Soil properties: bearing capacity, shear strength, and Atterberg limits

One-line orientation

Bearing capacity describes how much pressure soil can support. Shear strength describes resistance to sliding. Atterberg limits show how fine-grained soil changes as its water content changes.

Key points

• Bearing capacity: the maximum pressure the soil can bear. It is the key input for shallow vs deep foundation selection — weak soil drives you to deep foundations that reach a stronger stratum.
• Shear strength: resistance of soil particles to sliding past one another. In coarse-grained soil, density and confinement strongly affect strength. Fine-grained soil requires its own assessment of consistency, plasticity, moisture, and consolidation; particle size alone does not set one universal ranking.
• Atterberg limits — critical water contents at which a fine-grained soil changes state:
  • Plastic limit (lower / drier): moisture content at which the soil begins to behave plastically.
  • Liquid limit (higher / wetter): moisture content at which the soil changes from plastic behavior to liquid behavior.
  • Order, drier → wetter: plastic limit first, then liquid limit.
  • Plasticity Index (PI) = Liquid Limit − Plastic Limit — the moisture range over which the soil stays plastic. A larger PI = more plastic, more clay-rich, more shrink/swell-prone.

Atterberg limits: soil state along the moisture axis

Drier to wetter, soil crosses the plastic limit first, then the liquid limit.

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Exam trap: wetting crosses PL first (drier), LL second (wetter) — the exam reverses the sequence.

The tinted band’s width is the Plasticity Index (PI = LL − PL), the moisture range where soil stays plastic; a wider band means more clay-rich and more prone to shrink and swell.

Confusions / comparison

Limit	Where on the moisture scale	What it marks
Plastic limit (PL)	Lower / drier	Soil begins to behave plastically
Liquid limit (LL)	Higher / wetter	Soil changes from plastic behavior to liquid behavior
Plasticity Index (PI)	The band between PL and LL	PI = LL − PL = range of moisture where soil stays plastic

Soil condition	Shear strength	Load it can support
Dense coarse soil	Often higher because particles interlock under confinement	Confirm with testing
Loose coarse soil	Lower than a dense form of the same soil	Confirm with testing
Fine-grained soil	Depends on consistency, plasticity, moisture, and consolidation	Confirm with testing

Related

→ Soil types (this module): which soils are cohesive/plastic vs frictional · Soil liquefaction (this module): loss of shear strength under seismic load · Geotech testing (this module): SPT blow counts and lab tests that produce these numbers.