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Role of effective stress in shear strength

Short answer

Shear strength is controlled by effective stress because water has no shear strength; only grain contacts provide friction. The Mohr-Coulomb relation is τ_f = c' + σ' tanφ' = c' + (σ-u) tanφ', so increased pore pressure lowers effective stress and strength, while consolidation raises it.

What is the role of effective stress in shear strength of soil?

Shear strength of a soil is controlled by the effective stress, not by the total stress, because water has no shear strength and only the grain contacts carry friction. By Mohr–Coulomb in terms of effective stress:

τf=c′+σ′tan⁡ϕ′=c′+(σ−u)tan⁡ϕ′\tau_f = c' + \sigma' \tan\phi' = c' + (\sigma - u)\tan\phi'

A rise in pore pressure lowers σ′\sigma' and so lowers strength (as in liquefaction, quick sand and slope failure after rain). Increase of σ′\sigma' by consolidation raises strength.

✓ Answer checked by Claude Opus 5.5 (high effort)

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