In soil mechanics the total stress in the ground is shared between two parts: the pore water pressure in the voids and the effective stress carried through contact between the soil grains. The effective stress governs the strength and the settlement of the soil.
The three stresses
- Total stress (σ) — the total pressure at a given depth, caused by the weight of the overlying soil and any applied load.
- Pore water pressure (u) — the pressure of the groundwater in the pores between the grains.
- Effective stress (σ′) — the part of the stress transmitted through the grain contacts.
The relationship is Terzaghi’s classic principle:
- σ′ = σ − u
Effective stress is therefore the total stress minus the pore water pressure.
Why it matters
Only the effective stress provides the soil’s strength and stiffness; water cannot carry shear stress. This explains important practical cases:
- Dewatering — lowers the pore water pressure, so the effective stress rises and the soil settles further (settlement around excavations).
- Filling — extra soil weight first raises the pore water pressure; only when that water drains away does the effective stress increase and settlement occur (consolidation).
- Piping and quick conditions — if the pore water pressure becomes too high, the effective stress falls to zero and the soil loses its bearing capacity.
Understanding these stresses is essential for safely designing foundations, excavations, embankments and slopes.
Related terms
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