What is the effective stress principle?

Effective stress principle (Terzaghi) states that soil behavior is controlled by effective stress, not total stress. Effective stress (sigma') = Total stress (sigma) - Pore water pressure (u). Only particle-to-particle contact stress (effective stress) controls shear strength and consolidation. Below water table, pore pressure equals hydrostatic pressure. During loading of saturated clay, pore pressure initially carries load before dissipating and transferring to effective stress (consolidation).