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Effective stress

Only the part of the stress carried through the grain contacts gives the soil strength and compresses it. It is called effective stress and equals the total stress minus the pore pressure. This is Terzaghi's principle, the foundation of all geotechnics.

\sigma' = \sigma - u

effective stress

\gamma' = \gamma - \gamma_w

submerged unit weight

Symbols

$\sigma'$	effective stress	kPa
$\sigma$	total stress	kPa
$u$	pore pressure	kPa

Example

With $\sigma_v = 120$ kPa and $u = 40$ kPa:

$\sigma' = 80$ kPa.

If you use γ′ you have already removed the water. Then do not subtract u as well.

Practise effective stress and settlement for free →

← Total stress and pore pressure · Settlement →

Part of Geotechnical Engineering: Effective stress and settlement.