Shear strength

Soils fail in shear: grains slide and roll past each other along a surface. Foundations punching into the ground, slopes sliding and retaining walls tipping are all shear failures. Shear strength is therefore the property that decides bearing capacity, slope stability and earth pressure.

The Mohr–Coulomb criterion describes strength as τf = c′ + σ′ tan φ′, written in effective stresses: friction grows with the effective normal stress between grains, plus a small cohesion intercept for some soils. Effective stress matters because pore water carries part of the normal stress but cannot resist shear. Sands are usually free draining, so drained strength applies under static loading; under earthquake or other very rapid loading, loose saturated sands can behave undrained and may liquefy. Clays loaded quickly do not drain, so their short-term strength is the undrained shear strength cu, which does not change with total stress. Long-term stability of clay, after pore pressures settle, uses drained parameters and is often more critical for cuttings.

Strength is measured in the laboratory by the direct shear box, the triaxial test (unconsolidated undrained, consolidated undrained with pore pressure, or consolidated drained), the unconfined compression test for clays and the laboratory vane. On site, the vane shear test, cone penetration and standard penetration tests give strength indirectly. Dense sands and overconsolidated clays show a peak then soften to a lower residual strength, important for old landslides.