Geotechnical Engineering

Shear Strength (Soil)

The largest shear stress a soil can sustain, τ = c′ + σ′·tan φ′ by the Mohr-Coulomb criterion.

Soil fails in shear, and its strength comes from interparticle friction, which grows with effective normal stress, plus any cohesion. The Mohr-Coulomb envelope τ_f = c′ + σ′ tan φ′ is fitted to the failure circles from triaxial or direct shear tests.

Clean sands have essentially no cohesion and a friction angle around 30° to 40°. Saturated clays loaded quickly behave as if φ = 0, with an undrained strength s_u equal to half the unconfined compressive strength. Because strength depends on effective stress, rising pore pressure weakens soil without any change in load.

How much of the FE Civil exam is Geotechnical Engineering?