail in earthquakes · Geotech testing (this module): how the soil type is actually determined on site.