The shear strength ($\tau_f$) is defined as: $\tau_f = c' + \sigma' \tan \phi'$

• $c'$: Effective cohesion intercept.
• $\sigma'$: Effective normal stress on the failure plane.
• $\phi'$: Effective angle of internal friction.

For total stress analysis (undrained): $\tau_f = c_u + \sigma \tan \phi_u$ Often for saturated clays, $\phi_u = 0$, so $\tau_f = c_u = S_u$ (Undrained shear strength).

A sample is sheared along a predetermined horizontal plane. Normal stress is constant. Good for sands.

Sample is encased in a rubber membrane and subjected to confining pressure ($\sigma_3$). Axial load is increased ($\Delta \sigma_d$) until failure.

• UU (Unconsolidated-Undrained): Quick test, simulates end-of-construction.
• CU (Consolidated-Undrained): Pore pressures measured.
• CD (Consolidated-Drained): Slow test, no excess pore pressure.

A special case of triaxial test where confining pressure $\sigma_3 = 0$.

• $q_u$: Unconfined compressive strength.
• $c_u = q_u / 2$.