Effective stress is a fundamental concept in soil mechanics and geotechnical engineering that describes the portion of total stress in a soil mass that is carried by the solid soil skeleton, rather than the pore water. This concept applies broadly to granular media like sand, silt, and clay, as well as to porous materials such as rock, concrete, metal powders and biological tissues. It is crucial for understanding the mechanical behaviour of porous media, as effective stress governs both their strength and volume change (deformation).