** Effective stress ** is the normal force distributed per unit area that is transmitted from particle to particle in an aggregate of particles or rocks. that is, at p = 0) it provides the same tension or force behavior that is observed at p ≠ 0.[1]​ It is mainly applied in geological materials composed of particles, although the concept is valid for any porous material.

The effective tension is responsible, for example, for water escaping from wet sand when pressed. The effective tension between the sand grains increases by expelling the water inside. The study of the effective stress in the soil is essential to understand the stability of slopes, the settlement of a structure or the liquefaction of a soil, especially during an earthquake.

Karl von Terzaghi was the first to propose a formula for effective stress in 1936.[3] For him, the term "effective" meant the calculated stress "Strain (electricity)") that was effective in soils that moved or caused displacement. This tension represented the average of the tension supported by the soil skeleton.

The effective stress (σ') acting on a soil is calculated from 2 parameters, total stress (σ) and interstitial tension or water pore pressure (u) according to the following formula:.

Usually, for simple examples.