How is soil shear strength characterized using Mohr-Coulomb criterion?

Mohr-Coulomb criterion: tau_f = c + sigma'*tan(phi), where tau_f = shear strength, c = cohesion, phi = friction angle, sigma' = effective normal stress. For granular soils: c = 0, phi = 28-45 degrees depending on density. For clay: undrained strength cu (phi = 0 for total stress), or effective stress parameters c' and phi' from drained or CU tests. Strength parameters are determined from direct shear or triaxial tests. Peak, critical state, and residual strengths may differ significantly.