Effective stress

The effective stress can be defined as the stress, depending on the applied tension ${\displaystyle {\boldsymbol {\sigma }}_{ij}}$ and pore pressure ${\displaystyle p}$, which controls the strain or strength behaviour of soil and rock (or a generic porous body) for whatever pore pressure value or, in other terms, the stress which applied over a dry porous body (i.e. at ${\displaystyle p=0}$) provides the same strain or strength behaviour which is observed at ${\displaystyle p}$ ≠ 0.^[1] In the case of granular media it can be viewed as a force that keeps a collection of particles rigid. Usually this applies to sand, soil, or gravel, as well as every kind of rock and several other porous materials such as concrete, metal powders, biological tissues etc.^[1] The usefulness of an appropriate ESP formulation consists in allowing to assess the behaviour of a porous body for whatever pore pressure value on the basis of experiments involving dry samples (i.e. carried out at zero pore pressure).